Food Chemistry

FOOD SCIENCE AND TECHNOLOGY

FOOD CHEMISTRY

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FOOD SCIENCE AND TECHNOLOGY

FOOD CHEMISTRY

DONGFENG WANG HONG LIN JIANQIAN KAN LINWEI LIU XIAOXING ZENG AND

SHENGRONG SHEN EDITORS

New York

Copyright © 2012 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers‘ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. Additional color graphics may be available in the e-book version of this book.

Library of Congress Cataloging-in-Publication Data Food chemistry / editors, Dongfeng Wang ... [et al.]. p. cm. Includes index. ISBN: B 1. Food--Analysis. 2. Food--Composition. I. Wang, Dongfeng. TX531.F555 2011 664'.07--dc23 2011042522

Published by Nova Science Publishers, Inc. † New York

CONTENTS Preface

vii

Contributors

ix

About the Editors

xi

Chapter 1

Introduction Dongfeng Wang

1

Chapter 2

Water Jianqian Kan and Guoqing Huang

9

Chapter 3

Carbohydrates Dongfeng Wang, Jipeng Sun, Guoqing Huang, Xiaolin Zhou and Liping Sun

Chapter 4

Lipids Shengrong Shen, Dongfeng Wang and Undurti N. Das

107

Chapter 5

Proteins Hong Lin, Lisha Wu and Shuhui Wang

137

Chapter 6

Vitamins Yibin Zhou, Dongfeng Wang and Ping Dong

191

Chapter 7

Minerals Dongfeng Wang, Lina Yu, Haiyan Li, Bin Zhang, Shuhui Wang and Xingguo Liang

223

Chapter 8

Food Flavors Xiaoxiong Zeng and Guaoqing Huang

247

Chapter 9

Food Additives Linwei Liu and Shiyuan Dong

273

Chapter 10

Toxicants in Foods Wang Dongfeng, Guoqing Huang and Shuhui Wang

305

Index

35

353

PREFACE Foods consist of a large quantity of compounds, of which, some are original from plant or animal materials, some are new ones generated during processing or preservation, some are intentionally added by manufacturers, and some are contaminants produced during processing, preservation or packaging. These compounds undergo various changes during processing and storage and it is hence necessary to understand the effects of processing or storage on these compounds so as to enhance the nutrition, palatability and safety of foods. The purpose of Food Chemistry is to elucidate the structure, physicochemical properties, nutrition and safety of major food constituents and their changes occurred during processing and storage. Due to the extreme importance, Food Chemistry has been accepted as a major fundamental course for food-related majors. Though food chemistry has a history of more than 200 years, it developed into a relatively independent system in the late 1960‘s. Since then, the United States, Japan, Germany and other countries published several authoritative food chemistry textbooks, including Latest Food Chemistry edited by Hayashi Junzo and Kitamura Mitsuo (Japan), Food Chemistry by Sakurai Yoshito (Japan), Food Chemistry by Owen R. Fennema (United States), Food Chemistry by Belitz HD (Germany), Food Chemistry by Zhang Wang (China), and Food Chemistry by Dongfeng Wang (China). Of the works, the publications edited by Fennema and Belitz HD have been widely chosen by university students as textbook. However, the two books contain too many contents and part of them overlaps with those stated in Biochemistry and Organic Chemistry. Besides, the two books are too expensive for readers in developing countries. Hence, there is an urgent demand to publish a simplified Food Chemistry textbook that most university students can afford, which is the case of this book. This book presents the chemistry and properties of the six essential nutrients contained in foods, including water, carbohydrates, lipids, proteins, vitamins and minerals, and their changes occurred during food processing and storage. In addition, this book also deals with the chemistry and properties of flavors, food additives and toxic substances in foods. This book is simplified and cheaper than previously published books without reducing its academic level, and reflects the latest advances in food chemistry. This work can be used as a textbook by university students and especially suitable for students in developing countries and non-English speaking countries for bilingual delivery. The authors would like to thank the postgraduates of the Laboratory of Food Chemistry and Nutrition of Ocean University of China, including Mei Ding, Yan Li, Lu Yu, Xingya Li,

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Dongfeng Wang, Hong Lin, Jianqian Kan et al.

Xiang Gao, Wen Zhou, Zhe Xu, Min Wang, Mengqi Li, and Chunsheng Li, for assistance in literature collection and typesetting, and Ocean University of China for funding the publication.

CONTRIBUTORS Undurti N Das Jawaharlal Nehru Technological University, Kakinada-533 003, India Liping Sun College of Chemistry and Engineering, Kunming University of Science and Technology, Yunnan Province, China Jipeng Sun Third Institute of Oceanography State Oceanic Administration, Xiamen, China Lina Yu Shandong Peanut Research Institute, Qingdao, China Xiaoling Zhou Medical College of Shantou University, Shantou, Guangdong Province, China Bin Zhang School of Food and Pharmacy & Medical School, Zhejiang Ocean University, Zhushang City, Zhejiang Province, China Haiyan Li College of Food Science and Engineering, Ocean University of China, Qingdao, China Banping Wang College of Food Science and Engineering, Ocean University of China, Qingdao, China Xingguo Liang College of Food Science and Engineering, Ocean University of China, Qingdao, China

ABOUT THE EDITORS Dongfeng Wang is a professor of the College of Food Science and Engineering at the Ocean University of China. He has published many books related to food chemistry as editorin-chief, including Food Chemistry (2007), Advanced Food Chemistry (2009), Chemistry of Toxic Substances in Foods (2005), Technology of Experiment & Study of Tea Biochemistry (1997), Experiments on Food Quality & Food Safety (2004) and Technology of Experiments on Food Science and Engineering (2007). He has published over 120 original papers that reflect his research interests in food chemistry, tea biochemistry, carbohydrate chemistry, and preservation. He has received many teaching & academic honors, including The Second Prize for Advanced Science and Technology of China in 2010, The First Prize for Advanced Science and Technology from Ministry of Education of the People‘s Republic of China in 2009, Distinguished Teacher Awards from Shandong Province of China in 2006, and Award for Young Scientists from Anhui Province in 2000. Professor Wang received his BS degree of agriculture in 1982 from Anhui Agricultural College (Anhui, China), the MS degree of tea biochemistry in 1988 from Zhejiang Agricultural University (Zhejiang, China), and the PhD degree of inorganic biochemistry of food in 1999 from University of Science and Technology of China (Hefei, China). Jianquan Kan is a professor of the College of Food Science at Southwest University of China. He is the editor-in-chief of many books related to food chemistry, including The Practical Chemistry of Oil and Fat (1997), Food Chemistry (2002, revised in 2006 and 2008), Advanced Food Chemistry (2011), An Introduction to Food Safety (2009), Food Analysis (2011) and Experimental Methods (2011). He is the author or corresponding author of over 140 original papers covering food chemistry, food analysis and nutrition. He has been honored the Second-Class Prize of Chongqing Science and Technology Advancement (2009) and the Second Chongqing Academic and Technological Leader (2008). Professor Kan received the BS degree of chemistry from Nanchong Normal College (Sichuan, China) in 1986, the MS degree of Product Processing and Storage from Southwest Agricultural University (Chongqing, China) in 1992, and the PhD degree of Product Processing and Storage from Southwest Agricultural University (Chongqing, China) in 2003. Lingwei Liu is a professor of the College of Food Science & Engineering in Northwest A&F University, Yangling, ShaanXi, China. He has done intensive researches related to food chemistry, food analysis, nutrition and food safety. Professor Liu received the BS degree of food science from Northwest Agriculture University (ShaanXi, China) in 1982 and the PhD

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degree of food science from Northwest Agriculture & Forest University (ShaanXi, China) in 1995. Hong Lin received the BS degree in 1984, the MS degree in 1990, and the PhD degree in 1998 in seafood science from the Ocean University of China. Professor Lin is a famous expert in the seafood safety field and his research area covers novel marine organism-derived chemical and biological hazard discovery, quality control during seafood processing, and fast hazard detection method development. Professor Lin has been granted 4 invention patents, published more than 100 original articles, and edited 4 academic books, including Seafood Safety (2010), Aquatic Nutrition and Safety (2007), Effective Use of Aquatic Resources (2007), and Fish Preservation Technologies (2000). Xiaoxiong Zeng received the BS degree from Hunan Agricultural University in 1985, the MS degree from Zhejiang Agricultural University in 1988, and the PhD degree from Shizuoka University (Shizuoka, Japan) in 2000. Dr. Zeng is now a professor of the College of Food Science and Technology, Nanjing Agricultural University, China. He is one of the authors or corresponding author of over 100 original papers related to food chemistry, food biotechnology and glycobiology. Shengrong Shen is a professor of the School of Biosystems Engineering and Food Science of Zhejiang University. Professor Shen was granted the PhD degree by the Department of Biophysics of Zhejiang University in 1997. His research area includes structural analysis of such bioactive compounds as lipids, fatty acids, and polyphenols. He has published more than 100 original papers concerning food chemistry, food safety and applied nutrition in the latest 10 years. Besides, professor Shen has published 5 academic books related to food and health, tea biochemistry and food chemistry. Yibin Zhou is a professor of the Department of Food Science and Engineering at Anhui Agricultural University, Anhui, China. He had edited Food Chemistry (in Chinese) as an assistant, and is the author or corresponding author of over 40 original papers on carbohydrates, food engineering, and biotechnology. Guoqing Huang is a lecturer of the College of Food Science and Engineering, Qingdao Agricultural University, Qingdao, China. Shiyuan Dong is a lecturer of the College of Food Science and Engineering, Ocean University of China, Qingdao, China. Shuhui Wang is a PhD candidate in Biosystems Engineering Department College of Agriculture - Ginn College of Engineering, Aubum University, Auburn, AL 36849-5417, USA

In: Food Chemistry Editors: D.Wang, H. Lin, J. Kan et al.

ISBN: 978-1-61942-125-7 © 2012 Nova Science Publishers, Inc.

Chapter 1

INTRODUCTION Dongfeng Wang College of Food Science and Engineering, Ocean University of China, Qingdao, China

ABSTRACT Food Chemistry is a fundamental discipline for students, engineers, and professionals engaged in the food industry. This chapter provides an overview of this discipline, including its definition, purpose, development, and its role in food science and engineering.

1.1. Food Chemistry and Its History 1.1.1. What Is Food Chemistry Nutrients refer to the indispensable substance that provides nourishment essential for maintenance of life, growth and development of human being. The human body needs a lot of nutrients. Based on chemical structure, the nutrients can be divided into six major categories, including water, carbohydrates, proteins, lipids, vitamins and minerals. A minor difference between terms foodstuff and food should be noted first. Foodstuff refers to materials containing nutrients; while foods are materials that have been processed from foodstuff (ranging from simple cleaning to a modern factory processing) in order to meet people‘s nutritional and sensory requirements. In another word, a food shall be characterized by both nutrition and sensory satisfaction. The nutritional compositions of foods can be determined easily, but sensory satisfaction is a much complex issue and is related to the color, texture, and shape, flavor of foods in addition to the cultural background and dietary habits of consumers. The chemical compositions of foods are very complex (Figure 1-1). Of the components, some are intrinsic in animal or plant materials, some are generated during processing and storage, some are intentionally added by manufacturers, some are contaminants originated from the environment or microorganisms, and some are migrated from packing materials of

2

Dongfeng Wang

food. The purpose of Food Chemistry is to elucidate the structure, physical and chemical properties, nutritional value as well as safety of these components, their changes undergone during storage and processing, and the effects of these changes on food nutrition and palatability. The knowledge is of great importance in improving food quality, developing new food resources, evolving food processing and storage technologies, upgrading food packaging materials, and increasing food safety and quality.

Natural

Water Carbohydrates Proteins Lipids Minerals Vitamins Pigments Hormones Flavor components Toxic substances

Food components Natural additives Food Addtives Synthetic additives Unnatural

From processing Contaminants From environmental pollution

Figure 1-1. Composition of foods.

Food Chemistry is a comprehensive discipline and partially overlaps with chemistry, biochemistry, physical chemistry, botany, zoology, food nutrition, food safety, polymer chemistry, environmental chemistry, toxicology, molecular biology, and many other subjects. Food Chemistry associates the most closely with chemistry and biochemistry and it is the extension of the two subjects to the food area. However, the subjects have different contents and focuses. The chemistry subject deals mainly with the composition, property, and reactions of molecules, biochemistry focuses on the reactions and changes of various components in organisms under suitable or moderately suitable conditions, while food chemistry is interested in the changes of components occurred in such unsuitable conditions as freezing, heating, and drying, their interactions during these processes, and the effects of these changes on the nutrition, safety, and sensory properties (such as color, flavor, taste, and shape) of foods. 1.1.2. History of Food Chemistry It is a short time since Food Chemistry is accepted as an independent subject. However, the researches and reports related to this subject have been started since the last 1700s. Many components were separated from foods by chemists and botanists at that time and Researches on the Chemistry of Food by Justus von Liebig in 1847 is recognized as the first book related to food chemistry.

Introduction

3

As the trading of foods between regions and countries increased, both consumers and manufacturers had urgent needs on the information of water contents and the presence nonfood components in foods. Meanwhile, driven by the rapid development of analysis measures, the desire to understand the natural characteristic of foods also grew. In 1860, German scholars Hanneberg W. and Stohman F. invented a method for the simultaneous determination of water, crude fat, ash, and nitrogen contents. Several years later, diets containing solely proteins, lipids, and carbohydrates were found insufficient for maintaining life. In 1900s, with the advancement of analytical techniques and the biochemistry subject and the rapid development of the food industry, requirements on new food processing technologies and prolonged storage life emerged, which drove the quick development of food chemistry. During this period, a growing number of researches papers were published and the quantity of related journals increased significantly as well, including Archives of Biochemistry and Biophysics (initiated in 1942), Journal of Agricultural and Food Chemistry (initiated in 1953) and Food Chemistry (initiated in 1966). Due to the emergency of increasing deep and systematic publications, Food Chemistry gradually developed into an independent subject. Chinese scholars Yanbin Xia and Ruijin Yang divide the history of Food Chemistry into four stages. Stage one: Many natural components were separated from plants and animals and were identified, including lactic acid, citric acid, malic acid, and tartaric acid. The knowledge was not systematic yet and was reported mainly by chemists. Stage two: In the early 1900s (1820 ~ 1850), food chemistry developed quickly along with the development of agricultural chemistry and gained much importance in Europe. Specialized food chemistry laboratories were established and many professional journals related to food chemistry were issued. Meanwhile, adulteration became a serious issue and the need for impurity determination propelled the development of food chemistry. In this stage, Justus von Liebig invented an optimized method for quantitative analysis of organic substances and published Researches on the Chemistry of Food in 1847. Stage three: In the middle 1900s century, the British scientist Arthur Hill Hassall reported the microscopic images of pure and adulterated foods and food chemistry came into the microanalysis time. In 1871, Jean Baptis M.D.M. proposed that diets containing only proteins, carbohydrates and lipids were insufficient to sustain human‘s life. The interests on the nutritional requirements further accelerated the development of food chemistry. Until the first half of the 20th century, the majority of components in foods were identified and the number of literatures related to chemistry food increased markedly. Food chemistry then turned to be a mature and independent subject in mid-20th century. Stage four: Food chemistry is now in the fourth stage. With the rapid development of society, economy, science and technology, and the improvement of living standards, consumers raise higher requirements on food security, nutrition, palatability, and convenience. Meanwhile, to realize the transformation from traditional to scaled, standardized, and modernized processing of foods, more and more new technologies, materials, and equipment are used, which markedly drive the rapid development of food chemistry. Besides, the advancement of basic chemistry, biochemistry, instrumental analysis and other related subjects guarantee the rapid development of food chemistry. Food chemistry has become a most important subject for food scientists [1, 2].

4

Dongfeng Wang

1.1.3. Food Chemistry Textbooks A series of food chemistry textbooks were published between 1976 to 1985, including Latest Food Chemistry by Hayashi Junzo and Kitamura Mitsuo (Japan), Food Chemistry by Sakurai Yoshito (Japan), Food Chemistry by Owen R. Fennema (United States), and Food Chemistry by Belitz HD (Germany), in which, the works of Fennenma and Belitz HD contributed a lot to the development of food chemistry and has been widely chosen by university students as textbook. Food Chemistry has been chosen as a fundamental course for food related majors.

1.2. The Role of Food Chemistry in Food Science and Engineering Foodstuff undergoes various chemical and biochemical reactions during storage, transport, and processing. These reactions might yield products that are either beneficial to food nutrition and palatability or harmful to consumers. The knowledge of food chemistry is hence of extreme importance, because the purpose of this subject is to elucidate the changes of various food components occurred during storage, transport, and processing and the effects of these changes on food quality. In recent years, the control of composition, property, structure, and interaction of various food components, the chemical nature of the nutrition and palatability of complex food systems, and the exploitation of new food resources constitute the new contents of food chemistry. With the development of science and technologies and the extension of other fundamental subjects to the food industry, more and more toxic and harmful chemicals in foods are identified and food chemistry has turned to be the theoretical foundation for guaranteeing food quality and safety. Food chemistry plays an important role in food science and engineering and is developing quickly. 1.2.1. Role of Food Chemistry in Technology Advancement Nutrition, healthcare, safety, and enjoyment are the four fundamental attributes of foods required by the modern food industry. The theories and application research results of food chemistry are guiding the healthy and sustainable development of the food industry (Table 11). Practice has proved that, no the theoretical guidance of food industry, no the ever growing modern food industry. Table 1-1. Impact of food chemistry on technological advancement of the food industry [3, 4] Food Industry Basic food industry Storage and processing of fruits and vegetables

Application Flour improving; starch modification; new edible materials exploitation; high-fructose syrup; food enzymes; molecular basis of food nutrition; new sweetener and natural additive development; new oligosaccharide production; oil modification; vegetable protein isolates; functional peptides production; microbial polysaccharides and single cell protein development; development and utilization of wild, marine, an edible drug resources, etc. Chemical peeling; color protection; texture control; vitamin retention; deastringency and debittering; coating and waxing; chemical preservation; controlled atmosphere storage; bioactive packaging; enzyme-assisted juicing, filtration and clarification; chemical preservation, etc.

Introduction

5

Table 1-1. (Continued) Food Industry Storage and processing of meats

Beverage industry

Dairy industry Baking industry Edible oils and fats industry Condiments industry Fermented food industry Food safety Food inspection

Application Post-slaughter processing; juice preservation and tenderization; color protection and development; enhancement of the emulsifying capacity, gelling capacity, and viscoelasticity of meat; frozen denaturation of proteins; fresh meat packaging in supermarket; production and application of fumigation agent; artificial meat production; comprehensive utilization of viscera, etc Instant dissolution; ingredient floating and/or sinking inhibition; protein beverage stabilization; water treatment; juice stabilization; juice color protection; flavor enhancement; alcohol degree decrease; beer clarification; beer foamability and bitterness improvement; chemical nature and prevention of beer non-biological stability; off-flavor elimination; juice deastringency; soybean odor elimination, etc. Yoghurt and juice milk stabilization; chymosin substitute development; whey utilization; nutrition fortification of diary products; etc. High-efficiency leavening agent development; crispness improvement; bread color and texture modification; aging and mildewing inhibition; etc. Lipid refinement; lipid modification; development and utilization of DHA, EPA, and MCT; food emulsifier and anti-oxidant development; oil absorption reduction of fried foods; etc. Meat soup production; nucleotide-type flavor enhancers; organic iodinesupplemented salt; etc. Post-processing of fermented foods; flavor changes during postfermentation; comprehensive utilization of biomass and residues; etc. Source identification of exogenous toxicants and their prevention; identification of endogenous toxicants and their elimination; etc Formulation of inspection standards; rapid analysis; biosensor development; fingerprint preparation of products; etc.

Due to the rapid development of food chemistry, some important reactions, including the Millard reaction, caramelization, lipid auto-oxidation, starch gelatinization and aging, polysaccharide hydrolysis and modification, protein hydrolysis and denaturation, pigment discoloration, vitamin degradation, metal-catalyzed reactions, enzyme-catalyzed reactions, fat hydrolysis and transesterification, lipid thermo-oxidative decomposition and polymerization, flavor compound changes, action mechanisms of food additives, generation of harmful ingredients as well as postharvest physiology, are identified in foods. The knowledge on these reactions greatly enhances the development of the food industry. 1.2.2. Role of Food Chemistry in Human Nutrition and Health It has been more than two centuries since proteins, carbohydrates and lipids were identified as the three major nutrients for human. The two most important attributes of foods are to provide consumers with nutrition and sensory satisfaction. One of the objectives of food chemistry is to investigate the nutrition and flavor composition in food materials and processed foods and the interactions of the components occurred during processing and storage and effects of these interactions on food nutrition and palatability. The modern food

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Dongfeng Wang

chemistry should not only ensure the healthcare and enjoyment attributes of food components, but also guide consumers on rational diet selection. The concept of nutrition has evolved significantly due to social development and the change of the healthy status of consumers. How to reduce the incidences of diet-related diseases, such as cardiocerebrovascular diseases, cancers, and diabetes, has turned to be a new major task of food industry. In addition to the healthcare attribute, foods should also provide desirable flavors so that consumers enjoy the eating process. The emergence of biotechnologies and new food processing technologies guarantees the safety of foods. Contamination of foods by pollutants is currently a worldwide concern due to global environmental deterioration. The analysis and identification of trace and ultramicro substances are of vital importance to the nutrition value and the control of toxicants of foods. The development of food chemistry has been associated with the healthy status and civilization level of human.

1.3. Research Methods of Food Chemistry Each type of food contains a large number of components and is thereby a much complex system. Hence, the research methods of food chemistry are quite different from those of common chemistry subjects. In food chemistry, the knowledge on the chemical composition, physicochemical properties, and changes of food components must be associated with the nutrition, enjoyment, and safety of foods. The experimental design of food chemistry should reveal the complex composition of food systems and the changes of the nutrition value, enjoyment, and safety of foods during processing and storage. The interactions between food components and their changes occurred during storage and processing (such as ultra-high pressure, high temperature, freezing, presence or absence of oxygen) are extremely complex. Hence, many researches are carried out in simplified and stimulated models, which must be then verified in real food systems. The experiments of food chemistry include mainly physicochemical experiments and sensory evaluation experiments. Physicochemical experiments reveal the composition of foods and the structures of the components, including nutrients, toxicants, and flavors; while sensory experiments evaluate the texture, flavor, and color changes of foods through visual inspection. Foods or food materials undergo a series of changes during storage, transport, processing, and sales. The changes include: enzymatic and chemical reactions in raw and fresh materials; changes caused by water activity variance; component decomposition, polymerization, and denaturation under violent conditions (high temperature, high pressure, mechanical actions); oxidation induced by oxygen or other oxidants; photochemical reactions; and migration of packaging materials to foods. Of the changes, non-enzymatic browning, lipid oxidation and hydrolysis, protein hydrolysis and denaturation, protein cross-linking, oligosaccharide and polysaccharide hydrolysis, and change of the presence form of natural pigments and their degradation, are the most important reactions for the food industry. Of the reactions, some are desired, but some are unexpected and must be avoided during processing (Table 1-2). The mechanisms and control of these reactions constitute the key contents of food chemistry.

Introduction

7

Table 1-2. Part reactions occurred during food processing and storage and their influences on foods [3, 4] Reaction Nonenzymic browning Oxidation Hydrolysis

Isomerization Polymerization

Protein denaturation

Examples Color development in bakery foods Oxidation of lipids, vitamins, an phenols Hydrolysis of lipids, proteins, and carbohydrates

Influence on foods Desired or undesired color, smell, taste; loss of nutrition; harmful ingredients. Change color; desired flavor or off-odors and toxicants Increased soluble solids content; texture changes; desired color, flavor, taste, and nutrition; toxicity loss of certain components

cis-trans isomerization of lipids Foam and insoluble brown precipitate forming in frying Egg white coagulation; enzyme inactivation

Discoloration; formation or loss of certain functions Discoloration; loss of nutrition; off-odor development; toxicants formation Improved nutrition; toxicity loss of certain components

The research fruits and methods of food chemistry have been widely absorbed by the food industry and greatly promote the development of the food industry. In the last decades, some new subjects and research areas, such as structural chemistry, free radical chemistry, membrane separation, edible package, microencapsulation, extrusion, superfine comminution, bioactive packaging, supercritical extraction, molecular distillation, membrane catalysis, bioreactor, toxicant chemistry of foods, molecular nutrition, and nutria-genomics, have been established. These new technologies and subjects will undoubtedly facilitate the rapid development of the food industry, which in turn benefits the improvement of the food chemistry subject.

REFERENCES [1] [2] [3] [4]

Wang, DF. Food Chemistry.1st edition. Beijing: Chemistry Industry Press; 2007 Damodaran, S; Parkin, KL; Fennema, OR. Fennema’s Food Chemistry. 4th edition. New York: CRC Press; 2007. Kan, JQ. Food Chemistry. 1st edition. Beijing: China Agricultural University Press; 2002. Wang, Z. Food Chemistry. 1st edition. Beijing: China Light Industry Press; 2005.

In: Food Chemistry Editors: D.Wang, H. Lin, J. Kan et al.

ISBN: 978-1-61942-125-7 © 2012 Nova Science Publishers, Inc.

Chapter 2

WATER 1

Jianqian Kan1 and Guoqing Huang2

College of Food Science, Southwest University, Chongqing, China College of Food Science and Engineering, Qingdao Agricultural University, Qingdao, China

2

ABSTRACT Water is an important component in many foods. Its content and occurrence status significantly affect the flavor, texture, and stability of foods. This chapter deals with the various physical and chemical properties of water and ice and the interactions with other components in foods. Water occurs in multiple states due to interactions with solutes and the interactions significantly affect the bioavailability of water to chemical reactions and microorganisms. To distinguish the differences between water content and its bioavailability, the term water activity (aw) is proposed and its application in food stability predication are detailed. The relationship between water content and aw can be presented by moisture sorption isotherm (MSI), which is very useful in designing the concentration and dehydration processes of foods. In addition to aw, molecular mobility (Mm) has also been proposed to predict food stability. Its definition and its effect on food stability are also a concern of this chapter. Water is a predominant constituent in many foods (Table 1). Water in proper amount, location, and orientation profoundly influences the structure, appearance, and taste of foods and their susceptibility to spoilage. Because medium water supports chemical reactions and water is a reactant in hydrolytic processes, the removal of water from foods retards many reactions and inhibits the growth of microorganisms, thus improving the shelf lives of a number of foods. Through physical interaction with proteins, polysaccharides, lipids and salts, water contributes significantly to food texture. Water is essential to life: as an important governor of body temperature, as a solvent, as a carrier of nutrients and waste products, as a reactant and reaction medium, as a lubricant and plasticizer, as a stabilizer of biopolymer conformation, as a likely facilitator of the dynamic behavior of macromolecules, including their catalytic (enzymatic) properties, and in other ways yet unknown.

10

Jianqian Kan and Guoqing Huang Table 1. Water contents of some foods [1] Food Pork, raw, composite of lean Beef, raw, retail cuts Chicken, all classes, raw meat without skin

Water content (%) 53~60 50~70

Food

Water content (%) 10~13 20

Fish, muscle proteins

65~81

bananas

75

Berries, cherries, pears Apples, peaches, oranges, grapefruit Rhubarb, strawberries, tomatos Butter, margarine Milk powder

80~85

Cereal flour Honey Avocado, bananas, peas (green) Beets, broccoli, carrots, potatoes Asparagus, beans (green), cabbage, cauliflower, lettuce Bread

85~90

Biscuits

3~8

90~95

Tea

3~7

15 4

Edible oil

0

74

74~80 80~85 90~95 35~45

1. PHYSICAL AND CHEMICAL PROPERTIES OF WATER AND ICE 1.1. The Water Molecule and Its Association 1.1.1. The Water Molecule The water molecule is comprised of two hydrogen atoms interacting with the two sp3 bonding orbitals of oxygen, forming two covalent σ bonds. A schematic orbital model of a water molecule is shown in Figure 1.a and the appropriate van der Waals radii are shown in Figure 1.b.

Figure 1. Schematic model of a single HOH molecule: (a) sp3 configuration, and (b) van der Waals radii for a HOH molecule in the vapor state [1].

Water

11

In the vapor state, the bond angle of an isolated water molecule is 104.5°. The O-H internuclear distance is 0.96 Å and the van der Waals radii for oxygen and hydrogen are 1.40 and 1.2 Å respectively.

1.1.2. Association of Water Molecules Each water molecule has an equal number of hydrogen-bond donors and receptor sites and is able to hydrogen-bond with a maximum of four water molecules. The resulting tetrahedral arrangement is shown in Figure 2. The two unshared electron pairs (n-electrons or sp3 orbitals) of oxygen act as H-bond acceptor sites and the H-O bonding orbitals act as hydrogen bond donor site. The dissociation energy of this hydrogen bond is about 1125kJ/mol. As mentioned above, each water molecule can hydrogen bond with at most four water molecules and the resultant three-dimensional structure is quite stable. This structure is quite different from those formed by other small molecules that also involved in hydrogen bonding (such as NH3 and HF). Ammonia has three hydrogen-bond donors and one hydrogen-bond receptor, while HF has one hydrogen and three receptor sites. Both the two chemicals do not have equal numbers of donor and receptor sites and therefore can form only two dimensional hydrogen-bonded networks. The above mentioned polarization of H-O bonds is transferred via hydrogen bonds and extends over several bonds. Therefore, the dipole moment of a complex consisting of increasing numbers of water molecules is higher as more molecules become associated and is certainly much higher than the dipole moment of a single molecule. Proton transport takes place along the H-bridges. It is actually the jump of a proton from one water molecule to a neighboring water molecule. In this way a hydrate H3O+ ion is formed with an exceptionally strong hydrogen bond (dissociation energy about 100kJ/mol). A similar mechanism is valid in transport of OH- ions, which also occurs along hydrogen bridges (Figure 3).

Figure 2 Hydrogen bonding of water molecules in a tetrahedral configuration. Open circles are oxygen atoms and closed circles are hydrogen atoms. Hydrogen bonds are represented by dashed lines [1].

Figure 3. Proton transport in water [2].

12

Jianqian Kan and Guoqing Huang Table 2. Coordination number and distance between two water molecules [2]

Ice (0°C) Water (1.5°C) Water (83°C)

Coordination number 4.0 4.4 4.9

O-H…O Distance 0.276 nm 0.290 nm 0.305 nm

Table 3. Comparisons of the melting and boiling points of methanol, dimethyl ether, and water Formula H2O CH3OH CH3OCH3

Fp/°C 0.0 -98 -138

Kp/°C 100.0 64.7 -23

1.2. Structures of Water and Ice 1.2.1. The Structure of Water (Liquid) Due to the strong tendency of water molecules to associate through H-bridges, liquid water is highly structured as ice, but not sufficiently established to produce long-range rigidity. The major difference between liquid water and ice lies in the coordination number and the distance between neighboring water molecules (Table 4). The degree of intermolecular hydrogen bonding among water molecules is temperature dependent. Ice at 0°C has a coordination number of 4.0, with nearest neighbors at a distance of 2.76 Å. As the temperature increases, the coordination number increases from 4.0 in ice at 0°C, to 4.4 in water at 1.50°C, then to 4.9 at 83°C. Simultaneously, the distance between nearest neighbors increases from 2.76 Å in ice at 0°C, to 2.9 Å in water at 1.5°C, then to 3.05 Å at 83°C. The increase in the distance between nearest neighbors during ice-water transformation decreases the water density, while the increase in the coordination number increases water density. The maximum water density is observed in 3.98°C and then declines gradually. The hydrogen-bound water structure can be changed in the presence of dissolved salts or molecules with polar and/or hydrophobic groups. For example, in salt solutions the nelectrons occupy the free orbitals of the cations, forming ―aqua complexes‖. Other water molecules then coordinate through H-bridges, forming a hydration shell around the cation and disrupting the natural structure of water. In addition, hydration shells are also formed by polar groups through dipole-dipole interaction or H-bridges, again leading to the disruption of the structure of water. The three-dimensional hydrogen-bound structures of ice and water impart them with unique properties and extra energy is needed for disrupting the structures. Table 3. lists the comparisons of the melting and boiling points between methanol, dimethyl ether, and water.

Water

13

Figure 4. Unit cell of ordinary ice at 0°C. Circles represent oxygen atoms of water molecules. Nearestneighbor internuclear O-O distance is 2.76 Å; θ is 109° [3].

1.2.2. The Structure of Ice Ice is the orderly organized crystal of water molecules. The O-O internuclear distance between nearest neighboring water molecules in ice is 2.76 Å and the O-O-O bond angle is about 109°, which is very close to the perfect tetrahedral angle of 109°28'. As shown in Figure 4, each water molecule is associated with four other water molecules 1, 2, 3, and W'. Because pure water contains H3O+, OH–, and negligible isotope variants (such as those containing 16O, 1H, 17O, 18O, and 2H) in addition to ordinary water molecules, actual ice is not present as the perfect crystal shown in Figure 4. Due to the presence of H3O+, OH– and their dislocation, ice crystals suffer both orientational and ionic defects. Only at temperatures near -180°C or lower will all hydrogen bonds be intact, and as the temperature is raised, the mean number of intact (fixed) hydrogen bonds will decrease gradually. The amount and kind of solutes present in foods influence the quantity, size, structure, location, and orientation of ice crystals. The four major ice structures are hexagonal forms, irregular dendrites, coarse spherulites, and evanescent spherulites. The hexogonal form, which is most highly ordered, is found exclusively in foods, provided extremely rapid freezing is avoided and the solute is of a type and concentration that does not interfere unduly with the mobility of water molecules.

2. STATES OF WATER IN FOODS 2.1. Water-Solute Interactions Mixing of solutes and water results in altered properties of both water and solutes. Hydrophilic solutes change the structure and mobility of adjacent water, and water causes changes in the reactivity, and sometimes structure, of hydrophilic solutes. Hydrophobic groups of added solutes interact only weakly with adjacent water. Interactions between water and specific classes of solutes are considered below.

14

Jianqian Kan and Guoqing Huang

2.1.1. Interaction of Water with Ions and Ionic Groups Ions and ionic groups of organic molecules hinder the mobility of water molecules to a greater degree than do any other types of solutes. The strength of electrostatic water-ion bonds is greater than that of water-water hydrogen bonds, but is much less than that of covalent bonds. The normal structure of pure water (based on a hydrogen-bonded, tetrahedral arrangement) is disrupted by the addition of dissociable solutes. Water and simple inorganic ions undergo dipole-ion interactions. The example in Figure 5. involves hydration of the NaCl ion pair. In a dilute solution of ions in water, second-layer water is believed to exist in a structurally perturbed state because of conflicting structural influences of first-layer water and the more distant, tetrahedrally oriented ―bulk-phase‖ water. In concentrated salt solutions, water structure would be dominated by the ions. The ability of a given ion to alter net structure is related closely to its polarizing power (charge divided by radius) or simply the strength of its electric field. Ions that are small and/or multivalent (mostly positive ions, such as Li+, Na+, H3O+, Ca2+, Ba2+, Mg2+, Al3+, F–, and OH–) have strong electric fields and are net structure formers. These ions strongly interact with the four to six first-layer water molecules, causing them to be less mobile and pack more densely than HOH molecules in pure water. Ions that are large and monovalent (most of the negatively charged ions and large positive ions, such as K+, Rb+, Cs+, Cl–, Br–, I–, NO3– , BrO3–, IO3– and CIO4– have rather weak electric fields and are net structure breakers, although the effect is very slight with K+. These ions disrupt the normal structure of water and fail to impose a compensating amount of new structure. Ions, through their varying abilities to hydrate (compete for water), alter water structure, influence the permittivity of the aqueous medium, and govern the thickness of the electric double layer around colloids, profoundly influence the ―degree of hospitality‖ extended to other nonaqueous solutes and to substances suspended in the medium. Thus, conformation of proteins and stability of colloids (salting-in, salting-out in accord with the Hofmeister or lyotropic series) are greatly influenced by the kinds and amounts of ions present.

Figure 5. Likely arrangement of water molecules adjacent to sodium chloride. Only water molecules in plane of paper are shown [3].

Water

15

2.1.2. Interaction between Water and Neutral Groups Possessing Hydrogen-Bonding Capabilities Interactions between water and nonionic, hydrophilic solutes are weaker than water-ion interactions and about the same strength as those of water-water hydrogen bonds. Therefore, solutes capable of hydrogen bonding might be expected to enhance or at least not disrupt the normal structure of pure water. However, in some instances it is found that the distribution and orientation of the solute's hydrogen-bonding sites are geometrically incompatible with those existing in normal water. Thus, these kinds of solutes, such as urea, frequently have a disruptive influence on the normal structure of water. It should be noted that the total number of hydrogen bonds per mole of water may not be significantly altered by addition of a hydrogen-bonding solute that disrupts the normal structure of water. This is possible since disrupted water-water hydrogen bonds may be replaced by water-solute hydrogen bonds. Hydrogen bonding of water can occur with various potentially eligible groups (e.g., hydroxy1, amino, carbony1, amide, imino groups, etc.). This sometimes results in ―water bridges‖, where one water molecule interacts with two eligible hydrogen-bonding sites on one or more solutes. A schematic depiction of water hydrogen bonding (dashed lines) to two kinds of functional groups found in proteins is shown in Figure 8. A more elaborate example involving a three-HOH bridge between backbone peptide units in papain is shown in Figure 9.

Figure 8. Hydrogen bonding (dotted lines) of water to two kinds of functional groups occurring in proteins [3].

Figure 9. Examples of a three-molecule water bridge in papain; 23, 24, and 25 are water molecules [4].

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Jianqian Kan and Guoqing Huang

2.1.3. Interaction of Water with Nonpolar Substances The mixing of water and hydrophobic substances, such as hydrocarbons, rare gases, and the apolar groups of fatty acids, amino acids, and proteins, enhances the hydrogen bonding of water molecules in the vicinity of hydrophobic groups due to the repulsion with water. This process has been termed ―hydrophobic hydration‖. Because hydrophobic hydration is thermodynamically unfavorable, water would tend to minimize its association with apolar entities that are present. Thus, if two separated apolar groups are present, the incompatible aqueous environment will encourage them to associate, thereby lessening the water-apolar interfacial area. This process is thermodynamically favorable and is referred to as ―hydrophobic interaction‖. Two aspects of the antagonistic relationship between water and hydrophobic groups are: formation of clathrate hydrates and association of water with hydrophobic groups in proteins. A clathrate hydrate is an ice-like inclusion compound wherein water, the ―host‖ substance, forms a hydrogen-bonded cage-like structure that physically entraps a small apolar molecule known as the ―guest molecule.‖ The guest molecules of clathrate hydrates are characteristically low-molecular-weight compounds with sizes and shapes compatible with the dimensions of host water cages comprised of 20–74 water molecules. Typical guests include low-molecular-weight hydrocarbons and halogenated hydrocarbons; rare gases; short-chain primary, secondary, and tertiary amines; and alkyl ammonium, sulfonium, and phosphonium salts. Interaction between water and guest is slight, usually involving nothing more than weak van der Waals forces. Clathrate hydrates are the extraordinary result of water's attempt to avoid contact with hydrophobic groups. There is evidence that structures similar to crystalline clathrate hydrates may exist naturally in biological matter, and if so, these structures would be of far greater importance than crystalline hydrates since they would likely influence the conformation, reactivity, and stability of molecules such as proteins.

Figure 10. Schematic depiction of a globular protein undergoing hydrophobic interaction. Open circles are hydrophobic groups, ―L-shaped‖ entities around circles are water molecules oriented in accordance with a hydrophobic surface, and dots represent water molecules associated with polar groups [2].

Water

17

Because exposure of protein nonpolar groups to water is thermodynamically unfavorable, association of hydrophobic groups or ―hydrophobic interaction‖ is encouraged, and this occurrence is depicted schematically in Figure 10. Hydrophobic interaction provides a major driving force for protein folding, causing many hydrophobic residues to assume positions in the protein interior. Hydrophobic interactions also are regarded as being of primary importance in maintaining the tertiary structure of most proteins. It is therefore of considerable importance that a reduction in temperature causes hydrophobic interactions to become weaker and hydrogen bounds to become stronger.

2.1.4. Interaction of Water with Amphiphilic Substances Water functions as the dispersion medium of amphiphilic compounds, such as fatty acid salts, lipoproteins, glycolipids, polar lipids, and nucleic acids, in some foods. Water associates with the hydrophilic entities (COO–, OH, PO4–, –C=O, or those containing the nitrogen atom) and dissolves the compounds. Amphiphilic compounds occur as micelles in water and each micelle contains hundreds or thousands of the molecules. The apolar groups are directed to the interior of the micelles, while polar groups are distributed in the water environment.

2.2. Water in Foods Foods are composed of proteins, polysaccharides, minerals, pigments, and many other constituents in addition to water. These constituents interact with water and significantly affect the properties and status of water. Generally, the water in foods can be classed as ―bulk water‖ and ―bound water‖.

2.2.1. Bound Water ―Bound water‖ is water that exists in the vicinity of solutes and other nonaqueous constituents and binds to other solutes through covalent bonds. According to the binding strength, bound water is further divided into the following three types: Constitutional water: Water of this type is a constituent of other compounds and binds the most tightly. Water in hydrates belongs to this type. Monolayer water: Water of this type is the first layer water bound to the hydrophilic groups of solutes. The forces involved include mainly water-ion or water-polar association, followed by hydrogen bonding between water and solutes. Multilayer water: Water of this type refers to water distributed in multiple layers around nonaqueous components. The forces involved are water-water and water-solute hydrogen bonding. Multilayer water binds tightly to nonaqueous components, but the strength is lower than that of monolayer water. Besides, multilayer water has changed properties compared with ordinary water. 2.2.2. Bulk Water Bulk water or free water is not chemically bound to nonaqueous compounds and mainly includes water that is physically entrapped. Based on the physical interaction, bulk water is further divided into two types:

18

Jianqian Kan and Guoqing Huang

Entrapped water: Water of this type is entrapped by microstructures or ultrastructures and cannot flow freely as pure water. Capillary water: Water of this type is restricted in the gaps between cells or the capillaries of food structures. Capillary water has similar reduced fluidity and vapor pressure as entrapped water. As mentioned above, the states of water in foods depend on the composition of foods and the physical status of the components. Water states and contents significantly influence the structure, processing properties, and stability of foods. The differences between bulk water and bound water are shown in Table 4. According to Table 4, bound water and bulk water differ in the following: 1. Bound water associates with nonaqueous constituents more tightly and its vapor pressure is much lower than bulk water. More energy is required for removing bound water than bulk water and the removal of bound water might irreversibly degrade the flavor, texture, and other properties of foods. 2. Bound water freezes in much lower temperature than bulk water. This explains why plant seeds and microbial spores can survive low temperatures. In contrast, juicy fruits and vegetables have much higher water contents and their tissues are susceptible to damage by ice crystals in low temperatures. 3. Bound water cannot dissolve solutes. 4. Bulk water can be utilized by microorganisms, while bound water cannot. Table 4. Comparisons of bulk water and bound water Item General description Freezing point Solute solubilization capability Molecular movement compared with pure water Enthalpy of vaporization compared with pure water Percentage among total water in high-moisture (90%) foods

Bound water Occurs in vicinity of solutes and other nonaqueous constituents and includes constitution water, monolayer water, and multilayer water Not frozen even at temperatures lower than -40°C

Bulk water Locates far away from solutes and occurs as water-water hydrogen bonding Slightly lower than that of pure water

None

Yes

Markedly reduced or none

Changed slightly

Increased

Nearly not changed

Less than 0.03%

ca. 96%

3. WATER ACTIVITY Intensive researches have indicated that no relationship can be established between the water content of a food with its physiochemical properties or stability. It has also been

19

Water

observed that various types of foods with the same water content differ significantly in perishability. Thus, water content alone is not a reliable indicator of perishability. This situation is attributable, in part, to differences in the intensity with which water associates with nonaqueous constituents. The term ―water activity‖ (aw) was developed to account for the intensity with which water associates with various nonaqueous constituents. Experience shows that food stability, safety, and other properties can be predicted far more reliably from aw than from water content.

3.1. Definition and Measurement of aw The water activity (aw) is defined as follows: aw

P P0

RVP

ERH 100

(1)

where, RVP is the relative vapor pressure; P is the partial vapor pressure of food moisture at temperature T; P0 is the saturation vapor pressure of pure water at temperature T, and ERH is the equilibrium relative humidity at temperature T. Equation (1) applies only to ideal solutions and thermodynamically equilibrium systems and the values obtained are only approximate for food systems. The RVP of a food can be determined by placing it in a closed chamber for a time sufficient to achieve apparent equilibrium (constant weight) and then measuring either pressure or relative humidity in the chamber. The vapor pressures of the aqueous solution of solutes are generally lower than that of pure water and aw hence falls in the range 0~1.

3.2. Temperature Dependence of aw aw is temperature dependent, and the modified Clausius-Clapeyron equation (3) can be used to precisely present its relationship with the absolute temperature: d ln aw d (1/ T )

H R

(3)

where, T is the absolute temperature, R is the gas constant, and ⊿H is the isosteric net heat of sorption at the water content of the sample. By rearrangement, equation (2-4) can be obtained: Inaw

k

H R

1 T

(4)

where, R and T have the same meaning as those in Equation (2-3), ⊿H is the latent heat of vaporization of pure water (40.5372kJ/mol), and k is calculated from the following formula:

20 k

Jianqian Kan and Guoqing Huang Absolute temperature of the sample - Absolute temperature of pure water in vapor pressure p Absolute temperature of pure water in vapor pressure p

Plots of Inaw versus 1/T are not always linear over broad temperature ranges, and they generally exhibit sharp breaks with the onset of ice formation. Figure 11. is a plot of logaw versus 1/T, illustrating that (a) the relationship is linear at subfreezing temperatures, (b) the influence of temperature on RVP is typically far greater at subfreezing temperatures than at above-freezing temperatures, and (c) a sharp break occurs in the plot at the freezing point of the sample. Below freezing temperatures, the water activity (aw) can be calculated as follow:

aw

p ff

pice

p0 ( SCW )

p0 ( SCW )

(5)

where, pff is the partial pressure of water in partially frozen food, p0(SCW) is the vapor pressure of pure supercooled water, and pice is the vapor pressure of pure ice. Two important distinctions should be noted when comparing aw values at above- and below-freezing temperatures. First, at above-freezing temperatures, aw is a function of sample composition and temperature, with the former factor predominating. At subfreezing temperatures, aw becomes independent of sample composition and depends solely on temperature; that is, in the presence of an ice phase aw values are not influenced by the kind or ratio of solutes present. Hence, the knowledge of aw at a subfreezing temperature cannot be used to predict aw at an above-freezing temperature. Second, as the temperature is changed sufficiently to form or melt ice, the meaning of aw, in terms of food stability, also changes. For example, in a product at -15°C (aw =0.86), microorganisms will not grow and chemical reactions will occur slowly. However, at 20°C and aw 0.86, some chemical reactions will occur rapidly and some microorganisms will grow at moderate rates.

Figure 11. Relationship between relative vapor pressure and temperature for a complex food above and below freezing [5].

Water

21

4. MOISTURE SORPTION ISOTHERM 4.1. Definition and Zones of Moisture Sorption Isotherm A plot of water content (expressed as mass of water per unit mass of dry material) of a food versus aw at constant temperature is known as a moisture sorption isotherm (MSI). The MSI of a food system is of great importance for the following reasons: 1. The ease of dehydration during concentration or drying is aw dependent; 2. The migration of water between materials during blending must be avoided; 3. It determines whether the determination of the moisture barrier properties of packaging materials is necessary; 4. It can be used to predict the water content that inhibits microbial growth; 5. It can be used to predicate food stability. Shown in Figure 12. is a schematic MSI for a high-moisture food plotted to include the full range of water content from normal to dry. Omission of the high-moisture region and expansion of the low-moisture region, as is usually done, yields an MSI that is much more useful (Figure 13). Several substances that have MSIs of markedly different shapes are shown in Figure 14. Isotherms with an S shape are characteristic of most foods. Foods such as fruits, confections, and coffee extract that contain large amounts of sugar and other small soluble molecules, and are not rich in polymeric materials exhibit a J-type isotherm shown as curve 1 in Figure 14.

Figure 12. Schematic moisture sorption isotherm encompassing a broad range of moisture contents [3].

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Jianqian Kan and Guoqing Huang

Figure 13. Generalized moisture sorption isotherm for the low-moisture segment of a food (20°C) [3].

Figure 14. Resorption isotherms for various foods and biological substances. Temperature 20°C, except for number 1, which is 40°C: (1) confection (main component powdered sucrose), (2) spray-dried chicory extract, (3) roasted Columbian coffee, (4) pig pancreas extract powder, (5) native rice starch [6].

The MSI can be prepared in two ways. For high-moisture foods, the desorption isotherm can be obtained by plotting the water content versus aw during dehydration. For low-moisture foods, the resorption isotherm can be determined by plotting water content versus aw during addition of water to the foods. The shape and position of the isotherm are determined by several factors including sample composition, physical structure of the sample (e.g., crystalline or amorphous), sample pretreatments, temperature, and methodology To deeply understanding the meaning and usefulness of sorption isotherms it is sometimes appropriate to divide them into three zones as indicated in Figure 13.

Water

23

(1) Water present in Zone I of the isotherm is most strongly absorbed and least mobile. This water associates with accessible polar sites by water-ion or water-dipole interactions, is unfreezable at -40°C, has no ability to dissolve solutes, and is not present in sufficient amount to have a plasticizing effect on the solid. It behaves simply as part of the solid. The high-moisture end of Zone I (boundary of Zones I and II) corresponds to the ―Brunauer-Emmett-Teller (BET) monolayer‖ moisture content of the food. Zone I water constitutes a tiny fraction of the total water in a high-moisture food material. (2) Water added in Zone II occupies first-layer sites that are still available. This water associates with neighboring water molecules and solute molecules primarily by hydrogen bonding, is slightly less mobile than bulk water, and most of it is unfreezable at -40°C, it exerts a significant plasticizing action on solutes, lowers their glass transition temperatures, and causes incipient swelling of the solid matrix, leads to acceleration in the rate of most reactions. Water in Zones I and Zone II usually constitutes less than 5% of the water in a high-moisture food material. (3) Further addition of water (Zone III) causes a glass-rubber transition in samples containing glassy regions, a very large decrease in viscosity, a very large increase in molecular mobility, and commensurate increases in the rates of many reactions. This water is freezable, available as a solvent, and readily supports the growth of microorganism. Zone III water is referred to as bulk-phase water. The bulk-phase water of Zone III, either entrapped or free, usually constitutes more than 95% of the total water in a high-moisture food.

4.2. Hysteresis of MSI An MSI prepared by addition of water (resorption) to a dry sample will not necessarily be superimposable on an isotherm prepared by desorption. This lack of superimposability is referred to as ―hysteresis,‖ and a schematic example is shown in Figure 15. Typically, at any given p/p0, the water content of the sample will be greater during desorption than during resorption. MSIs of polymers, glasses of low molecular-weight compounds, and many foods exhibit hysteresis. The following explanations have been proposed for the occurrence of hysteresis: 1. Some moisture cannot be released during desorption due to the interaction with nonaqueous components. 2. Different vapor pressures are needed for evacuating or filling the moisture entrapped by capillaries. 3. The tissues of foods are changed during desorption. As a result, moisture cannot bind the same tightly to the tissues during resorption and higher aw is resulted in the same water content. 4. The magnitude of hysteresis, the shape of the curves, and the inception and termination points of the hysteresis loop can vary considerably depending on factors such as nature of the food, the physical changes it undergoes when water is removed or added, temperature, the rate of desorption, and the degree of water removal during desorption.

24

Jianqian Kan and Guoqing Huang Table 6. Water activity and growth of microorganisms in foods [7]

Range of aw

Inhibited Microorganisms

1.00–0.95

Pseudomonas, Escherichia Proteus, Shigella, Klebsiella, Bacillus, Clostridium perfringens, some yeasts

0.95–0.91

Salmonella, Vibrio parahaemolyticus, C. botulinum, Serratia, Lactobacillus, some molds, yeasts (Rhodotorula, Pichia)

0.91–0.87

Many yeasts (Candida, Torulopsis, Hansenula, Micrococcus)

0.87–0.80

Most molds (mycotoxigenic penicillia), Staphylococcus aureus, most Saccharomyces (bailii) spp., Debaryomyces

0.80–0.75 0.75–0.65

0.65–0.60

Most halophilic bacteria, mycotoxigenic aspergilli Xerophilic molds (Aspergillus chevalieri, A. candidus, Wallemia sebi),Saccharomyces bisporus Osmophilic yeasts (Saccharomyces rouxii), few molds (Aspergillus echinulatus, Monascus bisporus)

0.50

No microbial proliferation

0.40

No microbial proliferation

0.30

No microbial proliferation

0.20

No microbial proliferation

Foods generally within this range Highly perishable (fresh) foods and canned fruits, vegetables, meat, fish, and milk; cooked sausages and breads; foods containing up to approximately 40% (w/w) sucrose or 7% sodium chloride Some cheeses (Cheddar, Swiss, Muenster, Provolone),cured meat (ham), some fruit juice concentrates; foods containing up to 55% (w/w) sucrose or 12% sodium chloride Fermented sausage (salami), sponge cakes, dry cheeses, margarine; foods containing up to 65% (w/w)sucrose (saturated) or 15% sodium chloride Most fruit juice concentrates, sweetened condensed milk, chocolate syrup, maple and fruit syrups; flour, rice, pulses containing 15–17% moisture; fruit cake; country-style ham, fondants, high-ratio cakes Jam, marmalade, marzipan, glacé fruits, some marshmallows Rolled oats containing approximately 10% moisture; grained nougats, fudge, marshmallows, jelly, molasses, raw cane sugar, some dried fruits, nuts Dried fruits containing 15–20% moisture; some toffees and caramels; honey Pasta containing approximately 12% moisture; spices containing approximately 10% moisture Whole egg powder containing approximately 5% moisture Cookies, crackers, bread crusts, etc. containing 3–5% moisture Whole milk powder containing 2–3% moisture; dried vegetables containing approximately 5% moisture; corn flakes containing approximately 5% moisture; country style cookies, crackers

Water

25

Figure 15. Hysteresis of moisture sorption isotherm [5].

Figure 16. Relationships among relative water vapor pressure, food stability and sorption isotherms. (A) Microbial growth versus p/p0. (B) Enzymatic hydrolysis versus p/p0. (C) Oxidation (nonenzymatic) versus p/p0. (D) Maillard browning versus p/p0. (E) Miscellaneous reaction rates versus p/p0. (F) Water content versus p/p0. All ordinates are ―relative rate‖ except for F [3].

26

Jianqian Kan and Guoqing Huang

5. WATER ACTIVITY AND FOOD STABILITY It has been widely recognized that aw is a much better indicator of food stability than water content. The data in Figure 16 and Table 6. provide examples of these relationships.

5.1. Water Activity (aw) and Growth of Microorganisms in Foods Shown in Table 6. are various common microorganisms and the range of aw permitting their growth. Most bacterial growth is affected above water activity 0.90 and most yeast and molds, however, can grow above water activity 0.80. No microorganisms survive in water activity lower than 0.5.

5.2. Water Activity (aw) and Chemical and Enzymatic Reactions in Foods The relationship between water activity and the rates of chemical and enzymatic reactions is very complex. First, water is a reactant of many chemical and enzymatic reactions and its content significantly influences the balance of the reactions. Second, water can bind to polar or ionic groups through hydration and significantly affects their contact with other reactants. Third, many biomolecules swell in the presence of water and more reaction sites are exposed, leading to accelerated reaction. However, high water contents dilute solutes and retard the proceeding of reactions. As shown in Figure 16, all chemical and enzymatic reactions, except oxidation reactions, have the lowest reaction rates at the boundary of Zone I and Zone II, corresponding aw 0.2~0.3.

5.3. Water Activity (aw) and Lipid Oxidation Figure 6.(c) indicates the relationship between lipid oxidation rate and aw. Within a specific aw range, the rate of lipid oxidation decreases with the increase of aw. When aw further increases to the boundary of Zone II and Zone III, the lipid oxidation rate starts to rise. Generally, the lowest lipid oxidation rate is found in aw 0.35. The moisture in food might either enhance or suppress lipid oxidation. When aw is lower than 0.35, lipid oxidation is suppressed for the following reasons. Firstly, water covers the oxidizable sites and prevents their contact with oxygen. Secondly, water hydrates ions and eliminates the oxidation reactions initiated by ions. Thirdly, water hydrogen bonds with hydroperoxides and retards the oxidation induced by them. Finally, water facilitates the binding between free radicals and disrupts the chain reactions involving the free radicals. When aw is greater than 0.35, water enhances lipid oxidation though two ways. Water dissolves solutes and facilities their movement. Meanwhile, biomolecules swell in high water content and more accessible sites are exposed.

27

Water

5.4. Water Activity (aw) and Maillard Reaction Figure 16(c) illustrates the influence of aw on Maillard reaction. It could be seen that Maillard reaction occurs mainly in aw range 0.3~0.7. In low aw, the water-solute hydrogen bonding and the association of water with neighboring molecules retard the movement of solutes and subsequently suppress the Maillard reaction. As the aw increases gradually, reactants and products move more easily and the rate of Maillard reaction increase as a result. When aw exceeds a specific value, solutes are diluted and the Maillard reaction is consequently retarded.

5.5. Calculation of BET Monolayer Value As shown in Figure 16, all the chemical enzymatic reactions, except oxidation, occur the slowest in the boundary of Zone I and Zone II (corresponding to aw 0.2~0.3) and further decrease of the water content does not change the minimum rates. The water content at the first-encountered rate minimum is the ―BET monolayer‖ water content. The BET theory is a theory proposed by Brunauer, Emett, and Teller in 1938 and the theory was named after them. The BET monolayer value of a food provides a good first estimate of the water content providing maximum stability of a dry product. One can use the BET equation to compute the monolayer value:

aw m(1 - aw )

1 m1c

c 1 m1c

(10)

where, aw is water activity, m is water content (in g H2O/g dry matter), m1 is the BET monolayer value, and c is a constant. From this equation, it is apparent that a plot of versus aw, known as a BET plot, should yield a straight line. An example for native potato starch, with aw replaced by p/p0, is shown in Figure 17. The linear relationship, as is generally acknowledged, begins to deteriorate at p/p0 values greater than about 0.35. The BET monolayer value can be calculated as follows:

Monolyaer value (m1 )

1 (y intercept) (slope)

(11)

From Figure 17, the y intercept is 0.6. Calculation of the slope from Figure 17 yields a value of 10.7. Thus,

m1

1 0.6 10.7

0.088g H2O/g dry matter

In this particular instance, the BET monolayer value corresponds to a aw, of 0.2.

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Jianqian Kan and Guoqing Huang

Figure 17. BET plot for native potato starch (resorption data, 20°C) [8].

6. FREEZING AND FOOD STABILITY Although freezing is regarded as the best method of long-term preservation for most kinds of foods, the benefits of this preservation technique derive primarily from low temperature as such, not from ice formation. The formation of ice in cellular foods and food gels has two important adverse consequences: (1) All water converted to ice increases 9% in volume. Consequently, ice crystals formed in a disperse system can cause locally increased pressures, which can in turn cause mechanical damage. Hence water will leave the cells, which will shrink considerably; enzymes will release into solution, whereby they become active; this may result in a product of poor quality. (2) Nonaqueous constituents become concentrated in the unfrozen phase, the unfrozen phase changes significantly in properties such as pH, titratable acidity, ionic strength, viscosity, freezing point (and all other colligative properties), surface and interfacial tension, and oxidation reduction potential. In addition, solutes sometimes crystallize, supersaturated oxygen and carbon dioxide may be expelled from solution, water structure and water-solute interactions may be drastically altered, and macromolecules will be forced closer together, making interactions more probable. These changes in concentration-related properties often favor increases in reaction rates. Thus, freezing can have two opposing effects on reaction rate: lowering temperature, as such, will always decrease reaction rates, and freeze-concentration, as such, will sometimes increase reaction rates.

7. MOLECULAR MOBILITY AND FOOD STABILITY 7.1. Molecular Mobility In addition to water activity, molecular mobility (Mm) has also been used to predicate and control the stability of foods. Molecular mobility involves all the movements of food

Water

29

components during storage that are related to the stability and processability and includes: molecular movement or deformation caused by liquid movement or mechanical stretch; Brownian movements or atomic rotation caused by molecular diffusion; relative movement of materials in containers or pipelines. Some properties and behavioral characteristics of food that are dependent on Mm are shown in Table 7. Mm is mainly influenced by hydration and temperature. The water content and the interaction between water and nonaqueous components determine the fluidity of the liquid phase. As temperature is increased, the translational and rotational motion (Mm) becomes easier, while upon cooling to Tg, translational motion of polymer segments stop.

7.2. State Diagrams It is necessary to introduce the concept of state diagram before the discussion of the relationship between Mm and the stability of dried, partially dried, or frozen foods, State diagrams are supplemented phase diagrams, and contain equilibrium information as well as information on conditions of nonequilibrium and metastable equilibrium ―states‖, and are appropriate because foods that are dried, partially dried, or frozen do not exist in a state of thermodynamic equilibrium. A simplified temperature-composition state diagram for a binary system is shown in Figure 19. Table 7. Some properties and behavioral characteristics of foods that are governed by molecular mobility (diffusion-limited changes in products containing amorphous regions) [9] Dry or semidry foods Flow properties and stickiness Crystallization and recrystallization Sugar bloom in chocolate Cracking of foods during drying Texture of dry and intermediate moisture foods Collapses of structure during secondary (desorption) phase of freeze-drying Escape of volatile encapsulated in a solid, amorphous matrix Enzymatic activity Maillard reaction Gelatinization of starch Staling of bakery products caused by retrogradation of starch Cracking of bakery goods during cooling Thermal inactivation of microbial spores

Frozen foods Moisture migration (ice crystallization, formation of in-package ice) Lactose crystallization (―sandiness‖ in frozen desserts) Enzymatic activity Structural collapse of amorphous phase during sublimation phase of freeze-drying Shrinkage (partial collapse of foam-like frozen desserts)

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Figure 19. State diagram of a binary system. Assumptions: maximal freeze concentration, no solute crystallization, constant pressure, no time dependence. Tml is the melting point curve, TE is the eutectic point, Tms is the solubility curve, Tg is the glass transition curve, and Tg‘is the solute-specific glass transition temperature of a maximally freeze concentrated solution. Heavy dashed lines represent conditions of metastable equilibrium. All other lines represent conditions of equilibrium [1].

Most foods are so complex that they cannot be accurately or easily represented on a state diagram. Differential scanning calorimetry (DSC) can successfully determine the glass transition temperature (Tg) of simple polymer systems, but is inapplicable to complex food systems. The Tg of complex food systems is often determined by using dynamic mechanical analysis (DMA) or dynamic mechanical thermal analysis (DMTA). Glass transition temperature (Tg) is the temperature at which a supersaturated solution (amorphous liquid) converts to a glass, and is dependent on solute type and water content. Tg‘ is a special Tg that applies only to samples containing ice, and only when ice has been formed so maximum freeze-concentration occurs (very slow cooling). Below Tg or Tg‘ of a complex sample, all but small molecules lose their translational mobility while retaining limited rotational and vibrational mobility. In the glassy state, the food will have greater stability (shelf life). As long as the temperature remain below Tg‘, the composition of the system is virtually fixed. This implies physical stability: crystallization, for instance, will not occur. But some chemical reactions may still proceed, albeit very slowly because of the high viscosity and the low temperature.

7.3. Molecular Mobility, State Diagram, and Food Properties 7.3.1. Reaction Rates and Molecular Mobility Mm is causally related to diffusion-limited properties of foods that contain, besides water, substantial amounts of amorphous, primarily hydrophilic molecules, ranging in size from monomers to polymers. Foods of this type include starch-containing foods, such as pasta, boiled confections, protein-based foods, intermediate-moisture foods, and dried, frozen, or freeze-dried foods. The utility of the Mm approach for predicting many kinds of physical changes has been reasonably well established. However, situations do exist where the Mm

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approach is of questionable value or is clearly unsuitable. Some examples are (1) chemical reactions whose rates are not strongly influenced by diffusion, (2) desirable or undesirable effects achieved through the action of specific chemicals (e.g., alteration of pH or oxygen tension), (3) situations in which sample Mm is estimated on the basis of a polymeric component (Tg of polymer) and where Mm of small molecules that can penetrate the polymer matrix is a primary determinant of the product attribute of interest, and (4) growth of vegetative cells of microorganisms (p/p0 is a more reliable estimator than Mm). Examples of diffusion-limited reactions are proton transfer reactions, radical recombination reactions, acid-base reactions involving transport of H+ and OH-, many enzyme-catalyzed reactions, protein folding reactions, polymer chain growth, and oxygenation/deoxygenation of hemoglobin and myoglobin. At constant temperature and pressure, three primary factors govern the rate at which a chemical reaction will occur: a diffusion factor, D (to sustain a reaction, reactants must first encounter each other), a frequency-of-collision factor, A (number of collisions per unit time following an encounter), and a chemical activation-energy factor, Ea (once a collision occurs between properly oriented reactants the energy available must be sufficient to cause a reaction, that is, the activation energy for the reaction must be exceeded). For a reaction to be diffusion-limited, it is clear that factors A and Ea must not be rate-limiting. Diffusion-limited reactions typically have low activation energies (8–25 kJ/mol). When a food is cooled and/or reduced in moisture content so that all or part of it is converted to a glassy state, Mm is greatly reduced and diffusion-limited properties become stable.

7.3.2. Free Volume and Molecular Mobility The free volume of a food system decreases as the temperature decreases and the translational and rotational motion become more difficult, which affects the motion of the segments and local viscosity of polymers. When the temperature decreases to below Tg, the free volume decreases significantly and the translational motion of polymer segments stops. Hence, foods have stable diffusionlimited properties in temperatures below Tg. The increase of free volume, which is often unexpected, can be achieved by adding small molecular-weight solutes such as water or by increasing the temperature. Both the two practices improve the translational motion of solutes and are not beneficial for food stability. However, this relationship is applicable only to certain food systems and free volume cannot be used as a quantitative indicator of food stability to present. 7.3.3. Moisture Content and Tg The moisture content has special effect on the Tg of food systems. The Tg of water is as low as -135°C and water is a strong plasticizer. First, water has a much smaller size and moves more easily than other solutes such as polysaccharides, proteins, and lipids. The ease of motion provides space required for the movement of segments. Second, water interacts with the polar groups on other components and replaces partial inter-molecular or intra-molecular hydrogen bonds, which decreases the rigidity of the components and consequently reduces the Tg. Generally, the increase of moisture content by 1% decreases the Tg by 5~10°C.

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It should be noted that the plasticizing effect is valid only when water gets entrapped in the amorphous region of the components. In the absence of environmental effects, moisture content is the predominant factor that affects Tg, especially in low-moisture foods. Table 8. Relationship between the Tg and moisture content of pre-gelatinized starch and wheat starch Pre-gelatinized starch Moisture content Tg/°C 0.153 62 0.166 53 0.181 40 0.222 28 0.247 25

Native wheat starch Moisture content Tg/°C 0.151 90 0.164 67 0.178 59 0.221 40 0.256 33

For example, the Tg of the anhydrous mixture of 50% starch and 50% sucrose is about 60°C; when the moisture content increases to 2%, the Tg decreases to 20°C; when the moisture content further increases to 6%, Tg falls to as low as 10°C. Table 8. lists the relationships between the Tg and moisture content of native wheat starch and pre-gelatinized starch. It could be seen that the Tg of both the materials increases along the decrease of moisture content.

7.3.4. Carbohydrates, Proteins, and Tg Carbohydrates and proteins are major constituents in many foods and their presence and contents markedly influence the Tg of foods. Besides, the sizes of the components also affect the Tg. Generally, carbohydrates or proteins with higher average molecular weight have more compact structure, higher viscosity, lower free volume, and consequently higher Tg. Table 9. lists the Tg of maltodextrin with different DE and concentration. It is seen that Tg decreases as DE increases in the case of same moisture content. Generally, Tg is dependent on solute type and water content, while Tg‘ is solely solute type dependent. For glycosides and polyols with molecular weight less than 1200, Tg or Tg‘ increases as molecular weight rises. When the average molecular weight exceeds 3000 (DE of 6 or more for starch hydrolysis products), g becomes independent of MW, as shown in Figure 25. An exception occurs when biomolecules are present in the entanglement networks form. In this case, g continues to rise with increasing MW. Most biomolecules, including starch, maltodextrin, cellulose, hemicellulose, carboxymethyl cellulose, glucan, xanthan gum, gluten, glutenin, gliadin, zein, collagen, elastin, keratin, albumin, globulin, casein, and gelatin, have similar glass transition curve and Tg‘, which approaches -10°C.

7.3. Mm, State Diagram and Food Stability Knowledge on the relationship between Tg or Tg‘ and food components is of great importance for the processing and storage of the foods.

33

Water Table 9. Tg of maltodextrin with different Des DE5 Moisture content 0.00 0.02 0.04 0.11 0.18

Tg/°C 188 135 102 44 23

DE10 Moisture content 0.00 0.02 0.05 0.10 0.19

Tg/°C 160 103 84 30 -6

DE15 Moisture content 0.00 0.02 0.05 0.11 0.20

Tg/°C 99 83 65 8 -13

Figure 25. Relationship between average molecular weight and dextrose equivalent (DE) of commercial starch hydrolysis products with Tg‘ [10].

7.3.1. Temperature, Mm, and Food Stability Good correlation exists among temperature and Mm or viscosity for foods containing amorphous regions in temperature range 10~100°C. Most molecules are present in the glassy or rubbery state in temperatures below Tg or Tg‘. In this case, the motion of food components is restricted and food stability is increased. 7.3.2. Food Stability Predication Based on State Diagram The approximate stability of foods can be predicated according to the state diagram. When foods are stored in temperatures lower than Tg or Tg‘, the diffusion of molecules is restricted and the shelf life is markedly prolonged. In contrast, the foods are susceptible to spoilage in temperatures higher than Tg or Tg‘. Hence, temperatures below or approaching Tg or Tg‘ should be preferred during food storage. Generally, Mm is more effective in predicating diffusion-limited properties, such as the physiochemical properties of frozen foods and optimum freeze-drying conditions. However, aw is more effective in predicating non-diffusion-limited properties and microbial growth of

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ice-free foods. Because aw can be measured conveniently and quickly, it is still the major indicator for judging food stability.

REFERENCES Fennema, OR. Food Chemistry. 3rd edition. New York: Marcel Dekker; 1996. Damodaran, S; Parkin, KL; Fennema, OR. Fennema’s Food Chemistry. 4th edition. New York: CRC Press; 2007. [3] Belitz, HD; Gorsch, W. Food Chemistry. 2nd edition. Berlin: Springer-Verlag, 1999. [4] Berendsen, HJC. Specific interactions of water with biopolymers. In: Franks, F. Water A Comprehensive Treatise. New York: Plenum Press, 1975; 293-349. [5] Fennema, OR. Enzyme kinetics at low temperature and reduced water activity. In: Crowe, JH; Clegg, JS. Dry Biological Systems. New York: Academic Press, 1978; 297322. [6] Ferry, JD. The evaluation of water activity in aqueous solutions from freezing point depression. International Journal of Food Technology, 1980, 16, 21-30. [7] Beuchat, LR. Microbial stability as affected by water activity. Cereal Foods World, 1981, 26, 345-349. [8] Van den Berg, C. Vapour Sorption Equilibria and Other Water-Starch Interactions; A Physico-Chemical Approach, PhD thesis, Wageningen: Wageningen Agricultural University, 1981. [9] Slade, L; Levine, H. Beyond water activity: Recent advances based on an alternate approach to the assessment of food quality and safety. Critical Reviews in Food Science and Nutrition. 1991, 30, 115–360. [10] Levine, H., and L. Slade (1986). A polymer physico-chemical approach to the study of commercial starch hydrolysis products (SHPs). Carbohydrate Polymers, 6, 213–244. [1] [2]

In: Food Chemistry Editors: D.Wang, H. Lin, J. Kan et al.

ISBN: 978-1-61942-125-7 © 2012 Nova Science Publishers, Inc.

Chapter 3

CARBOHYDRATES

1

Dongfeng Wang1, Jipeng Sun2, Guoqing Huang1,3, Xiaolin Zhou4 and Liping Sun5 College of Food Science and Engineering, Ocean Universityof China, Qingdao, China 2 Third Institute of Oceanography, State Oceanic Administration, Xiamen, China 3 College of Food Science and Engineering, Qingdao Agricultural University, Qingdao, China 4 Department of Biology, Shantou University Medical College, Shantou, China 5 College of Chemical Engineering, Kunming University of Science and Technology, Kunming, China

ABSTRACT Carbohydrates account for 3/4 of the dry weight of terrestrial plants and algae and can be found in all the plants, animals and microorganisms that human can eat. Carbohydrates are one of the major components in foods. The compounds not only provide human beings with energy, but also impart foods with desired textures and tastes. Carbohydrates undergo various changes during food processing and storage and yield substantial compounds that affect the flavor, quality and safety of foods. This chapter deals with the classification of carbohydrates and their most important functional properties, in which, special attention is paid to the non-enzymatic browning reaction, its influences on food quality, and its control. The structures, proportions, and applications of most important polysaccharides and oligosaccharides in foods are then detailed one by one. Dietary fiber as an important healthy diet component is also introduced at the end of this chapter.

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INTRODUCTION Carbohydrates Classification Carbohydrates are natural organic compounds converted from carbon dioxide and water through photosynthesis of plants. According to the number of monosaccharide units, carbohydrates are divided into monosaccharides, oligosaccharides and polysaccharides. Monosaccharides are the simplest sugars in structure and can no longer be hydrolyzed. A monosaccharide often contains three, four, five, or six carbon atoms and its functional group might be the aldehyde or keto group. An oligosaccharide generally consists of 2 to 20 monosaccharide units and can be hydrolyzed to simple sugars. Oligosaccharides are often found in glycoproteins or lipopolysaccharides. According to monosaccharide composition, oligosaccharides are further divided into homo-oligosaccharides and hetero-oligosaccharides. A homo-oligosaccharide is composed of only one type of monosaccharide, such as maltose and dextrin with degree of polymerization less than 20. In contrast, a hetero-oligosaccharide consists of two or more types of monosaccharide units. Polysaccharides, with degree of polymerization greater than 20, are formed by dehydration of multiple monosaccharide units and consist of homo-polysaccharides, such as cellulose and starch, and hetero-polysaccharide such as seaweed and tea polysaccharides. Polysaccharides can also be classified into plant, animal and microbial polysaccharides according to their origins or storage and functional polysaccharides according to their biological functions. Polysaccharides contain multiple hydroxyl groups and can covalently attach to the side chains of proteins or peptides to form glycoproteins or protein polysaccharides. Polysaccharides also react with carboxyl-containing molecules to form esters such as lipopolysaccharide (LPS) and sulfate polysaccharides. Besides, polysaccharides can complex with transition metals due to the presence of hydroxyl groups. These polysaccharide derivatives are generally referred to as polysaccharide complexes.

Carbohydrates in Foods Starch is one of the most abundant carbohydrates in plant-derived foods and is the most abundant in seeds, roots and tubers. Glycogen is found in animal-derived foods especially in muscle and liver, and it is structurally similar to amylopectin. Starch is insoluble in aqueous solutions and does not contribute to the sweetness of foods, unless it was hydrolyzed into oligosaccharides or glucose. The majority of plant-derived foods contain only a small amount of free sugars and most sugars are present as starch. For example, maize contains only 0.2%~0.5% D-glucose, 0.1%~0.4% D-fructose and 1%~2% sucrose. Free sugars not only provide the sweet taste for foods, but also participate in flavor and color formation during thermal processing. An increase in free sugar content during processing improves food quality. For example, to increase the sweetness of sweet corn, sweet corn must be harvested before sugars are converted to starch. Many fruits are often harvested before they are fully mature for two reasons. Firstly, the high rigidity of immature fruits facilitates their transport and storage. Secondly, starch is

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converted to sucrose or other sweet sugars during transport and storage. This change makes the fruits sweet and soft. The post-harvest ripening is the reverse of the starch synthesis process in the grain, tuber and root of plants. The contents of water-soluble sugars in processed foods are generally higher than the corresponding materials, because sugars are intentionally added by manufacturers to meet the requirements of consumers on flavor and color.

Carbohydrates and Food Quality Carbohydrates are the main constitutes in foods and are closely related to the nutrition, color, taste, texture and functionality of foods. Carbohydrates are essential nutrients for human body and 70% of the energy needed by human body is provided by carbohydrates. Reducing sugars with free aldehyde or ketone contribute to the formation of colors and flavors in food thermal processing and thus affects food quality. Besides, many free sugars are sweet and positively affect the mouth feel of foods. Some carbohydrates, such as guar gum and carrageenan, have special viscoelastic properties and can give pleasant textures for foods. Cellulose, pectin, and many other macromolecules can provide desired food texture and adjust the intestine flora as dietary fibers. In addition, some polysaccharides or oligosaccharides, such as lentinan and tea polysaccharides, have specific physiological functions and can be directly added to foods as functional ingredients.

PHYSICOCHEMICAL PROPERTIES AND FUNCTIONS OF CARBOHYDRATES Carbohydrate Structure Monosaccharide Monosaccharide generally contains 5 or 6 carbon atoms with the general formula Cn(H2O)n. Monosaccharides are asymmetric and optically active. Taking glyceraldehyde as an example, the C atom in position 2 is chiral and glyceraldehyde therefore has two enantiomers. D-glyceraldehyde is dextrorotatory and is distinguished by the prefix ―+‖ or ―d‖ and Lglyceraldehyde is levorotatory and is labeled with prefix ―-‖ or ―l‖. Monosaccharides derived from D-glyceraldehyde are termed D-ketones and those from L-glyceraldehyde are termed Lketones. The carboxyl group of a monosaccharide ring can react with a free hydroxyl group in the same molecule to yield more stable 5- or 6-membered hemiacetal or hemiketal, called lactol. Lactol formation provides a new chiral center. Thus, there are two additional diastereomers for each pyranose or furanose. These isomers are called anomers and are denoted as α- or βforms. Most naturally occurring simple sugars are present in the D form. Hence, it is sometimes unnecessary to indicate the configuration of these sugars. Monosaccharides can produce various biologically important derivatives after chemical modification. Monosaccharide derivates that have been identified in foods include

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monosaccharide phosphates, deoxy monosaccharides, glucosamines, aldonic acids, uronic acids, saccharic acids, ascorbic acid, sugar alcohols, myo-inositol, glycosides, etc.

Sugar Alcohol, Inositol and Glycoside Sugar alcohols are the hydrogenated products of monosaccharides and are also known as polyols. Most sugar alcohols are the reducing products of their corresponding monosaccharides except mannose, which occurs naturally in algae with high contents. Most sugar alcohols are white crystals and soluble in water. Table 3-1. Isomers of inositol

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Carbohydrates

Sugar alcohols have lower calorific values and are sweeter than their monosaccharide precursors. Sugar alcohols do not undergo the typical reactions of sugars and are stable against heating and pH variations. Sugar alcohols share the same chemical properties as common alcohols and are not involved in the Maillard reaction. Inositol is a cyclic hexatomic alcohol and has 9 stereoisomers (Table 3-1), of which, 7 are mesomeric and 2 are optically active. Among the isomers, only the myo isomer is biologically active. Inositol is often present as free form in the muscle, heart, liver, and lung of animals. The hydroxyl groups in inositol can react with phosphate acid to produce phosphoinositides. In higher plants, all the six hydroxyl groups in inositol are phosphated to inositol hexaphosphate. Phosphoinositides can complex with Ca2+ and Mg2+, forming calcium and magnesium salts of phytic acid. Glycosides are the condensation products of monosaccharides and non-saccharide ligand through the hemiacetal hydroxyl of monosaccharide. The bond between sugar and ligand is referred to as the glycosidic linkage. Glycosides contain one furanose or pyranose ring and the new chiral center might be present in the alpha or beta form. Most glycosides occur in the beta form in nature.

Oligosaccharides Oligosaccharides are water-soluble and occur widely in nature. Generally, naturally occurring oligosaccharides contain less than six monosaccharide units, of which, most are disaccharides and trisaccharides. For example, sucrose and maltose are disaccharides, and affinose is a trisaccharide. Some high-molecular weight oligosaccharides, such as cyclodextrins (or schardinger dextrin), have gained wide applications in the food industry. Cyclodextrins consist of 6~8 Dglucopyranose units, corresponding to α-, β-, and γ- cyclodextrins respectively. In addition to molecular weight, the three cyclodextrins differ in their water solubility and cavity size, as shown in Table 3-2. X-ray diffraction reveals that α-cyclodextrin is a highly symmetric cylinder. Six C6 hydroxyl groups are located in the bottom of the cylinder and 12 C2 and C3 hydroxyl groups are arranged on the cylinder top. The inner wall of the cylinder is covered with C-H groups and hence more hydrophobic than the external surface. Cyclodextrins are used to stabilize hydrophobic substances by entrapping them in the cavity during food processing. Table 3-2. Chemical and physical properties of cyclodextrins Item Number of glucose residues Molecular weight Solubility in water at 25°C (g/l) Optical rotation Inner diameter of cavity (nm) Cavity height (nm)

α-cyclodextrin 6 972 145 +150.5 0.57 0.67

β-cyclodextrin 7 1135 18.5 +162.5 0.78 0.70

γ-cyclodextrin 8 1297 232 +174.4 0.95 0.70

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Polysaccharides Structure The degrees of polymerization (DP) of polysaccharides range from 21 to several thousands. Polysaccharides consist of one type of structural unit (homoglycans) or multiple types of structure units (heteroglycans) and the structural units can be linked in a linear (such as cellulose and amylose) or branched pattern (such as amylopectin and glycogen). The monosaccharide units generally occur periodically in polysaccharides and each periodic repeat contains one or more alternative structural units. An example of the exceptions is the carbohydrate components in glycoproteins, in which the monosaccharide compositions are nonperiodic all along the chain. The DPs of polysaccharides are heterogeneous, that is, polysaccharides have no fixed molecular weight (MW) and display the Gauss distribution. The heterogeneity of polysaccharide molecular weight is associated with the metabolic status of organisms. For example, the MW of glycogen is closely dependent on the blood glucose level of animals. When the blood glucose level is low, the liver glycogen is hydrolyzed and glycogen is cleaved to small segments. In contrast, when the blood glucose level increases, glycogen is synthesized in the liver and the glycogen MW is increased. In addition, many polysaccharides are present as complexes, such as glycoproteins, glycopeptides and glycolipids. In this case, the MWs of polysaccharides are determined by much more factors than polysaccharide alone. Conformation Polysaccharides are either straight or branched molecules, but they have much more complex conformations. Some typical conformations are elucidated in the following by taking glucans and some other polysaccharides as examples. Extended or stretched ribbon-type conformation This conformation is typical for 1,4-linked β-D-glucopyranosyl residues (Figure 3-1), for instance that in cellulose fibers. This formula shows that the stretched chain conformation is due to the zigzag geometry of monomer linkages involving oxygen bridging. The chain may be shortened or compressed to enable formation of H-bonds between adjacent resides and thus contribute to conformational stabilization. In this type of conformation, if the number of monomers in turn is denoted as n and the pitch (advancement) in the axial direction per monomer unit is denoted as h, n ranges from 2 to ±4 and h equals the length of a monomer unit. Thus, the chain given in Figure 3-2(a) has n value of -2.55 and h value of 5.13 Å. A strongly plated ribbon-type conformation might also occur, as shown by a segment of a pectin chain (1,4-linked α-D-galactopyranosyl-uronate units) and an alginate chain (1,4linked α-L-gulopyranosyluronate units), as shown in Figure 3-3.

Figure 3-1. Conformation of 1, 4-linked β-D-glucopyranosyl residue.

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Carbohydrates

Figure 3-2. Conformations of some β-D-glucans. Linkages: a 1→4, b 1→3, c 1→2.

(a) Peetin

(b) Alginate

Figure 3-3. Plated ribbon-type conformation of pectin and alginate.

Since alginate contains multiple oxygen atoms, it can complex with many transition metals. As shown in Figure 3-3(b), Ca2+ stabilizes the conformation of alginate. In this case, two alginate chains are assembled in a conformation which resembles an egg box, which is referred as the egg box type of conformation (Figure 3-4). Hollow helix-type conformation This conformation is typical for 1,3-linked β-D-glucopyranose units and occurs in the polysaccharide lichenin, for example, as shown in Figure 3-5 (a). The formula shows that the helical conformation of the chain is imposed by a U-form geometry of the monomer linkages. Amylose (1,4-linked α-D-glucopyranosyl residues) also has such a geometry, and hence a helical conformation (Figure 3-5 (b)).

Figure 3-4. Sketch map of egg box type of conformation.

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(a) Lichenin.

(b) Amylose

Figure 3-5. Conformation of lichenin and amylase.

a

b

c

Figure 3-6. Stabilization of helical conformations. a, clathrate compounds; b, coiled double or triple helices; c, nesting.

The number(n) of monomers per turn and the pitch in the axial direction per residue (h) might differ significantly in this conformation. The value of n varies from 2 to ±10, while h can be near its limit value of 0. The conformation of a β(1-3)-glucan, with n value of 5.64 and h value of 3.16 Å, is shown in Figure 3-2(b). The helical conformation can be stabilized in various ways. When the helix diameter is large, inclusion (clathrate) compounds can be formed (Figure 3-6(a)). More extended or stretched chains, with smaller helix diameter, can form double or triple stranded helices (Figure 3.6(b)), while strongly-stretched chains, in order to stabilize the conformation, have a zigzag, plated association and not stranded (Figure 3-6(c)). Crumpled-type conformation This conformation occurs with, for example, 1,2-linked β-D-glucopyranosyl residues (Figure 3-2 (c)). This is due to the wrinkled geometry of the monomer O-bridge linkages. Here, the n value varies from 4 up to −2 and h is 2–3 Å. The conformation reproduced in Figure 3-2 (c), the n = 2.62 and h = 2.79 Å. The likelihood of such a disorderly form associating into more orderly conformations is low. Polysaccharides of this conformational type play only a negligible role in nature. Loosely-jointed conformation This is typical for glycans with 1,6-linked β-D-glucopyranosyl units, because they exhibit a particularly great variability in conformation. The great flexibility of this glycan-type conformation is based on the nature of the connecting bridge between the monomers. The bridge has three free rotational bonds and, furthermore, the sugar residues are further apart.

Carbohydrates

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Figure 3-7. Conformations of β-D-galactopyranosyl-4-sulfate and 3,7-anhydro-α-D-galactopyranosyl-2sulfate residues in ι-carrageenan.

Figure 3-8. Biosynthesis of ι-carrageenan.

Conformations of heteroglycans The examples considered so far have demonstrated that a prediction is possible for a homoglycan conformation based on the geometry of the bonds of the monomer units which maintain the oxygen bridges. It is more difficult to predict the conformation of a heteroglycan with a periodic sequence of several monomers, which implies different types of conformations. Such a case is shown by ι-carrageenan, in which the β-D-galactopyranosy l-4sulfate units have a U-form geometry, while the 3,6-anhydro-α-Dgalactopyranosyl-2-sulfate residues have a zigzag geometry (Figure 3-7). Calculations have shown that conformational possibilities vary from a shortened, compressed ribbon band type to a stretched helix type. X-ray diffraction analyses have proved that a stretched helix exists, but as a double stranded helix in order to stabilize the conformation. Interchain interactions As mentioned above, the periodically arranged monosaccharide sequence in a polysaccharide can be interrupted by nonperiodic segments. Such sequence interferences result in conformational disorders. This will be explained in more detail with ι-carrageenan.

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Initially, a periodic sequence of altering units of β-D-galactopyranose-4-sulfate (Figure 3-8, I) and α-D-galactopyranose-2,6-disulfate (Figure 3-8, II) is built up in carrageenan biosynthesis: When the biosynthesis of the chain is complete, an enzyme-catalyzed reaction eliminates sulfate from most of α-D-galactopyranose-2,6-disulfate (Figure 3-7, II), transforming the unit to 3,6-anhydro-α-D-galactopyranose-2-sulfate (Figure 3-7,III). This transformation is associated with a change in linkage geometry. Some II-residues remain in the sequence, acting as interference sites. While the undisturbed, ordered segment of one chain can associate with the same segment of another chain, forming a double helix, the nonperiodic or disordered segments cannot participate in such associations (Figure 3-9). In this way, a gel is formed with a three dimensional network in which the solvent is immobilized. The gel properties, e. g., its strength, are influenced by the number and distribution of α-D-galactopyranosyl-2,6-disulfate residues, i.e. by a structural property regulated during polysaccharide biosynthesis. The example of the ι-carrageenan gel-building mechanism, involving a chain–chain interaction of sequence segments of orderly conformation, interrupted by randomly-coiled segments corresponding to a disorderly chain sequence, can be applied generally to gels of other macromolecules. Besides a sufficient chain length, the structural prerequisite for gel-setting ability is interruption of a periodic sequence and its orderly conformation. The interruption is achieved by insertion into the chain of a sugar residue of a different linkage geometry (carrageenans, alginates, pectin), by a suitable distribution of free and esterified carboxyl groups (glycuronans) or by insertion of side chains. The inter-chain associations during gelling (network formation), which involve segments of orderly conformation, can then occur in the form of a double helix (Figure 3-10(a)); a multiple bundle of double helices (Figure 3-10(b)); an association between stretched ribbon-type conformations, such as an egg box model (Figure 3-10(c)); some other similar associations (Figure 3-10(d)); or, lastly, forms consisting of double helix and ribbon-type combinations (Figure 3-10(e)).

Figure 3-9. Schematic representation of a gel setting process.

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Figure 3-10. Interchain aggregation between regular conformations. a Double helix. b double helix bundle. c egg-box. d ribbon–ribbon. e double helix, ribbon interaction.

Physiochemical Properties Solubility Most monosaccharides, such as sugar alcohols, glycosides, and oligosaccharides, are water soluble. At 20°C, up to 195 g of sucrose can dissolve in 100 g of water. The solubility of sugar alcohols varies significantly with species. For example, sorbitol has a higher solubility than sucrose and reaches up 220 g per 100g water, while that of mannitol, erythritol and isomaltitolto is only 17, 50, and 100 g per 100g water, respectively. Sugar alcohols absorb much more heat than sugars when dissolution and thus produce cooling sensation in the mouth. Sugar alcohols are added to candies and chewing gums to provide the cooling sensation. The solubility of glycosides is closely correlated with their ligands. Generally, glycosides are more water soluble than corresponding ligands. For example, flavonoids are usually insoluble, but the corresponding glycosides are soluble. Flavonoids provide foods with different colors and tastes in the soluble glucoside form. Each sugar unit in polysaccharides contains an average of three hydroxyl groups and each hydroxyl group can hydrogen bond with one or more water molecules. In addition, the oxygen atoms in the sugar ring and glycosidic bond can also form hydrogen bonds with water. Therefore, monosaccharide units in polysaccharides can be completely solvated and most polysaccharides have strong water-holding capabilities and are highly hydrophilic. Polysaccharides in foods affect the movement of water and significantly influence the functional properties of foods. The presence of polysaccharides does not increase the penetrability or significantly reduce the freezing point of water, although polysaccharides can be solvated by water. Therefore, polysaccharides are good frozen stabilizers. Taking starch solution as an example, when a starch solution is frozen, a two-phase system is formed, of which, one phase is composed of crystal water and another phase is in the glass state consisting of 70% starch and 30% unfrozen water. Due to the extremely high polysaccharide concentration in the glassstate phase, the viscosity is high and the movement of unfrozen water in the glass-state phase is restricted. In addition, polysaccharides are concentrated in low temperatures. In this case,

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the mobility of water is further restricted and water molecules can no longer adsorb to crystal nucleus or the active sites for crystal growth. Both high and low molecular weight polysaccharides can effectively protect food texture and structure from being damaged during frozen storage. Some polysaccharides occur in a highly ordered form. In these molecules, the chains are tightly bound to form crystals. Because the number of exposed hydroxyl groups is significantly reduced, these polysaccharides are insoluble in water unless the inter-chain hydrogen bonds are broken. Taking cellulose as an example, the structural unit β-Dglucopyranosyl residues are arranged orderly and linearly along the chain and form interchain hydrogen bonds with parallel chains. The crystal regions of cellulose are not water soluble and are very stable. The majority of carbohydrates is not present as crystals and readily dissolves or swells in water. Water-soluble polysaccharides and their derivates used in the food industry are often termed gums or hydrophilic colloids. The solutions of most macromolecular polysaccharides are viscous and the viscosity depends on the molecular size, shape, net charge and conformation in solution. The thickening and gelling properties of polysaccharides significantly affect the quality of foods.

Hydrolysis Hydrolysis of Glycosides Though glycosides occur in low contents in foods, they impart important physiological effects and functionality to foods. For example, nature saponins are strong foaming agent and stabilizer, flavonoids produce bitterness and color for foods. In addition to a small amount of sweet glycosides such as stevioside and osladin, most glycosides taste bitter or astringent, particularly when the ligand is larger than methyl. When glycosides are hydrolyzed, their solubility is reduced and the bitterness or astringency is alleviated. O-glycoside bonds are stable in neutral and weak alkaline solutions, but easily hydrolyzed in acidic conditions. Most glycosides in foods (except for strong acidic foods) are stable. In the enzymatic hydrolysis of glycosides, the sugar component is transformed to highly active half-chair conformation and the glycosidic bond is weakened. Then, a proton is transferred from the enzyme to an oxygen atom in glycoside. When the oxygen atom is separated from the carbon atom, a positively-charged carbon ion is generated. This carbon ion then reacts with the negatively-charged -COO- group in the enzyme and is temporarily stabilized until it is completely hydrolyzed by reacting with the -OH- group in solvent. N-glycosidic bond is not as stable as O-glycosidic bonds and is susceptible to hydrolysis in water. For example, glycosylamines are unstable in water and can be hydrolyzed to colored products through a series of reactions. These reactions are the main reason of Maillard reaction initiation (to be discussed in Section 3.2.4). Thioglycosides, which contain S-glycosidic bonds, occur naturally in mustard and horseradish and are very stable and water-soluble. Thioglycosides can be hydrolyzed by thioglucosidase to produce isothiocyanates, as shown in Figure 3-11. Cyanogenic glycosides are another category of glycosides that significantly affect food safety. Cyanogenic glycosides are widely present in apricot, cassava, sorghum, bamboo, and lima beans and can yield toxic hydrocyanic acid upon hydrolysis. Amygdalin and

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mandelonitrile are the most important cyanogenic glycosides. The complete hydrolysis of amygdalin generates D-glucose, benzaldehyde, and hydrocyanic acid (Figure 3-12). Table 3-3 lists the main thio-glycosides and their hydrolysates. Excessive intake of cyanogenic glycosides leads to cyanide poisoning. In addition to enzyme activity and environmental acidity, the hydrolysis of glycosides is also affected by glycodic bond conformation, substitution of the sugar ring, and sugar ring size. Generally, glycosides with β glycodic bond are hydrolyzed faster than those with α glycodic bond. Substitution on the sugar ring reduces the hydrolysis rate and furanosides are hydrolyzed faster than corresponding pyranosides (Table 3-4). The hydrolysis rate of glycosides increases rapidly with elevated temperature, as shown in Table 3-4. Hydrolysis of oligosaccharides and polysaccharides Similar to glycosides, oligosaccharides are hydrolyzed easily by acid and enzymes and are stable in alkaline solutions. Sucrose can be hydrolyzed by acid to yield equamolar mixture of glucose and fructose that is called invert sugar.

Figure 3-11. Hydrolysis of thioglycosides by thio-glucosidase.

Figure 3-12. Hydrolysis of amygdalin.

Table 3-3. Main thioglycosides found in foods and their hydrolysates Glycoside Amygdalin and prunasin Linamarin Vicianoside Linarin

Occurrence Almond kernel and dried alpinia japonica Lima bean, linseed (flax), cassava Tare Sieve (black bean) and Chickpea horsebean

Hydrolysates D-Glucose + hydrocyanic acid + benzaldehyde D-Glucose + hydrocyanic acid + acetone Vicianose + hydrocyanic acid + benzaldehyde D-Glucose + hydrocyanic acid + acetone (to be confirmed)

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Dongfeng Wang, Jipeng Sun, Guoqing Huang et al. Table 3-3. (Continued) Glycoside Lotaustralin Dhurrin Sinigrin Glocoside

Occurrence Arabicus of lotus Broomcorn and corn Black mustard Brassicaceae

Hydrolysates D-Glucose + hydrocyanic acid + lotoflavin D-Glucose + hydrocyanic acid + salicylide D- Glucose + propyl isorhodanide + KHSO4 D-Glucose+ 5-ethenyl-2-oxazolidinethione, or goitrogen+ KHSO4

Table 3-4. Effect of temperature on the hydrolysis rate of glucosidesa Ka 70°C Methyl-α-D-pyran-glycoside 2.82 Methyl-β-D-fructofuranoside 6.01 a Note: First order reaction rate constant, ×106 sec-1. Glycosides (in 0.5mol/L H2SO4)

80°C 13.8 15.4

93°C 76.1 141.0

Polysaccharides are also prone to acidic or enzymatic hydrolysis, accompanying reduced viscosity and increased sweetness. In the food industry, α-amylase and glucose glucoamylase have been widely used in corn starch hydrolysis to produce D-glucose. The hydrolysate is further treated by D-glucose isomerization to yield a balanced mixture of 54% D-glucose and 42% D-fructose, which is known as fructose syrup. This mixture has replaced sucrose as a low-cost sweetener in many foods.

Oxidation Carbohydrates with reducing free aldehydes or ketoses that can be converted to aldehyde groups can be oxidized into aldonic acid in the presence of weak oxidants in alkaline conditions. When strong oxidants are present, both the aldehyde and primary hydroxyl of aldoses are oxidized to carboxyl and aldaric acids are produced as a result [3, 18].

Figure 3-13. Oxidation of D-glucose by glucose oxidase.

Some enzymes catalyze the oxidation of aldoses. For example, dehydrogenases oxidize the primary hydroxyl of some aldoses to produce uronic acids. D-glucuronic acid, Dgalacturonic acid, and D-manuronic acid are the components of many heteropolysaccharides. D-glucose can be oxidized to D-gluconic acid by glucose oxidase. Figure 3-13 illustrates the preparation of D-gluconic acid and gluconolactones. D-gluconic acid-δ-lactone can be transformed to γ-lactone and both can be hydrolyzed into D-gluconic acid at room

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temperature. As the hydrolysis proceeds, the pH value decreases gradually. Hence, gluconolactones can be used as a mild acidifier. Gluconolactones have gained applications in meat, dairy and soy products and especially baked goods as a leavening agent.

Reduction The carbonyl groups of monosaccharides can be reduced to the hydroxyl group. Reduction of a ketose yields to two sugar alcohols isomers due to the formation of a new chiral carbon atom. Figure 3-14 shows sugar alcohols produced by the reduction of glucose and fructose. Esterification and Etherification Due to the presence of hydroxyl groups, sugars can be esterified by organic acids or some inorganic acids, such as D-glucose-6-phosphate and D-fructose-1,6-diphosphate (Figure 315). The ester derivatives of starch, such as starch succinate, have found wide applications in the food industry. Another typical example is sucrose fatty acid ester, which is a commonly used an emulsifier.

Figure 3-14. Reduction of D-glucose and D-fructose.

Figure 3-15. Esterification of glucose and fructose by phosphorus acid (left: D-glucose; right: Dfructose).

Sugars can also be etherified, but naturally occurring sugar ethers are not as diverse as sugar esters. Etherification of polysaccharides can significantly improve their functional properties. Carboxymethyl cellulose (CMC) is an important ester of cellulose and is soluble in water. CMC has been widely used in the food industry as thickening agent or emulsion stabilizer.

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Properties of Carbohydrates Hydroscopicity Carbohydrates contain abundant hydrophilic hydroxyls and can bind water through hydrogen bonding. For example, when monosaccharides or oligosaccharides are placed in environments of different relative humidity (RH), the compounds can absorb moisture from air (Table 3-5). Table 3-5. Moisture adsorption of different sugars (%) Sugar D-glucose D-fructose Sucrose Anhydrous maltose Hydrous maltose Anhydrous lactose Anhydrous lactose

20°C in different RHs 60%, 1h 60%, 9d 0.07 0.07 0.28 0.63 0.04 0.03 0.80 7.0 5.05 5.1 0.54 1.2 5.05 5.1

100%, 25d 14.5 73.4 18.4 18.4 Not measured 1.4 Not measured

Figure 3-16. Hydroscopicity and water holding capacity of tea polysaccharide in different RHs (left: RH=81%; middle: RH=43%; right: RH=43%).

All sugar alcohols, except mannitol and isomaltulose, exhibit hygroscopicity especially in high RH environments. The hygroscopicity of sugar alcohols varies with their purity and lowpurity compounds have relatively higher hygroscopicity. Sugar alcohols are used as moisturizing agent in cream foods and soft pastries. Polysaccharides also absorb moisture from air and hence have good water holding capacity (Figure 3-16). The hydroscopicity of carbohydrates, which is often referred to as moisture retention capacity, is the most important attribute and determines their applications in foods. For example, to overcome the sticky problem of icing sugar powders after packaging, sugars with low hydroscopicity, such as lactose and maltose, are preferred in such products.

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Viscosity and Gelling Property Definition of Viscosity Viscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or tensile stress. Viscosity can be measured with various types of viscometers and rheometers, such as capillary viscometer, rotational viscometer, falling ball viscometer and vibration-type viscometer. The aqueous solutions of monosaccharides, sugar alcohols, oligosaccharides and soluble macromolecular polysaccharides are viscous. Many factors affect the viscosity of carbohydrates. Internal factors include the average molecular weight size and shape of molecular chains and external factors are carbohydrates concentration, environmental temperature, etc. Viscosity of Polysaccharide Solutions Polysaccharides (gums, hydrocolloids) are primarily used to thicken and/or gel aqueous solutions and otherwise to modify and/or control the flow properties and textures of liquid foods and beverages and the deformation properties of semisolid foods. They are generally used in concentrations 0.25–0.50%, indicating their great ability to produce viscosity and to form gels. The viscosity of a polysaccharide solution is related with the molecular size, shape, net charge and conformation of the polysaccharide in solution. Polysaccharides are often present as random coils in solutions (Figure 3-17) and the specific conformations are closely related to their compositions and connection modes. Straight-chain and branched-chain polysaccharides of the same degree of polymerization (DP) differ markedly in viscosity in aqueous solutions. Compared with their linear counterparts of equal molecular weights and equal concentrations, solutions of branched polysaccharides have a lower viscosity. It is assumed that the viscosity reflects the ―effective volume‖ of the macromolecule. The ―effective volume‖ is the volume of a sphere with diameter determined by the longest linear extension of the molecule. Molecules with larger effective volumes undergo collision more frequently than those with smaller effective volumes. Hence, linear polysaccharides can provide high viscosity even in low concentrations.

Figure 3-17. Random coiled structure of polysaccharides.

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Figure 3-18. Effective volume of straight-chain and branched-chain polysaccharides with equal molecular weights in solution.

Branched polysaccharides (amylopectin, glycogen) are more soluble in water than their perfectly linear counterparts since the chain–chain interaction is less pronounced and there is a greater extent of solvation of the molecules. This is especially the case for highly branched polysaccharides, because they have much less effective volumes than their linear counterparts, as shown in Figure 3-18. In addition to the DP, unfolding degree, and rigidity, the viscosity of polysaccharides is also affected by the shape and flexibility of polysaccharides after solvaiton. The charge carried by solvated polysaccharides significantly influences the viscosity of the solution. For example, straight-chain polysaccharides containing carboxyl, sulfate hemiester or phosphate groups are often negatively charged. The electrostatic repulsion between the molecules unfolds the molecules and increases their effective volume, leading to increased viscosity. Hence, pH has significant impact on the viscosity of polysaccharide solutions, since it is closely related to the charge statues of the molecules. The viscosity of polysaccharide solutions decreases with the increase of temperature, except xanthan solution, whose viscosity remains nearly unchanged in the temperature range 0~100°C. Besides, the viscosity of xanthan gum solutions decreases with shear rate increase. Hence, xanthan gum solutions are pseudoplastic. A practical use would be in salad dressing: xanthan gum makes it thick enough at rest in the bottle to keep the mixture fairly homogeneous, but the shear forces generated by shaking and pouring thins it, so it can be easily poured. When it exits the bottle, the shear forces are removed and it thickens back and clings to the salad.

Gelation Gelation is another important property of polysaccharides. In food processing, polysaccharides or proteins and other macromolecules can form a sponge-like threedimensional network gel structure through hydrogen bonding, hydrophobic interaction, Van der Waals attraction, ionic cross bridges, entanglement or covalent bonding (Figure 3-19). Liquids containing small-sized solutes and polymers fill the holes of the gel network.

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Gels possess the properties of both solids and liquids. They are not as flowable as continuous liquids or rigid as orderly organized solids. Instead, gels are viscoelastic semisolids and can retain their shape even in the presence of external stresses. Polysaccharide gels generally contain only about 1% polymer—that is, they may contain as much as 99% water. Examples of polysaccharide gels are dessert gels, aspics, structured fruit pieces, structured onion rings, meat-analog pet foods, and icings. The firmness of a gel depends on the extent of junction zone formation. If the conjunction zone area is not long enough, polysaccharide chains cannot bind tightly. In this case, the chains can be separated by pressure or high temperature due to increased mobility of the chains. Such gels are easily damaged and are heat-labile. If the conjunction area contains long chain segments, the interaction between chains is strong enough to withstand applied pressure or thermal stimulation. Gels of this type are hard and stable. Therefore, gels of different hardness and strength are available by controlling the length of the conjunction zone. Branched-chain polysaccharides or heteropolysaccharides cannot bind to each other well as linear molecules and large enough conjunction areas cannot be formed between these molecules. Hence, gels cannot be formed by branched-chain or hetero- polysaccharides. This is also the case for charged molecules, such as those containing carboxyl groups, because the Coulomb repulsion between chains hinders the formation of conjunction zone.

Figure 3-19. A diagrammatic representation of the type of three-dimensional network structure found in gels.

The selection of a specific polysaccharide for a particular application depends on the viscosity or gel strength desired, required rheology, pH of the system, temperatures during processing, interactions with other ingredients, desired texture, and cost.

Flavor Retention Aroma release from food matrix and the subsequent delivery of flavor to the olfactory and gustatory receptors is greatly dependent on the type of food ingredients and on the physicochemical properties of the aroma compound. It has been shown that macromolecules, such as polysaccharides, are involved in the retention of volatile compounds. Polysaccharides influence the volatility of aroma compounds and their partitioning between different phases [1].

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The use of carbohydrates may induce a significant decrease in flavor perception and/or release. Some factors affecting the retention and release of volatile flavor compounds by carbohydrates are depending on physicochemical properties of flavor compounds, type of carbohydrates and their concentrations. Firstly, high molecular weight flavor compounds tend to retain in carbohydrate than low molecular weight flavor compounds. Additionally, long linear chain length molecules will be retained in polysaccharide matrix higher than short chain molecules or aromatic one. Among the volatile flavor compounds such as alcohol, aldehyde, ester and ketone, alcohol are usually the best retained in carbohydrates. The retention of polar (hydrophilic) volatiles flavor compounds is expected to be very low in carbohydrates complex. The second factor is depending on type of carbohydrates. Each type of carbohydrate resents different structure that influence on the interaction between flavor compounds and its structure and also the retention and release. Thirdly, the concentration of carbohydrates generally shows that an increase in the concentration of sugar is proportional of the release of flavor compounds due to the salting out effect. On the other hand, an increase in polysaccharide concentration leads to a decrease the release of flavor compounds due to complexation and viscosity effect of that polysaccharide themselves. This knowledge can be used to optimize product quality in term of flavour retention during preparation or processing and its release during eating [2].

Browning and Food Flavor The non-enzymatic browning reaction of carbohydrates yields abundant volatile substances in addition to melanoidin and imparts peanut, coffee bean, and bakery foods with special flavors. Some compounds, such as maltol (3-hydroxy-2-methyl-4H-pyran-4-one) and ethyl maltol (3-hydroxy-2-ethyl-4H-pyran-4-one), not only exhibit special flavors on their own, but also enhance the flavors of other components in foods. Maltol lowers the threshold of sucrose by a factor of two. Ethyl maltol is more effective as a sweetness enhancer than maltol. Maltol enhances the sweet taste of foods, especially the sweetness produced by sugars, and is able to mask the bitter flavor of hops and cola. Ethyl maltol enhances the same sensation but is 4- to 6-times more powerful than maltol. In contrast to maltol, ethyl maltol is not a natural constituent in foods. Table 3-6. Relative sweetness of sugars and sugar alcohols to sucrose in 10% aqueous solution Sugar/sugar alcohol Sucrose

Relative sweetness 100

Sugar/sugar alcohol D-Mannitol

Relative sweetness 69

Galactitol D-Fructose D-Galactose D-Glucose Invert sugar Lactose Maltose

41 114 63 69 95 39 46

D-Mannose Raffinose D-Rhamnose D-Sorbitol Xylitol D-Xylose

59 22 33 51 102 67

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Sweetness Sweetness is a value relative to that of sucrose under the same conditions, which is often referred to as 100. All sugars, sugar alcohols and oligosaccharides are sweet (Table 3-6) with varying intensity of sweetness and some glycosides and polysaccharide complexes are excellent sweeteners. For example, the sweetness of honey and most fruits is contributed by sucrose, D-fructose or D-glucose. The sweetness perceived by consumers varies with the composition, conformation, and physical aspects of sugars. The intensities of sweetness of sugar alcohols differ markedly from those of corresponding sugars. For example, sorbitol is sweeter than glucose and that of xylitol is greater than xylose. Generally, all sugar alcohols, except xylitol, are less sweet than sucrose. Sugar alcohols are usually incompletely absorbed into the blood stream from the small intestines which generally results in a smaller change in blood glucose than sucrose. This property makes them popular sweeteners among diabetics and people on low-carbohydrate diets. As an exception, erythritol is actually absorbed in the small intestine and excreted unchanged through urine, so it has no side effects at typical levels of consumption.

Nonenzymatic Browning Reactions Nonenzymatic browning, or oxidative browning, is a chemical process that produces a brown color in foods without the participation of enzymes. The two main forms of nonenzymatic browning are caramelization and Maillard reaction. Ascorbic acid is also involved in nonenzymatic browning under certain conditions. Because phenols are important components in some foods and these compounds readily undergo autooxidation to yield brown color, the nonenzymatic browning of phenols is also discussed in this section.

Types and Process of Nonenzymatic Browning Reactions Maillard Reaction and Its Process The Maillard reaction is a form of nonenzymatic browning and involves the complex reactions between reducing sugars and amino acids/proteins. French chemist Louis-Camille Maillard described the reactions for the first time in 1912. John Hodge then named the reaction after Maillard and summarized the reaction process for the first time in 1953, as shown in Figure 3-20. The process of the Maillard reaction is often divided into three stages. The initial stage of the Maillard reaction involves the condensation of a carbonyl group, for example from a reducing sugar such as glucose, with a free amino group, typically the epsilon amino group of lysine residues within proteins. This glycation reaction results in the formation of an unstable Schiff base (aldimine) that spontaneously rearranges to the more stable 1-amino-1-deoxy-2-ketose (ketoamine), which is also known as the Amadori product after the Italian scientist Mario Amadori. When the initial sugar is glucose, the Amadori product is commonly known as fructoselysine (FL) (Figure 3-21).

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Figure 3-20. Maillard reaction scheme recommended by John Hodge [3].

Amadori products are degraded via various pathways in the intermediate stage, leading to the formation of furfurals, reductones and fragmentation products (carbonyl and hydroxycarbonyl compounds). Furfural or hydromethylfurfural (HMF) formation (Figure 322) is favored under acidic conditions, while alkaline media favor the production of reductones (Figure 3-23). The products of 1-amino-1-deoxy-2-ketose fragmentation in alkaline media, such acetol, diacetyl, pyruvaldehyde, etc, are able to react with amino acids via the Strecker degradation (named after the German chemist Adolph Strecker, Figure 3-24) to give Strecker aldehydes of the amino acids and aminoketones; the latter subsequently condense to form pyrazines. Strecker aldehydes and pyrazines contribute to aroma formation in heated foods.

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Figure 3-21. Scheme of the initial stage of the Maillard reaction.

Figure 3-22. Formation of hydromethylfurfural from 1-amino-1-deoxy-D-fructose.

Figure 3-23. Formation of reductones from 1-amino-1-deoxy-D-fructose.

Figure 3-24. Scheme of the Strecker degradation.

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Figure 3-25. Scheme of the aldolisation of aldehydes and ketones.

The majority of the compounds displaying color are formed in the final stage of the Maillard reaction. Furfurals, reductones, aldehydes either undergo aldol condensation without the intervention of amino compounds or react with amino compounds as other intermediates, leading to the ultimate reaction products known as melanoidins. Hodge defined melanoidins as ‗brown, nitrogenous polymers and copolymers‘. The structures of melanoidins remain unknown till now. The pigment generated in the Maillard reaction is soluble in the early stage of polymerization and exhibits no characteristic absorption pick in the visible range. Infrared spectrum and chemical composition analysis indicate that melanoidins contain unsaturated bonds, heterocycles, and complete amino acid residues. The reaction mechanism of the Maillard reaction proposed by John Hodge is largely unchanged after 60 years and is still widely cited. However, the hypothesis has some flaws. Firstly, the mechanism presents only the general process of the Maillard reaction and the details on the reaction are not described. Secondly, some new reactions have been identified by other researchers. For example, the work of Japanese scientists Namiki and co-workers found that the carbonyl fission products can also be formed directly from N-substituted glycosylamine through a free radical-mediated pathway, which is known as the Namiki pathway [4].

Mechanism of Caramelization Heating of carbohydrates without the presence of nitrogen-containing compounds causes a complex group of reactions termed caramelization. Unlike the Maillard reaction, caramelization of carbohydrates is thermolysis as compared to the reaction with amino acids. Mild heating or thermolysis in the early stage leads to anomeric shifts, ring size change, glycosidic bond breakdown, and new glycosidic bond formation. Mostly, thermolysis causes dehydration of the sugar molecule with introduction of double bonds or formation of anhydro rings. Caramelization yields hundreds of flavor compounds and these compounds impart foods with pleasant color and flavor. The caramels derived from different sugars have similar compositions. Generally, the products of caramelization include caramel, which is the polymerization product of sugar dehydration, and aldehydes, ketones, phenols, etc. The caramelization process is divided into two stages. Caramel formation Figure 3-26 illustrates the formation of caramel by taking sucrose as the example. It can be seen that caramelization is the removal of water from a sugar, proceeding to isomerization and polymerization of the sugars into various high-molecular weight compounds.

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The aqueous solutions of caramels are colloidal and the isoelectric points range from 3.0~9.0, with some exceptions with pI lower than 3. The presence of acids or certain salts facilitates the reaction. The caramel prepared by heating sucrose solution in the presence of acid or acidic ammonium salts has been widely used in food coloration. The caramelization of sucrose consists of three steps: Step 1: Caramelization of sucrose starts with the melting of the sugar at high temperatures followed by foaming (boiling). At this stage sucrose decomposes into glucose and fructose. This is followed by a condensation step, in which the individual sugars lose water and react with each other to form the isosaccharosan (Figure 3-27). Isosaccharosan loses the sweetness of sugar and tastes bitter instead.

Figure 3-26. General mechanism for thermal promoted caramelization of sucrose.

Figure 3-27. Structure of isosaccharosan.

Step 2: This step involves further dehydration reactions. Isosaccharosan dehydrates and condensates to caramelan. Caramelan is one of the three main products of sucrose caramelization. Caramelan are lightly brown and tastes bitter with formula of C24H36O18. Caramelan melts at 138°C and is soluble in water and ethanol.

Step 3: This step includes both fragmentation reactions (flavor production) and polymerization reactions (color production). With respect to color production, caramelan is further dehydrated to form caramelen. If the mixture is further heated, polymerization reaction occurs and the insoluble caramelin is generated. The caramelan melts at 154°C, tastes bitter, and is soluble in water with formula of C36H50O25. Caramelin appears dark brown and are not soluble in water with formula of C125H188O80.

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The presence of iron enhances the color of caramels. Phosphates, inorganic salts, alkalines, citric acid, ammonia, and ammonium sulfate catalyze sugar caramelization. Ammonia and ammonium sulfate increase the yield of caramel. However, the addition of ammonia and ammonium sulfate cause the generation of 4-methylimidazole at high temperatures, which is an anticonvulsants and can cause nervous system damage after longterm consumption. Ammonia and ammonium sulfate have been prohibited in the production of caramels. Caramel flavors from thermal fragmentation a. Formation of aldehydes in acid media. In acid media, aldoses or ketones undergo enolization to yield hexose-1,2-enediol when heated, followed by a series of dehydration reactions, as shown in Figure 3-28. b. Formation of aldehydes in alkaline media. In alkaline conditions, the intermediate 1,2-enol hexose, such as fructose, is produced via the tautomery of reducing sugars and is then fragmented when heated, as shown in Figure 329.

Figure 3-28. Formation of aldehydes from sugars in acid media and thermal condition.

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Figure 3-29. Thermal fragmentation of 1, 2-enediol-hexose in alkaline media.

Mechanism of Nonenzymatic Browning of Ascorbic Acid Ascorbic acid is a well-known natural antioxidant. It can be readily oxidized in two ways. In the presence of oxygen, ascorbic acid is oxidized to dehydroascorbic acid, which is then dehydrated to yield 2,3-diketogulomic acid (DKG). DKG is further decarboxylated to yield xylosone and subsequently reductones, which participate in the reactions in the intermediate and final stages of the Maillard reaction. Ascorbic acid degrades very quickly in the presence of oxygen and the specific rate is related to the level of dissolved oxygen and extraneous gas. If the food matrix contains components with reduction potential higher than that of ascorbic acid, ascorbic can be oxidized to dehydroascorbic acid even in the absence of oxygen. In both cases, dehydroascorbic acid is then transformed to DKG in the presence of water. DKG further undergoes decarboxylation and dehydration to yield aldofuranoses or reductions. These products can participate in the Maillard reaction, leading to the formation of brown pigments (Figure 3-21). Ascorbic acid is slowly oxidized to dehydroascorbic acid in acidic solutions with pH < 5 and this process is reversible. Mechanism of Polyphenols Browning Some plant-derived foods contain high levels of phenolic compounds. For example, the content of polyphenols in green tea is up to 30%. The phenolic hydroxyl group of polyphenols is very susceptible to oxidation, especially in alkaline conditions. Polyphenols undergo auto-oxidation at high temperature and high moisture conditions and the oxidation products exhibit different colors. Polyphenols are an important cause of food browning and the browning mechanism is elucidated by taking catechin as an example.

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Figure 3-30. Mechanism of ascorbic acid browning.

Structure of catechin Catechin possesses two benzene rings (called the A- and B-rings) and a dihydropyran heterocycle (the C-ring) with a hydroxyl group on carbon 3, as shown below:

Catechin contains multiple phenolic hydroxyl groups and is very susceptible to oxidation, polymerization, and condensation. Catechin is white crystal and is oxidized to yellowishbrown compounds in the air. Catechin is soluble in water, ethanol, methanol, acetone, and acetic anhydride, partially soluble in ethyl acetate and acetic acid, and insoluble in chloroform and anhydrous ether.

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Mechanism of oxidation The hydroxyl groups in catechin have different activities with respect to auto-oxidation. Hydroxyl groups in adjacent and vicinal positions are oxidized easily, while the –OH in carbon 3 on ring C cannot be oxidized. The mechanism of the nonenzymatic auto-oxidation of catechin is very complex and not well understood to the present. It is generally accepted that the process involves two reactions. The first is the formation of quinine. Quinine is unstable and rapidly undergoes condensation. The condensation product in the early stage is soluble, light yellow and tastes bitter. As the reaction proceeds, the intermediates are further condensed to brown nitrogenous polymer in the presence of amino compounds. The general process is illustrated in Figure 331.

Effects of Nonenzymatic Browning on Foods Nonenzymatic browning reaction is one of the most important reactions during the storage and processing of foods. A large number of food components, such as carbohydrates, amino acids, phenolic compounds, and ascorbic acid, are involved in the reaction. The reaction yields multiple volatiles and nonvolatile compounds, which significantly influence the color, aroma, taste, nutrition, and safety of foods.

Figure 3-31. Mechanism of nonenzymatic browning of catechin.

Effects on Color The composition of nonenzymatic browning reaction products is very complex. Many components affect the color of foods and their molecular weights range from several hundreds to up to more than 100000 Dolton. Several coloring compounds have been isolated from different model systems.

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A yellow compound is isolated from the Maillard reaction product of the xylose – Lys model system and MS and NMR reveals that its possible structure is one of the following two [5]:

(1) Furan-2-carboxaldehyde – L-alanine model system Two red products are isolated and identified from the Maillard reaction product and their structures are shown below [6]:

(2) Xylose – L-alanie model system A white compound is isolated:

(3) Glucose – propylamine model system in ethanol solution. A yellow product is identified:

(4) Xylose – alanie model system in the presence of carbonyl compounds. Two yellow compounds are isolated:

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(5) One red compound is isolated:

(6) Starting materials, as well as reaction conditions, markedly affect the elemental composition and structure of melanoidins. Thus far, the structures of the melanoidins have not been elucidated, although some structural insights have been gained from model reactions. In a glucose/amino acid Maillard reaction system, the carbonyl compound reacts mainly via the Amadori product to form several deoxyosones which are able to react with each other in an aldol-type condensation to form a basic melanoidin skeleton of amino-branched sugar degradation products. Figure 3-32 shows the possible structure of a melanoidin formed from 3-deoxyhexosuloses in this way.

Figure 3-32. Part of possible melanoidin structure formed from 3-deoxyhexosulose involving amino compounds [7].

Figure 3-33. Strecker degradation of L-Lys with dicarbonyl compound.

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Figure 3-34. pH changes of various sugar-amino acids solutions heated at 100°C. From up to down: glucose, lactose, lactose + alanine, glucose +alanine, lactose + glycine, glucose + glycine, glucose + lysine, lactose+ lysine.

Effects on Food Aroma and Taste The intermediate and final products of nonenzymatic browning reactions markedly influence the aroma and taste of foods. Under high temperatures, carbohydrates undergo dehydration, fragmentation, isomerization, and oxidation/reduction, giving rise to formic acid, acetic acid, lactic acid, acetol, acetoin, diacetyl, pyruvic acid, and many other products. The dicarbonyl compounds generated in non-enzymatic browning can initiate the changes of other food components. For example, amino acids are deaminated and decarbonylated to yield abundant aldehydes, as shown in Figure 3-33: Non-enzymatic browning can produce both desirable and undesirable flavors. Maltol and isomaltol in breads gives the caramel-like flavor for breads and 4-hydroxy-5-methyl-3(2H)furanone delivers the flavor of roasted meat and is used as flavor and sweetness enhancer, while some pyrazine and aldehyde compounds are the source of undesired burned taste in some foods. CO2 is a product of the Strecker degradation and its volume released is proportional to the moiety of dicarbonyl compounds. Besides, the reducing ketones and aldehydes produced can be readily oxidized to acidic compounds. Therefore, non-enzymatic browning reaction reduces the acidity of foods. Figure 3-34 shows the changes of pH in different model systems. Antioxidation The antioxidant activity of Maillard reaction products (MRPs) was firstly reported by Franzke and Iwainsky in 1954. The two researchers found that the thermal reaction product of glycine and glucose improved the stability of margarine against oxidation [8]. However, this research did not attract much attention until 1980s. The antioxidant components in various MRPs are not identified yet and the antioxidant activity of MRPs is studied mainly by using various model systems.

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Elizalde et al. found that the volatile compounds of the MRP of the glucose-glycine system showed a significant antioxidant activity and prolonged the induction period of soybean oil thermoxidation by up to 3 times in relation to the control [9]. Bedinghaus and Ockerman compared the lipid oxidation inhibitor activities of fifteen MRPs prepared by heating one of three sugars (glucose, xylose, and dihydroxyacetone) with one of five amino acids (arginine, histidine, leucine, lysine, and tryptophan) by using cooked ground pork patties as the model food. It was found that the most effective MRPs were xylose-lysine, xylose-tryptophan, dihydroxyacetone-histidine, and dihydroxy-acetone-tryptophan when compared to controls [10]. Yamaguchi et al. obtained a fraction from the MRP of the xylose – glycine system through multiple chromatographic steps and found that it is more effective in preventing linoleic acid oxidation than BHA and propyl gallate [11]. Yoshimura et al. investigated the MRP of the glucose-glycine system on the inhibition toward active oxygen and fount that it inhibited more than ca. 90% of active oxygen species existing in the form of hydroxyl radicals (•OH) [12]. Morales and Jiménez-Pérez investigated the DPPH· scavenging activities of MRPs produced by heating glucose or lactose with lysine, alanine or glycine. It was found that all the MRPs were effective DPPH· scavenging agents. Browning was not directly related to the free radical scavenging properties of MRPs formed at prolonged heating conditions and fluorescence measurement is more effective than browning to follow the formation of MRPs with free radical scavenging activities, as shown in Figure 3-35. Although the antioxidant effects of MRPs have been well recognized, their application as effective antioxidants in foods is still limited due to insufficient knowledge on the structure and antioxidant mechanism of MRPs. Early researches indicated that antioxidant capability of MRPs is contributed by the intermediate reductones of the Maillard reaction and the metal chelating ability of MRPs. Latest researches indicate that MRPs also have strong active oxygen scavenging capabilities and can reduce peroxides.

Note: A: up to down: glucose+alanine, glucose+glycine, glucose+lysine. B: up to down: lactose+alanine, lactose+glycine, lactose+lysine. Figure 3-35. Free radical scavenging activity in sugar/amino acid mixtures heated at 100°C up to 24 h [13].

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Effects on Nutrition One of the most obvious negative consequences of the Maillard reaction in food is the loss of nutritive value of proteins due to decreased digestibility, destruction and/or biological inactivation of amino acids, including essential amino acids like lysine and tryptophan, inhibition of proteolytic and glycolitic enzymes, and interaction with metal ions. Lysine is the most liable to loss in non-enzymatic browning, followed by alkaline amino acids, including L-Arg and L-His. In addition to sugar-amino acid reactions, Strecker degradation is also involved in amino acid loss. The formation of complex macromolecules reduces the solubility of proteins and reduces their nutritional value. The effect of the Maillard reaction on amino acid availability has been investigated by using rainbow trout (Salmo gairdneri) as the model. It was found that arginine and lysine exhibited the greatest losses in the mixture of fish protein isolate and glucose stored for 40 d at 37°C and the apparent digestibility and absorption of individual amino acids, particularly lysine, was lower in trout fed browned protein than in those fed the control protein [14]. Vitamin C is also involved in browning and therefore suffers loss during the process. The products of non-enzymatic browning reduce the bioavailability of minerals. Whitelaw et al. employed the dialysis procedure to determine the effect of the Maillard reaction on apparent 65Zn availability. In the presence of 65ZnCl2, the amino acids glycine, Dleucine, L-proline, L-lysine and L-glutamic acid were combined with D-glucose and autoclaved (110°-120°C, 15 atm, 10 min) to produce high molecular weight 65Zn binding compounds that were not dialyzable (6-8KD). Experiments in stimulated gastrointestinal digestive conditions revealed that the Maillard reactions significantly reduced the bioavailability of Zn [15]. Harmful Compounds The safety of MRPs has attracted wide attentions in recent years. With the development of new instrumental analysis measures, more and more harmful compounds have been isolated and identified. For example, mutagenic compounds have been found in instant and caffeine-free coffee and the compounds consist of dicarbonyl compounds, methylglyoxal, diacetyl and glyoxal, among which the methylglyoxal presented highest mutagenic activity [16]. Also in both fried and grilled meat and fish, mutagenic compounds were identified, mainly stemming from heterocyclic amines [17]. The reaction mechanism seems to have a major influence on the mutagenicity of the reaction products. For instance, ketose sugars showed a higher mutagenic activity than the corresponding aldose sugars [18]. However, due to the complexity of non-enzymatic browning reactions and the poor stability of intermediates, only very few harmful compounds have been well elucidated, of which, acrylamide is the most intensively studied. Acrylamide is a well-known cancerogen and can cause neurologic damage. Acrylamide has been detected in nearly all the foods, but fried and roasted foods that are processed at high temperatures have much higher acrylamide contents, as presented in Table 3-7.

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Carbohydrates Table 3-7. Content of acrylamide in some foods Foods Potato chips French fries Biscuit Fried bread American breakfast Cornflakes Bread

Acrylamide contents (μg/kg) Median Minimum ~ maximum 1,200 330-2,300 450 300-1,100 410 glucose. Among carbonyl compounds, -hexenal has the highest browning rate, followed by dicarbonyl compounds and ketones in sequence. Alkaline amino acids undergo browning more readily than acidic and neutral amino acids and those with the amino group locating at the end or ε- position are more susceptible to browning than those with amino group at the α- position. Non-enzymatic browning also occurs between proteins and carbonyl compounds, but the reaction is slower than that of peptides. Temperature and Time Reaction temperature significantly affects browning. When the temperature increases by 10 °C, the browning rate can be 3~5 times higher. Generally, non-enzymatic browning occurs rapidly at temperatures higher than 30 °C. The degree of browning is also affected by reaction time. When the mixture of disaccharide or monosaccharide and amino acid is heated at 100°C, the absorbance of the solutions at 420nm increases with heating time, indicating that the formation of the brown color is positively correlative with the reaction time (Figure 3-36). In addition to the color and flavor of foods, reaction temperature and time influence the generation of harmful compounds as well. For example, when equimolar amount of asparagine and glucose is heated, the formation of acrylamide is detected in 120 °C and the content continues increasing with temperature elevation until 170 °C. The formation of acrylamide generated in the mixtures of glucose and glutamine, methionine, or asparagine heated at 180 °C as a function of time (5~60 min) reveals that the contents of acrylamide varies with amino acids. The highest acrylamide content is found in the glucose/asparagine system and the highest content appears in the 5th minute. When the reaction proceeds, the level of acrylamide keeps decreasing. In the glucose/glutamine system, the highest

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acrylamide formation is detected within 10 min and the content remains unchanged thereafter. While in the glucose/methionine system, the formation of acrylamide keeps increasing in the first 30 min and then remains unchanged in remaining time.

pH Medium pH affects multiple reactions of non-enzymatic browning. Aqueous solutions of sugar (xylose or glucose) and amino acid (glycine or lysine monohydrochloride), are heated without pH control for up to 120 min and the total reaction products are analyzed by HPLC and compared with those of the corresponding model systems maintained at pH 5 throughout heating. For xylose-lysine, seven of these peaks are common to the systems heated both with and without pH control for 15 min, while no peak is common to both glucose-lysine systems heated for 120min, indicating different products are formed under the conditions [19]. Generally, the browning rate between sugars and amino acids increases with pH when the medium pH is greater than 3.5 and is reversely proportional to pH values in the range 2.0~3.5. Hence, browning can be suppressed by reducing pH. This is why browning does not occur readily in acidic foods such as pickled vegetables. Sulfites inhibit nonenzymatic browning by reacting with carbonyl intermediates, thereby preventing their further reaction to form brown pigments [20].

Water Content and Metals The rate of non-enzymatic browning is a function a water activity. Generally, nonenzymatic browning occurs readily in the water activity range 10%~15% and is suppressed in water activity less than 3%. Moderate water content facilitates the mobility of solutes, but too high water contents lead to solute dilution and consequently reduced browning. Metal ions are also involved in the process of non-enzymatic browning, as evidenced by the fact that the browning of grapefruit juice is inhibited by the addition of a chelating agent EDTA [21]. Cu(I), Cu(II), Fe(II), and Fe(III) speed up the browning of ascorbic acid and phenols, but other metals, such as Pb, Zn, and Sn, seem to have little effect.

Figure 3-36. The time course of the browning develop in heated sugars -amines solutions at 100°CUp to down: glucose+lysine, lactose +lysine, glucose+glycine, lactose+ glycine, glucose+alanine, lactose +alanine, lactose, and glucose.

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High Pressure High pressure (100-1000 MPa), which is gaining increasing importance as a foodprocessing technology particularly in combination with moderate temperatures (30-60 °C), may influence the Maillard reaction and thereby affects the flavor, color, and nutritional value of foods. It has been proposed that high pressure exerts its influence on non-enzymatic browning by changing the medium pH. The effect of high pressure on Maillard reaction has been investigated by using the glucose-lysine model system over a range of pH values (5-10) at 60 °C either under atmospheric pressure or at 400 MPa. The results obtained showed that high pressure affected in different ways the different stages of the Maillard reaction and that such effects were strongly influenced by pressure-induced changes in the pH of the systems. In unbuffered media, at an initial pH 8.0, the formation of Amadori rearrangement products (ARP) was not considerably affected by pressure, whereas the intermediate and advanced stages of the Maillard reaction were suppressed, suggesting a retardation of the degradation of the ARP. In buffered media, at pH values 8.0, pressure slowed the Maillard reaction from the initial stages. These effects are attributed to the pH drop caused by the pressure-induced dissociation of the acid groups [22]. Control of Non-Enzymatic Browning The non-enzymatic browning has both beneficial and adverse impacts on food qualities. Researchers have developed rational approaches to minimize adverse consequences of browning reactions and optimize beneficial ones. Non-enzymatic browning can be prevented by chemical or biochemical methods [23]: Sulfhydryl compounds Sulfur-containing amino acids such as cysteine, N-acetylcysteine, and the tripeptide glutathione play key roles in the biotransformation of toxic compounds by actively participating in their detoxification. These antioxidant and antitoxic effects are due to a multiplicity of mechanisms including their ability to act as (a) reducing agents, (b) scavengers of reactive oxygen (free radical species), (c) destroyers of fatty acid hydroperoxides, (d) strong nucleophiles that can trap electrophilic compounds and intermediates, (e) precursors for intracellular reduced glutathione, and (f) inducers of cellular detoxification. Under certain conditions, SH-containing compounds may be as effective as sodium sulfite in preventing nonenzymatic browning of both apples and potatoes. Acetylation of amino groups Modifications of amino groups prevent them from participating in browning reactions. For example, treatment of foods with the enzyme transglutaminase will transform lysine amino groups to amide groups. The former initiate browning, whereas the latter do not. Antioxidants Oxygen seems to be required for some nonenzymatic browning reactions. Hence, antioxidants could suppress browning in some foods. Besides, antioxidants can trap or

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prevent the formation of intermediates in the Maillard and related reactions, thus prevent the formation of undesirable compounds during food processing. Deglycation An extract from soil microorganisms catalyzed the deglycation of α- and εfructosyllysines to lysine. This finding suggests that these purified enzymes could be used to prevent or reverse Maillard reactions in foods and in vivo provided they are safe in other regards. Non-enzymatic browning can also be prevented by controlling processing conditions: Low temperature Non-enzymatic browning occurs slowly in low temperatures. Storage at low temperatures can delay non-enzymatic browning in foods. Sulfurous acid The condensation product of carbonyl group and sulfurous acid can react with R-NH2 and the resultant product cannot be transformed to Schiff‘s base. Hence, SO2 and sulfites suppress non-enzymatic browning. pH Non-enzymatic browning does not occur readily in acidic conditions as in alkaline conditions. Product concentration Products with lower concentrations suffer lighter browning. For example, because lemon juice is more susceptible to browning than orange juice, the concentration factor of lemon juice is 4:1, which is lower than 6:1 of orange juice. Insensitive sugar The presence of free carbonyl groups is essential for non-enzymatic browning. Replacement of reducing sugar with sucrose can prevent browning. Removal of sugar Some foods contain only trace amount of sugars and these sugars can be removed to prevent non-enzymatic browning. For example, glucose oxidase and catalase have been used to remove the trace glucose in dried egg yolk and dehydrated meat. Calcium salts Calcium can complex with amino acids to form precipitates.

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IMPORTANT OLIGOSACCHARIDES AND POLYSACCHARIDE IN FOODS Oligosaccharides and polysaccharides occur widely in the nature and some of them contribute largely to the quality and nutrition of foods. Oligosaccharides and polysaccharides have gained important applications in the food industry as thickening agent, gelling agent, crystallization inhibitors, clarifying agent, stabilizing agent, film forming agent, flocculating agent, controlled-release agent, expansive agent and encapsulation agent. Some carbohydrates posses important physiological functions and many functional foods have been developed based on them.

Oligosaccharides Oligosaccharides occur naturally in a variety of foods, especially plant-derived foods, such as vegetables, grains, legumes, and algae. Oligosaccharides have also been identified in animal-derived foods, including milk, honey and insects. Sucrose, maltose, lactose and cyclodextrin are the most important oligosaccharides for the food industry. Some oligosaccharides, such as fructo-oligosaccharides, xylo-oligosaccharides, and Konjac oligosaccharides, have obviouse physiological functions and can be used as the effective ingredients of functional foods. Cellobiose, maltose, isomaltose, gentiobiose and trehalose are common disaccharides. All these disaccharides, except trehalose, contain a free semi-acetal group and hence are reducing sugars. Sucrose, lactose, lactulose, and melibiose are hybrid-oligosaccharides and each contains a reducing group except sucrose. Of the sugars, lactose deserves special attention. Lactose is found notably in milk and non-fermented dairy products and accounts for 2~8% of milk by weight. In small intestine, lactose is hydrolyzed by lactase to D-glucose and D-galactose, which are then absorbed by the human body. However, some consumers lack the lactase and cannot digest or metabolize lactose. These consumers might suffer abdominal pain, bloating, flatulence, diarrhea, nausea, and acid reflux and this is called lactose intolerance or lactase deficiency. Some natural foods contain functional oligosaccharides. These compounds cannot be absorbed by human body and provide very few energy, but can enhance the proliferation of intestinal bifidobacteria and prevent dental caries and colon. Some important functional oligosaccharides are described below. Soybean oligosaccharide Soybean oligosaccharide has once been regarded as an antinutritional factor and has been reported to increase the incidence of diarrhea in rats. However, recent researches reveal that soybean oligosaccharide has the potential as new functional ingredients in functional foods. Soybean oligosaccharide has recently been found to be ―probiotic material‖ and have been approved by the Food and Drug Administration as generally recognized as safe material in the USA. Composition analysis indicates that soybean oligosaccharide was composed of

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galactose (65.3%), mannose (15.6%), fructose (7.8%) and glucose (8.7%) [24]. Intake of 3~5 g soybean oligosaccharide per day is sufficient to enhance the proliferation of bifidobacteria. Fructo-oligosaccharide Fructo-oligosaccharides (FOS) are fructose oligosaccharides containing a single glucose moiety. FOS are mainly composed of 1-kestose (GF2), nystose (GF3), and 1-β-fructofuranosyl nystose (GF4), in which fructosyl units (F) are bound at the β(2 → 1) position of sucrose molecule (GF). FOS occur naturally in various fruits and vegetables, such as bananas, onions, chicory root, garlic, asparagus, barley, wheat, jícama, tomatoes, and leeks, but commercial FOS are often prepared by transfructosylation from sucrose and chemical or enzymatic hydrolysis of inulin. FOS are about 0.4 and 0.6 times as sweet as sucrose and have been used in the pharmaceutical and food industries as functional sweeteners. FOS with low polymeric grade have better therapeutic properties than those with a high polymeric degree. They present properties such as low caloric values, non-cariogenic properties, decreased levels of phospholipids, triglycerides and cholesterol, help gut calcium and magnesium absorption, and are used as prebiotics to stimulate the bifidobacteria growth in the human colon [25]. Figure 3-37 illustrates the structure of several FOS. Xylo-oligosaccharide Xylo-oligosaccharides (XOS) consist of 2~7 xylose residues connected through by the β (1→4) glycosidic bond. XOS are about 40 times as sweet as sucrose. XOS have good thermal stability and are not decomposed in acidic conditions (pH2.5~7) when heated. XOS can be used in yogurt, lactobacillus drinks, carbonated drinks and other acidic beverages. Commercial XOS consists mainly of xylose, xylobiose, and less amount of polymers with DP greater than 3. Xylobiose is the main constituent of XOS and its content determines the quality of XOS products. XOS can be prepared by the enzymatic (xylanase) hydrolysis of xylan-rich materials, such as corncob, bagasse, cottonseed hull, and bran. Many fungi and bacteria can produce xylanase, of which, the endoxylanase produced by Chaetomiu globosum has been used in the industrial production of XOS. XOS cannot be digested but can selectively activate bacterial reproduction within intestines and hence are prebiological substances. It can obviously improve intestinal microecological balance, proliferate bifid bacteria and gastric function. Chito-oligosaccharides Chito-oligosaccharides (COS) are oligomers of N-acetyl-D-glucosamine and Dglucosamine connected through the β-1, 4 glycosidic bond. COS are water soluble in contrast to chitosan and chitin. COS carry positive charge, which allows COS to bind to negatively charged cell surface strongly. This property contributes to many physiological functions of COS, such as antitumor, immunostimulatory, and anti-inflammatory. Besides, COS stimulate the proliferation of Bifidobacteria bifidium and Lactobacillus sp. [26].

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Figure 3-37. Structure of FOS.

Other oligosaccharides Palatinose (6-O-α-D-glucopyranosyl-D-fructose), also referred as isomaltulose, is a reducing disaccharide identified during the processing of sugar from sugar cane. It is completely digested and provides the same caloric value as sucrose, but it is non-cariogenic and digested much slower, leading not only to a low glycemic response but also to a prolonged glucose supply, indicating its potential as a parenteral nutrient acceptable to both diabetics and non-diabetics. Its ingestion selectively promotes the growth of beneficial bifidobacteria amongst the human intestinal micro flora. It is more stable than sucrose, which facilitates the maintenance of its sweetness and taste in fermented foods and beverages. It has been suggested as a non-cariogenic alternative to sucrose, and as such is currently widely used as a sugar substitute in foods. This disaccharide has a sweet taste and very similar physical and sensory properties to sucrose [27]. Lactulose (4-O-β-D-galactopyranosyl-D-fructose) is a synthetic disaccharide and is also termed isomerized lactose, because it can be prepared from the isomerization of lactose. It is a Bifidus factor in nutrition and is a very important humanizing factor in infant formula and is added to commercial infant formula products and various milk products. This sugar has greater sweetness and solubility than lactose and if produced economically, it could widely be used in baking and confectionery applications [28].

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Starch and Glycogen Starch Starch is the major constituent of many foods and is the most important carbohydrate source for human nutrition. Corn, wheat, potato, sweet potato, and rice are the most important materials for starch production. Table 3-8 lists the contents of starch in some crops. Starch can be divided into two fractions: amylose and amylopectin. Natural starches are the mixture of amylose (10~20%) and amylopectin (80~90%). The percentages of amylose and amylopectin in starches derived from different sources are shown in Table 3-9. Gelatinization does not occur easily in starches with high amylose contents and the gelatinization temperature can reach up to 100 °C. However, gelatinized amylose is unstable and is prone to aging in contrast to the high stability of gelatinized amylopectin. Due to the unique physicochemical properties and nutritional functions, starches are of incomparable importance to human beings. Starch and its derivatives have been widely used as thickening agent, bonding agent and stabilizing agent and as the materials of pudding, soup, sauce, vermicelli, infant foods, pie, and mayonnaise. Chemistry structure Amylose is a linear polymer of α-D-glucopyranosyl residues connected through the 1→4 glycosidic bond. The number of repeated glucose subunits is usually in the range from 300 to 3000, but can be up to many thousands. For example, the polymerization degree of wheat starch is in the range 500~6000, while in potato it can rise up to 4500. Amylose also contain few α(1→6) bonds, accounting for 0.3%~0.5% of total glycosidic bonds. Based on X-ray diffraction diagrams, native starches can be divided into types A, B, and C. An additional form, called the V-type, occurs in swollen granules. While types A and B are real crystalline modifications, the C-type is a mixed form. The A-type is largely present in cereal starches, and the B-type in potatoes, amylomaize, and in retrograded starches. The Ctype is not only observed in mixtures of corn and potato starches, but is also found in various legume starches [29]. Table 3-8. Contents of starch in some materials (%) Species Unpolished rice Corn Barley Kidney bean Sweet potato (fresh)

Content 73 70 40 49 19

Variety Potato Wheat Sorghum Buckwheat noodles Pea

Content 16 66 60 72 58

Table 3-9. Percentages of amylose and amylopectin in some starches (%) Starch High amylose corn starch Corn starch Waxy corn starch

Amylose 50~ 85 26 1

Amylopectin 15~50 74 99

Starch Potato starch Cassava starch Wheat starch

Amylose 21 17 28

Amylopectin 79 83 72

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Type B amylose shows the left-hand double helices structure, which are packed in a parallel arrangement. Hydroxyl groups are located outside of the chains and the double helix is stabilized by the hydrogen bridges between amylose molecules. The internal channel of the helix is hydrophobic and hence can enclose only hydrophobic compounds, such as lipids. The A-type is very similar to the B-type, except that the central channel is occupied by another double helix, as shown in Figure 3-38. Amylopectin is the highly branched polymer of glucose. The backbone chain is composed of glucose units linked in a linear way with α(1→4) glycosidic bonds and branching takes place through α(1→6) bonds. In amylopectin, α(1→6) bonds account for 4%~5% of total glycosidic bonds.

Figure 3-38. Unit cells and arrangement of double helices (cross section) in type-A amylose (left) and type-B amylose (right) [29].

Due to the presence of multiple linear branches, amylopectin has multiple reducing ends and are hydrolyzed by enzymes faster than amylose. The branches of amylopectin are arranged in parallel or double helix, as shown in Figure 3-39. Amylopectin contributes to the crystalline structure of starch granules. The molecular weight of amylopectin ranges from 5×107~5×108. The ratio of amylopectin in starches generally exceeds 75% and the value can reach up to 99% in some species, such as waxy corn starch, as shown in Table 3-9. Starches derived from potato also contains a high phosphorus content (0.06~0.1%). Hence, potato starches are slightly negatively charged and swell more rapidly in warm water due to coulombic repulsion. Gelatinization Starch granule: Starches are present as granules in plant cells. Starch granules can be round, oval, and polygonal and their sizes range from 0.001~0.15 mm, depending on the plant

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species, of which, potato starch granules have the largest size and cereal starch granules have the smallest size, as shown in Figure 3-40. Polarization microscopy observation and X-ray diffraction find double refraction and X-ray diffraction in starch granules, indicating the presence of crystalline structure alternate layers of crystal region and amorphous region (Figure 3-41 I). About 70% of the mass of a starch granule is regarded as amorphous and about 30% as crystalline. The amorphous regions contain mainly amylose and the crystalline regions consist primarily of amylopectin. Amylose can enclose fatty acids and hydrocarbons due to the presence of the hydrophobic internal channel and the complexes are termed inclusion complexes. Amylose occur as double helices as in starch granules. Amylose and amylopectin are arranged radially in starch granules, as shown in Figure 3-41 II.

Figure 3-39. Structural models (I, II) for amylopectin with parallel double helices. III is an enlarged segment of I or II.

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Figure 3-40. Shapes of starch grains in electron microscope (×1200).A, Green bean starch (mean grain size: 0.016nm); B, Potato starch (mean grain size: 0.049nm; C, Common corn starch (mean grain size: 0.013nm); D, Sweet potato starch (mean grain size: 0.017nm).

Figure 3-41. Sketch of crystallization section and amorphous section of starches (I), and radial shape of amylose and amylopectin in starches (I, II).

Gelatinization: Due to the inter-molecular hydrogen bonding, starch is not soluble in cold water in spite of the abundance of hydroxyl groups. When the suspension is heated, the vibration of starch molecules increases and the hydrogen bonding between molecules is disrupted. As a result, more hydroxyl groups are exposed and starch-water hydrogen bonding is formed instead. With the diffusion of water into starch granules, many long chains are separated and the confusion degree increases markedly. Meanwhile, the number and size of

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6ISCOSITY"RABENDERUNITSX 

crystal regions reduce significantly. When the suspension is further heated, the swelling becomes reversible. In this case, random coils are observed in amylopectin due to hydration and the ordered structure of starch is destroyed. This process is termed gelatinization and the temperature at which irreversible changes occur is called the gelatinization temperature. Starch gelatinization can be divided into three phases. In phase I, water diffuses into granules and absorbs to the polar groups in amorphous regions below the gelatinization temperature. The starch can restore to their original form if dehydrated. In phase II, when the gelatinization temperature is reached, bulk water enters starch granules and the granules swell significantly. Due to the increase of volume, the fraction between swollen starch increases and the suspension becomes viscous, which can be followed by Brabender amylograph. In this phase, water molecules enter the microcrystalline areas and the original arrangement is disrupted. When the temperature further increases, the viscosity of the suspension increases accordingly. In phase III, the swollen starch granules are disintegrated and the suspension viscosity drops sharply, as shown in Figure 3-42. It could be seen that the shape of the curve varies greatly for different starches. The gelatinization of starch is affected by the following factors: Water activity: Gelatinization occurs readily in medium with high water activity. The presence of high concentration of sugars significantly suppresses gelatinization, because the molecules can compete for water with starch.















 4IMEMIN



Figure 3-42. Gelatinization properties of various starches. Brabender viscoamylograph. 40 g starch/460 ml water, temperature programming: start at 50 °C, heated to 95 °C at a rate of 1.5°C/min. Held at 95°C for 30 min – potato, - - - waxy corn, −−− normal corn, and ••• amylomaize starch.

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Helix amylose

Fatty acid Figure 3-43. Scheme of the lipid-starch inclusion complex.

Starch structure: Starches with higher ratio of amylose is less susceptible to gelatinization and their gelatinization temperature might reach up to over 100 °C. Salts: Salts in high concentrations suppress the gelatinization of starch, but those in low concentrations have no the inhibitory effect, except potato starch, because salts affect the charge carried on phosphorus groups. Lipids: Lipids can be enclosed inside the helix of starch (Figure 3-43). The enclosed lipids cannot be easily removed from the helix and can prevent the diffusion of water into starch granules. Hence, any lipids that can complex with starch can prevent starch swelling and gelatinization. The addition of C16~18 monoacylglycerol increases the gelatinization temperature, reduces the gelation temperature, and weakens the gel strength. pH: When the medium pH is lower than 4, starch is hydrolyzed to dextrins, which reduces the suspension viscosity. Hence, crosslinked starch instead of natural starch should be preferred as thickening agent in acidic foods. Starch gelatinization is not affected in the pH range 4~7. Starch gelatinization occurs rapidly in medium with pH greater than 10, but such high pH is not found in foods. Amylase: In the initial phase of gelatinization, starch granules start to absorb water and swell and amylase is not inactivated yet. Amylase can hydrolyze starch and accelerate gelatinization. This is why new rice is gelatinized more easily than old rice, because the former contains higher amylase activity. Retrogradation When a hot starch paste is cooled down, the suspension generally transforms to a viscoelastic gel and starch becomes insoluble. The process of soluble starch becoming insoluble is called retrogradation. The presence of conjunction zones in the gel indicates that retrogradation is actually the recrystallization of starch molecules. The qualities of many foods are deteriorated during retrogradation. For example, the staling of breads and the loss of viscosity of soups and sauces are partially caused by starch retrogradation. The retrogradation of starch is influenced by the following factors: Amylose to amylopectin ratio: Amylose is more susceptible to retrogradation due to its linear structure, of which, amylose with DP of 100~200 is the most resistant to retrogradation. Starch concentration: Retrogradation occurs readily in high starch concentrations due to frequent molecular collision, but water content of less than 10% hinders the process. The highest retrogradation rate is generally found in the water content of 30~60%. Presence of inorganic salts: Inorganic salts hinder the repositioning of starch molecules. The hindering effects of some common ions are SCN-, PO43-, CO3-, I-, NO1-, Br-, CI-, Ba2+, Ca2+, K+, and Na+ in the decreasing order.

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Medium pH: Starch undergoes retrogradation readily in the pH range 5~7 and the process is inhibited in alkaline or acidic solutions due to charge repulsion. Temperature: The optimum temperature for starch retrogradation is 2°C~4°C and retrogradation does not occur in temperatures above 60°C or below -20°C. Cooling speed: When starch paste is cooled slowly, starch molecules have sufficient time to align and retrogradation occurs readily. If the paste is cooled rapidly, the water in the paste crystallizes quickly, which prevents the approaching of starch molecules. Hence, retrogradation occurs slowly in rapid cooling. Presence of other ingredients: Lipids and emulsifying agents prevent retrogradation. Compounds such as glyceryl monopalmitate (GMP), other monoglycerides and their derivatives, and sodium stearoyl 2-lactylate (SSL) are often added into doughs of bread and other baked goods to prevent starch retrogradation and increase shelf life. Hydrolysis of starch Starches can be randomly hydrolyzed by acids and enzymes. Under mild conditions, starch is only partially hydrolyzed by acids. This process is called thinning and the products are termed acid-modified or thin-boiling starch. Acid-modified starch has increased gel transparency and strength and is less susceptible to retrogradation. Acid-modified starch has wide applications and can be used as film forming agent and adhesive agent in products such as pan-coated roasted nuts and candy. Besides, they are also used as encapsulating agents and flavor carriers. Enzymatic hydrolysis has been used in the production of commercially important syrups. In the production of high-fructose corn syrup, corn is firstly hydrolyzed by α-amylase and glucoamylase to obtain high purity D-glucose. D-glucose isomerase is then added and Dglucose is converted to D-fructose. The final product is the mixture of 58% D-glucose and 42% D-fructose. High-fructose corn syrup is commonly used as sweetener in soft drinks. The degree of hydrolysis of starch is measured by dextrose equivalency (DE), which is defined as the percentage of reducing sugar in the syrup. DE is related to the degree of polymerization and is calculated in the following equation:

(1) Glucose polymers with DE less than 20 are defined as maltodextrins and those with DE in the range 20~60 are termed corn syrup. Table 3-14 lists the functional properties of some hydrolysis products of starch. Modified starch Physical, chemical and biochemical modifications have been implemented to enhance the functional properties of starches and modified starches have gained wide applications in the food, pharmaceuticals, and chemical industry. Important modified starches that are widely used in the food industry are described below. Low viscosity starch: Starch of this type is also termed acid-modified starch. When starch is exposed to acid below the gelatinization temperature, hydrolysis occurs only in the

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amorphous region and the crystal region nearly remains intact. The modified starch produced under such conditions is not soluble in cold water but is readily soluble in boiling water. Compared with native starch, low viscosity starch has decreased viscosity and gel strength of hot paste and elevated gelatinization temperature. Low viscosity starch can be used as thickening and film forming agents. Pre-gelatinized starch: This product is obtained by heating starch suspension above the gelatinization temperature and then dried with drum drying, spray drying or extrusion. Pregelatinized starch can dissolve and form gel in cold water. Pre-gelatinized starch can be used in elder and infant foods, surimi products, ham, sausage, and bakery foods. Besides, pregelatinized starch has gained applications in cooking-free instant foods due to its solubility in cold water. Table 3-14. The functional properties of hydrolysis products from starch Properties enhanced by greater Properties enhanced in products of less hydrolysis A conversion B Sweetness Viscosity production Hygroscopicity and humectancy Body formation Freezing point depression Foam stabilization Flavor enhancement Sugar crystallization prevention Fermentability Ice crystal growth prevention Browning reaction Note: A high DE syrups; B low DE syrups and maltodextrins.

Etherified starch: All the three free hydroxyl groups in each D-glucopyranose unit can be etherified. Hydroxyethyl starches of low degree of substitution (DS) have reduced gelatinization temperature, increased swelling rate, and lowered tendency of pastes and gels to retrogradation. Hydroxyalkyl starches, such as hydroxypropyl starch, can be used in salad dressings, pie fillings, and other foods as thickening agent. Esterified Starch: Starch can be esterified by acidic orthophosphates, acid pyrophosphates, and tripolyphosphates to produce various starch esters. Compared with native starch, monostarch phosphates gelatinize in lower temperatures and those with DS greater than 0.07 can swell in cold water and have increased paste viscosity and transparency, and decreased retrogradation. These properties are quite similar to those of potato starch, which has high phosphorus content. Because monostarch phosphates have good freezing-thawing stability, they are preferred in frozen foods, such as frozen broth and frozen cream pie, as thickening agent to unmodified starch. Starch can also be esterified by various organic acids, such as acetic acid, long-chain fatty acids (C6~C26), succinic acid, adipic acid, and citric acid. These esters are superior in thickening and paste transparency and stability to native starch and can be used in bakery products, soup powder, sauce, pudding and frozen foods as thickening agent and stabilizer, as well as in dehydrated fruit as protective agent and in flavor processing as encapsulating material. Crosslinked starch: Starch can react with sodium trimetaphosphate, phosphorus oxychloride, epichlorohydrin, and acid anhydride to yield crosslinked derivates. Crosslinking

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prevents starch granule swelling and increases the stability against heating. Crosslinking by phosphorus increases the stability of swollen starch granules, but the paste of distarch phosphates is opaque in contrast to that of monostarch phosphate. Table 3-15. Comparisons of the properties of native and modified starches Starch

Amylose/Amylopectin ratio 1:3

Gelatinization temperature range (°C) 62~72

Properties

0:1

63~70

High-amylose starch Acidmodified starch Hydroxyethyl starch Monostarch phosphate Crosslinked starch

3:2-4:1

66~92

Variable

69~79

Low tendency of paste to retrogradation Less birefraction of granule than common starch granules Reduced viscosity of hot paste

Variable

58~68 (DS0.04)

Variable

56~66

Variable

Acetylated starch

Variable

Higher than unmodified starch, depending on the degree of crosslinking. 55~65

Common starch Waxy starch

Poor freezing-thawing stability

Increased paste transparency and decreased retrogradation Reduced gelatinization temperature and retrogradation. Reduced peak viscosity and increased paste stability Increased paste transparency and stability

Starches with high degree of crosslinking are very stable against high temperature, low pH and mechanical vibration and their gelatinization temperature is proportional to the degree of crosslinking. Some starches with high degree of crosslinking do not swell even in boiling water. Crosslinked starches are mainly used in infant foods, salad dressing, fruit pie filling and cream-style corn as thickeners and stabilizers. They also provide resistance to gelling and retrogradation, show good freeze-thaw stability, and do not undergo syneresis or weep on standing. Oxidized starch: Starch is hydrolyzed and oxidized when its aqueous suspension is incubated with sodium hypochlorite below the gelatinization temperature and the carboxyl group occurs once every 25 to 50 glucose residues in the resulting oxidation products. Oxidized starch has relative low viscosity even in high concentrations and is used as thickening agent in salad dressing and mayonnaise. Compared with low viscosity starch, oxidized starch is less susceptible to retrogradation or forming opaque gel. Table 3-15 lists the nature of a variety of starch before and after modification.

Glycogen Glycogen is also called animal starch and is the major storage carbohydrate in muscle and liver tissue of animals. Because glycogen accounts for only 0.02%~0.1% of fresh animal

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tissues, it is less important for the food industry than starch. Glycogen is much branched than starch and is similar to amylopectin in structure.

Cellulose and Hemicellulose Cellulose Cellulose is the major component of plant cell walls and often associates with hemicellulose, pectin, and lignin. The mode and degree of their association significantly affect the texture of plant-derived foods. Cellulose cannot be digested by the enzymes in human digestive tract and hence is a good source of dietary fiber. Cellulose is a linear homopolysaccharde consisting of D-glucopyranose units that are connected through β-D-1,4-glycodic bonds. Native cellulose contains both crystal and amorphous zones, of which, amorphous zone is more susceptible to solvent and chemical reagent action. This difference has been used in the preparation of microcrystalline cellulose, in which the amorphous zone is hydrolyzed by acids and the acid-resistance crystal zone is remained. The molecular weight of microcrystalline cellulose ranges from 30~50kDal and its tradename is avieol. Microcrystalline cellulose is insoluble in water and is often added to lowcalorie foods as fillings and rheology control agents. The degree of polymerization (DP) of cellulose varies with the plant origin and ranges from 1000~14000. Due to the large molecular weight and the presence of crystal structures, cellulose is insoluble in water and its swelling power or the ability of absorb water is poor or negligible. Carboxymethyl cellulose (CMC) CMC is the most widely used derivative of cellulose. It is obtained by treating alkaline cellulose with chloroacetic acid. The DS of commercially important CMC ranges from 0.3~0.9 and DP from 500 to 2000. CMC has long and rigid chains and carries positive charge. CMC solution is viscous and stable due to the electrostatic impulsion. However, these properties depend on the DS and DP of the product. Low-DS (≤ 0.3) CMC is insoluble in water but soluble in alkaline solutions, while high-DS (> 0.4) are water soluble. Besides, the solubility and viscosity are also affected by medium pH. CMC with DS of 0.7~1.0 is used to increase the viscosity of foods. Their aqueous solutions exhibit the characteristics of non-Newtonian fluid and the viscosity decreases with temperature elevation. CMC solution is soluble in pH range 5~10 and is most stable in pH 7~9. Monovalent cations form soluble complex with CMC, divalent cations reduces CMC solubility, whereas trivalent cations can cause gelling or precipitation of CMC. CMC can be used to improve the solubility of such food proteins as gelatin, casein, and soybean protein by complexing with these molecules. CMC can maintain the stability of the dispersion system of proteins even in their isoelectric points. Due to the excellent rheologic properties, safety, and indigestibility, CMC is added to jellies, paste fillings, spreadable process cheeses, salad dressings, and cake fillings as binding and thickening agent. Meanwhile, because CMC has strong water binding capacity, it is

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widely used in ice creams and other foods to prevent ice crystal formation. CMC also increases the volume and elongate shelf life of cakes and other bakery foods, improves the mouthfeel of sucrose and prevent CO2 escape from low-calorie carbonated beverage. Methylcellulose (MC) and hydroxypropyl methylcellulose (HPMC) MC is the etherified derivative of cellulose and is prepared by treating alkaline CMC with chloroform. The DS of commercially important MC lies in the range 1.1~2.2. MC is characterized by its gelling properties. When a MC solution is heated, the initial viscosity drops with rising temperature and then increases sharply. This can be explained by the breakdown of hydration layer around MC molecules upon heating, which increases the hydrophobic interactions between MC molecules. Electrolytes such as NaCl and nonelectrolytes such as sucrose and sorbierite reduce the gelling temperature of MC by competing for water molecules. MC cannot be digested by human body and is a calorie-free polysaccharide. HPMC is prepared by incubating cellulose with methyl chloride and cyclopropane in alkaline solutions. The DS of commercial HPMC ranges from 0.002 to 0.3. The initial viscosity of HPMC solution decreases with temperature elevation and the formation of gel is reversible at specific temperature, which is similar to that of MC. The gelling temperature and gel strength are related to the type of substation groups, DS and concentration of the soluble gel. The hydroxypropyl group stabilizes the hydration layer of HPMC and thereby increases the gelling temperature. Changing the proportion of methyl to hydroxypropyl substituents can vary the jelling temperature within a wide range. MC and HPMC increase the water retention and absorption capabilities of foods and prevent excessive oil absorption in fried foods. MC can be added to functional foods as fillings and dehydration and shrinkage inhibitors. The two derivatives are added to salad dressings as thickening and stabilizing agents and to various foods as edible coatings and fat substitutes.

Hemicellulose Hemicelluloses occur along with cellulose in the cell walls of plant cells and consist of multiple monomers in contrast to cellulose, such as xylose, mannose, galactose, rhamnose, and arabinose in addition to glucose. The compositions of hemicelluloses vary with the plant origin and tissue. For example, wheat and rye contain mainly arabinoxylans, while β-glucans predominate in barley and oats. In the food industry, hemicellulose is mainly added to bakery foods to increase the water binding capability of flour, improve the kneading quality of dough by reducing the energy required by kneading, and increase bread volume. Breads containing hemicellulose have delayed hardening time compared with those without hemicellulose. Hemicellulose is an important source of dietary fiber.

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Pectin Pectins are polymers consisting of D-galactopyranosyluronic acids connected through the α-1,4-glycodic linkage. In addition to galacturonic acid, pectins might also contain rhamnose, galatose, arabinose, and other sugars. Pectins are present in the cells and intercellular layers of plants. Pectins originated from various sources differ mainly in their contents of methoxyl groups or degrees of esterification (DE). The DE of pectins is defined as the percentage of the number of esterified D-galacturonic acid residues among all D-galacturonic acid residues. Pectins with DE greater than 50% are termed high-methoxyl pectin (HM) and those with DE less than 50% are called low-methoxyl pectin (LM). Protopectins are the highly methyl esterified and insoluble form of pectins and are found in immature fruits and vegetables. Pectins with low DS are called pectinic acids and can be converted from protopectins by the hydrolysis of protopectinase and pectin methyl esterase. Pectinic acids are either colloidal or soluble in water depending on the degree of polymerization and degree of methyl esterification, of which, water soluble pectinic acids are also referred as low-methoxyl pectin. Pectinic acids can be completely hydrolyzed by pectin methyl esterase to pectic acids. Various pectinases participate in the post-harvesting ripening of plants. During this process, protopectinase converts protopectins to colloidal or soluble pectinic acids. Pectinesterase removes methoxyl groups from pectins to produce poly-D-galacturonic acid or pectic acid, which is then hydrolyzed by polygalacturonase to yield D-galacturonic acid units. These enzymes work together in the ripening of fruits and are important for the texture formation of fruits and vegetables. Pectin is an important polysaccharide with applications in foods, pharmaceuticals, and a number of other industries. Its importance in the food sector lies in its ability to form gel in the presence of divalent ions such as Ca2+ or a solute at low pH. In the case of HM, gels are formed only in low pH and high sugar concentrations. Generally, HM concentration 1%, pH 2.8~3.3 and sucrose concentration 58%~75% facilitate the gelation of HM. In HM, the crosslinking of pectin molecules involves a combination of hydrogen bonds and hydrophobic interactions between the molecules. Low pH suppresses the dissociation of carboxylic groups and the loss of charges minimizes the electrostatic impulsion between HM molecules. Meanwhile, sugars compete for water molecules and reduce the solvation of HM chains, facilitating the hydrogen bonding between HM molecules and consequently their gelation. HM gels can maintain their original properties even when heated at 100 °C. The strength of pectin gels is positively proportional to the molecule weights and intermolecular association of pectins. Generally, the gelation time increases as the degree of methyl esterification increases from 30%~50%, because the raise of carbomethoxy groups increases the steric hindrance for hydrogen bonding between pectin molecules. Pectins with degree of methyl esterification in the range 50%~70% have enhanced hydrophobic interactions and therefore gel in shorter time. The gelling characteristics of pectins are the function of degree of methyl esterification, as shown in Table 3-16. In the case of LM, the presence of divalent ions, such as Ca2+, is a prerequisite for their gelation and the ions function as the bridge between LM molecules. Increasing the concentration of Ca2+, which is the only divalent cation allowed in the food industry, increases the gelling temperature and gel strength of LM, which is similar to the role of Ca2+ in the formation of the egg-box structure in alginate gels. LM is not as sensitive to pH as HM and can gel in a wider pH range 2.5~6.5. Though sucrose is not a prerequisite for LM

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gelation, the addition of 10%~20% sucrose markedly improves the texture of gels. LM gels are less rigid or elastic than common pectin gels, but sugars or plasticizing agents improve the properties. LM pectin, since it does not require sugar for gelation, is used to make dietetic jams, jellies, and marmalades.

Agar Agar is a gelatinous polysaccharide extracted from some species of Rhodophyceae by a hot water extraction process.

Structure and Properties Agar is a linear heteropolysaccharide consisting of alternate β-D-galactopyranose and 3,6-anhydro-α-L-galactopyranose that are connected through 1→4 and 1→3 linkages respectively. Table 3-16. Effects of degree of methyl esterification on the gelling properties of pectin Degree of methyl esterification >70 50~70 5-methyl-tetrahydrofolate > 10-methyl-tetrahydrofolate > tetrahydrofolate. Table 6-11. Loss of folic acid in some foods during processing [11] Foods Eggs Liver Atlantic plaice Cauliflower Carrot Meat Grapefruit juice Tomato juice

Corn Flour Meat or vegetables

Processing methods Oil fried or cooking Cooking Cooking Boiling Boiling γ-radiation Canned or storage Canned Storage in dark for 1 year Exposure to light for 1 year Refining Milling Canned storage for 3 years Canned storage for 5 years

Loss of folic acid activity (%) 18~24 None 46 69 79 None Negligible 50 7 30 66 20~80 Negligible Negligible

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Figure 6-23. Structure of vitamin B12..

The loss of folic acid in foods varies with food species and processing technologies, as listed in Table 6-10.

2.12. Vitamin B12 Vitamin B12 contains cobalt and is therefore also called cobalamine. To present, at least five members of this family have been identified, but vitamins B12 sometimes refer solely to cyanoccbalamin. Liver has the highest vitamin B12 content, followed by milk, meat, egg, and fish. Vitamin B12 is not found in plants. Because only bacteria can synthesize vitamin B12, vitamin B12 in animal tissues derives either from foods or intestinal microbes. Vitamin B12 are bound to proteins in foods and are absorbed only after it is dissociated from the complex by heating or protease hydrolysis. (1) Structure. Vitamins of the B12 family are polycyclic compounds and contain one trivalent cobalt ion (Figure 6-23). According to the structure the R group, cobalamide is divided into cyanocobalamin (R = CN), hydroxocobalamin (R = OH), adenosylcobalamin (R = Ado), nitrosocobalamin (R = nitroso), methylcobalamin (R = CH3), etc. (2) Properties. The compounds of the B12 family are pink needle crystals and melt in temperatures higer than 320°C. The compounds are soluble in water, ethanol and propanol, but insoluble in chloroform. Vitamin B12 in either crystals or aqueous solutions (pH 4.5~5) are quite stable, but are readily destroyed strong acids/bases, sunlight, oxidants and reducing agents. (3) Physiological functions and deficiency.

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213

Vitamin B12 is the coenzyme of many enzymes and participates in a variety of reactions. For example, it is the carrier of methyl and is involved in the biosynthesis of methionine and thymine. Besides, vitamin B12 increases the bioavailability of folic acid. Vitamin B12 synthesized by intestinal microbes must combine with the special mucoprotein (also known as endogenous factor) secreted by gastricism for its adsorption. Lack of the endogenous factor retard the absorption of vitamin B12 and causes pernicious anemia and lesions in nervous system, tongue and gastricism. (4) Stability and degradation. Vitamin B12 in aqueous solutions is stable in pH range 4~6 in dark environments and room temperature, but it is quantitatively destroyed in alkaline solutions upon heating. Reducing agents in low levels protect vitamin B12 from destruction, but those in high concentrations have opposite effects. Trivalent iron salts can stabilize vitamin B12, but bivalent iron salts rapidly destroy the vitamin.

2.13. Lipoic Acid Lipoic acid occurs either in the reduced and oxidized forms and their structures are shown in Figure 6-24). Lipoic acid, which is necessary for the growth of microorganisms and protozoa and dissolves in water, is generally classified as a vitamin B. Lipoic acid is rich in liver and yeast, and the human body can synthesize it. Lipoic acid is a coenzyme of pyruvate dehydrogenase and α-ketoglutarate dehydrogenase, plays the role as acyl transfer. Lipoic acid is water soluble. It is essential for the growth of microorganisms and protozoa and is generally considered a member of the B family. Human can synthesize lipoic acid on their own. Lipoic acid is the coenzyme of pyruvate dehydrogenase and α-ketoglutarate dehydrogenase and participates in acyl transfer.

2.14. Vitamin C (1) Structure. Vitamin C, or ascorbic acid, is an acid derivative of hexose. Vc occurs widely in fresh fruits and vegetables. Only L-ascorbic acid is bioactive. The structure of vitamin C is shown in Figure 6-25. (2) Properties. Vc is a colorless sliced crystal and melts in 190-192°C. It is soluble in water and ethanol and its aqueous solutions taste sour. Vc is unstable against heating, sunlight exposure, and metal ions such as such as Cu2+, Fe3+. However, it is quite stable in oxalic acid or metaphosphoric acid solutions. Vc is a strong reducing agent and can be reversibly oxidized to the dehydrogenated form (Figure 6-25). Vitamin C has been widely used as an antioxidant in the food industry. L-ascorbic acid can reduce the blue dye 2, 6-dichlorophenol-indophenol to the colorless leuco and react with 2,4-dinitrophenylhydrazine to yield colored hydrazone. These reactions can be used for the qualitative and quantitative analysis of Vc.

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Figure 6-24. Structure of lipoic acid [15].

Figure 6-25. Structure of Vitamin C.

(3) Physiological functions and deficiency. Vc enhances the formation of supportive tissues and intercellular adhesion molecules. Vc functions as both hydrogen donor and receptor and plays important role in the redox reactions in human body. In addition, Vc is also involved in disease resistance and detoxification. Because human body cannot synthesis Vc, Vc deficiency causes a variety of symptoms, of which, the most notably symptom is scurvy. The symptoms in the early stage of Vc deficiency include local inflammation of skin, loss of appetite, dyspnea and general fatigue. In the later phase, the patients might suffer microvessel fracture in visceral, subcutaneous tissue, metaphyseal and gingiva and even death. (4) Stability and degradation. The stability of Vc depends largely on temperature, pH, oxygen, enzymes, metal ions such as Cu2+ and Fe3+, aw, initial concentration, and Vc to dehydrogenated Vc ratio. Although the nonenzymatic browning reaction of Vc occurs slowly in anaerobic conditions, Vc is degraded in the pathway shown in Figure 6-26 in weak acid and weak alkaline conditions. Diketo-gulonic acid can be further decomposed to produce a variety of compounds, including reducing ketones, furfural, and furan-2-carboxylic acids. In the presence of amino acids, Vc, dehydroascorbic acid, and their degradation products undergo the Maillard reaction, as shown in Figure 6-27.

R-CO-CO-COOH

Figure 6-26. Formation of diketo-gulonic acid through the degradation of Vc.

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215

Figure 6-27. Vc browning pathway [16].

Figure 6-28. Relation of Vc content in cabbage and blanching time [17].

Vc has been used as natural antioxidant in foods. For example, because Vc can reduce oquinone compounds and thus inhibit enzymatic browning, it has been used as a bread improver. In addition, Vc can protect folic acid and other oxidizable compounds from oxidation. Vc is also an effective free radical scavenger. It can remove singlet oxygen, reducing oxygen and carbon-centered radicals, and regenerate other anti-oxidants such as copherol. The changes of Vc content during food storage in an indicator of food quality change. Because Vc is water soluble and is sensitive to heat, pH, and oxygen, processing and storage cause serious Vc loss in foods. Figure 6-28 shows the effects of heating time on Vc

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content, Figure 6-29 indicates the retention of Vc in various processing methods, and Figure 6-30 illustrates the relationship between Vc destruction rate and aw.

Figure 6-29. Remained Vc content after various processes.

Figure 6-30. Relation between water activity and the rate of Vc destruction. O: Orange juice crystal; ●: △ Sucrose solution; : Corn and soybean mixture; □: Wheat flour.

Treatment of food materials with sulfur dioxide reduces the loss of Vc. Besides, sugar and sugar alcohols protect Vc from oxidative degradation, possibly due to their combination with ions.

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3. CHANGES OF VITAMIN CONTENTS IN FOODS 3.1. Stability of Vitamins The contents of various vitamins in foods are affected by many factors, such as maturity of raw materials, growth environment, soil conditions, fertilizer and water management, illumination time and intensity, as well as post-harvest or post-slaughter handling. The properties of the vitamins have been reported by many publications; however, only few efforts have been made on the changes of the vitamins in complex food systems during storage and processing. The stability of various vitamins is summarized in Table 6-12.

3.2. Effect of Maturity on Vitamin Contents According to Table 6-13, the maturity of tomato significantly affects the content of Vc.

3.3. Change of Vitamins During Postharvest Processing and Storage The nutrient compositions in foodstuff change significantly after harvest or slaughter. Many enzymes, especially endoenzymes such as oxidase and hydrolase, are released from cells.

Table 6-12 Stability of vitamins [11] Items

Light

Oxidant

Reductant

Heat

Moisture

Acid

Base

VA

+++

+++

+

++

+

++

+

VD

+++

+++

+

++

+

++

++

VE

++

++

+

++

+

+

++

VK

+++

++

+

+

+

+

+++

VC

+

+++

+

++

++

++

+++

B1

++

+

+

++

++

+

+++

B2

+++

+

++

+

+

+

+++

B5

+

+

++

+

+

+

+

B6

++

+

+

+

+

++

++

B12

++

+

+++

++

++

+++

+++

Folic acid Biotin

++

+++

+++

+

+

++

++

+

+

+

+

+

++

++

Note: +; good stability; ++, moderate stability; +++, poor stability.

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Yibin Zhou, Dongfeng Wang and Ping Dong Table 6-13. Contents of Vc in tomato as a function of maturity tomato [18]

Weeks after bloom

Color

Vitamin C content (mg%)

2

Average weight of a single fruit/g 33.4

green

10.7

3 4 5 6 7

57.2 102.5 145.7 159.9 167.6

green green-yellow red-yellow red red

7.6 10.9 20.7 14.6 10.1

These enzymes might change the chemical structure and activity of vitamins. For example, dephosphorylation of VB6, thiamine or riboflavin significantly affects the activities of the vitamins The most important factors that affect vitamin stability during storage and processing are temperature and time, because the activities of enzymes are directly relevant to temperature and reaction time. For example vitamin C oxidase specifically catalyzes the oxidation of vitamin C; the loss of beta carotenes in dehydrated food is 15% after storage for 10 days and the value reaches up to 98% after storage for 60 days. Foods undergo multiple reactions during storage and processing and these reactions significantly affect the sensory properties and vitamin contents of the foods. For example, lipid oxidation produces hydrogen peroxide and epoxide, which initiate the oxidation of carotenoid, tocopherol, and vitamin C; furthermore, hydrogen peroxide is decomposed to carbonyl compounds, which can react with thiamine, vitamin B, and pantothenic acid and lead to loss. In addition, carbonyl compounds generated in non-enzymatic browning of carbohydrates also partially contribute to the loss of some vitamins. The stability of vitamins is storage method dependent (Table 6-14).

3.4. Losses of Vitamins during Cereal Grinding Cereals are often grinded before further processing. Grinding generates heat and hence is a potential threat of vitamin stability. Figure 6-31 shows that effect of grinding time (indicated by flour yield) on the retention of various vitamins.

3.5. Losses of Vitamins During Rinsing and Blanching Rinsing and blanching causes the loss of water-soluble and heat-liable vitamins. For example, blanching at 100°C for 2 min leads to the loss of 65% of vitamin C in cabbage and vitamin C is completely lost after the 10 min of blanching. Steaming and boiling lead to different vitamin losses and water-soluble vitamins are better retained in steaming than in boiling (Table 6-15).

219

Vitamins Table 6-14. Losses of vitamins in different storage technologies [18] Methods

Frozen storage Storage after sterilization

Number of vegetable speciess 10(2) 7(3)

Loss rate (%) (1) Vitamin B2 Vitamin B5

Vitamin A

Vitamin B1

Vitamin C

12(4)

20

24

24

26

0-50 10 0-32

0-61 67 56-83

0-45 42 14-50

0-56 49 31-65

0-78 51 28-67

Note: (1) All vegetables were heated and dehydrated before storage. (2) Vegetables including asparagus, lima bean, kidney bean, brocoli, cauliflower, green pea, potato, spinage, cabbage and tender corn. (3) Vegetables treated with hot water including asparagus, lima bean, kidney bean, green pea, potato, spinage, and tender corn; (4) Average value.

Figure 6-31. Relationship between vitamins retention ratio and wheat flour yield [20].

Table 6-15. Retention of various vitamins in potato slices during steaming and boiling [21] Vitamins Vitamin C Vitamin B1 Vitamin B5 Vitamin B6 Folic acid

Boiling 60% 88% 78% 77% 66%

Steam 89% 90% 93% 97% 93%

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3.6. Losses of Vitamins During Chemical Treatment Various chemicals are often added during the storage and processing of foods and these compounds affect the stability of some vitamins. For example, decolourants in wheat flour causes oxidation and consequently loss of some sensitive vitamins; sulfur dioxide, sulphites, bisulfite, and pyrosulfites are inhibitors of both enzymatic and non-enzymatic browning reactions and protect vitamin C from oxidation. Nitrite is added as a color fixative and preservative in bacons. This compound can react with vitamin C and destroy thiamine and folic acid.

REFERENCES [1] [2]

[3] [4] [5]

[6] [7] [8] [9] [10] [11]

[12]

[13]

[14]

Chandler, LA; Schwartz, SJ. HPLC separation of cis-trans carotene isomers in fresh and processed fruits and vegetables. Food Science, 1987, 52, 669-672. Pesck, CA; Warthesen, JJ. Kinetic model for photoisomerization and concomitant photo-degradation of ß-carotenes. Journal of Agricultural and Food Chemistry, 1990, 38, 1313-1315. Dellamonica, ES; McDowell, PE. Comparison of beta-carotene content of dried carrots prepared by three dehydrated process. Food Technology, 1965, 19, 1597-1599. Van Niekerk, PJ. Determination of vitamins, in HPLC in Food Analysis. 2nd ed. San Diego: Academic Press, 1988. Thompson, JN; Hatina, G. Determination of tocopherols and tocotrienols in foods and tissues by high performance liquid chromatography. Journal of Liquid Chromatography, 1979, 2, 327-344. Herbert, V. Present Knowledge in Nutrition. 6th edition. Washington DC: International Life Science Institute – Nutrition Foundation, 1988. Machlin, LJ. Vitamin E in commercial processing on nutrients. In Machlin LM. Handbook of Vitamins. 2nd ed. New York: Marcel Dekker, 1991 Cort, WM; Borenstein, JH. Nutrient stability of fortified cereal products. Food Technololgy, 1976, 30, 52-62. Dennison, DJ; Kirk, J. Storage stability of thiamin and riboflavin in a dehydrated food system. Journal of Food Processing and Preservation, 1977, 1, 43-54. Freed, MS; Brenner, S; Wodicka, VO. Prediction of thiamin and ascorbic acid stability in canned stored foods. Food Technology, 1949, 3, 148-151. Emilia, L; Jana, K. Vitamin losses: Retention during heat treatment and continual changes expressed by mathematical models. Journal of Food Composition and Analysis, 2006, 19, 252–276. Woodcock, EA; Warthesen, JJ. Riboflavin photochemical degradation in pasta measured by high performance liquid chromatography. Food Science, 1982, 47, 545555. Weisinger, H; Hinz, HJ. Kinetic and thermodynamic parameters for Schiff base formation of vitamin B6 derivatives with amino acids. Archives of Biochemistry and Biophysics, 1984, 235, 34-40. Bonjour, JP. Biotin. In: Machlin LJ, ed. Handbook of Vitamins. 2nd edition: New York: Marcel Dekker, 1991.

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[15] Flavia. NI. Lipoic acid: a unique antioxidant in the detoxification of activated oxygen species. Plant Physiology and Biochemistry, 2002, 40, 463–470. [16] Tannenbaum, S. Ascorbic acid. In: Fennema, O. (Ed.), Principles of Food Science Part I Food Chemistry. Marcel Dekker, New York, 1976, 477–544. [17] Plank, R. Handbuch der Kältetechnik. Bd. XIV. Berlin: Springer-Verlag, 1966. [18] Maleski, W; Markakis, P. A research note. Ascorbic acid content of developing tomato fruit. Journal of Food Science, 1971, 36, 537-539. [19] Lund, D. Nutritional Evaluation of Food Processing. 3rd editon. New York: Van Nostrand Reihhold, 1988. [20] Moran, R. Biotin. Present Knowledge in Nutrition. International chemical form. Nutrition, 1959, 122, 533-545. [21] Zhao, H. The loss of vitamin in the process of cooking and food processing. Foreign medical and hygiene anthology, 2003, 30, 221.

In: Food Chemistry Editors: D.Wang, H. Lin, J. Kan et al.

ISBN: 978-1-61942-125-7 © 2012 Nova Science Publishers, Inc.

Chapter 7

MINERALS Dongfeng Wang1, Lina Yu2, Haiyan Li1, Bin Zhang1, Shuhui Wang3 and Xingguo Liang1 1

College of Food Science and Engineering, Qingdao, China 2 Shandong Peanut Research Institute, Qingdao, China 3 Biosystems Engineering Department, College of Agriculture - Ginn College of Engineering, Aubum University, Auburn, AL, US

ABSTRACT Among the 92 natural chemical elements identified to present, 81 of them have been found in human body and 25 elements are found to be essential to life, which are termed essential minerals. Essential minerals are widely involved in metabolisms in human body as cofactors or activators of enzymes and some play important role in maintaining osmotic pressure and cell membrane integrity. Besides, some minerals are important constituents of human tissues, such as calcium and phosphorus in bone and teeth. This chapter concerns the classification, distribution, property, and function of minerals and their occurrence in foods. Besides, the changes of minerals in foods during harvesting, processing, and storage are also highlighted in this chapter.

1. INTRODUCTION 1.1. Definition and Classification To present, 115 chemical elements have been discovered, including 23 artificial and 92 natural ones. Of the 115 elements, 81 have been discovered in human bodies. According to their importance to human body, elements are divided into 3 classes: Essential elements Essential elements support the normal biochemical processes of human body. Deficiency of elements of this class leads to impaired functions of human body and the functions can be

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restored in the early stage of the deficiency if the elements are supplemented in time. Essential elements have special physiologic functions and cannot be replaced by other minerals. To present, 29 elements have been found essential for the human body, including oxygen (O), carbon (C), hydrogen (H), nitrogen (N), calcium (Ca), phosphorus (P), potassium (K), sodium (Na), chlorine (Cl), sulfur (S), magnesium (Mg), ferrum (Fe), fluorine (F), zinc (Zn), cuprum (Cu), vanadium (V), stannum (Sn), selenium (Se), manganese (Mn), iodine (I), nickel (Ni), molybdenum (Mo), chromium (Cr), cobalt (Co), bromine (Br), arsenic (As), silicon (Si), boron (B), and strontium (Sr). The former 11 elements account for up to 99.95% of the total amount of the 29 elements in human body and are termed main or macro elements. The remaining 18 elements are trace elements and account for only 0.05% of the total amount. Potential beneficial elements or supplementary elements Elements of this class are beneficial to the physiological activities of human body when their contents are in normal levels. However, the elements are toxic when they are present in high levels. Rubidium (Rb), aluminum (Al), niobium (Nb), zirconium (Zr), lithium (Li), and some rare earth elements (REEs) belong to this class. Toxic elements Toxic elements are also referred to as contamination elements or toxic trace elements. These elements exert their toxicity even in very low concentrations. Bismuth (Bi), stibium (Sb), beryllium (Be), cadmium (Cd), mercury (Hg), plumbum (Pb), and thallium (Tl) are important toxic elements. The elements except C, H, O, and N are termed minerals. Based on the contents in foods, minerals are divided into main minerals, trace minerals, and ultra-trace minerals. Main minerals include Na, K, Ca, P, Mg, S, and Cl; trace minerals include Fe, F, I, Zn, Se, Cu, Mn, Cr, Mo, Co, and Ni; and ultra-trace minerals include Al, As, Ba, Bi, B, Br, Cd, Cs, Ge, Hg, Li, Pb, Rb, Sb, Si, Sn, Sm, Ti, Sr, Tl, W, and V.

1.2. Distribution of Minerals in Human Body The toxicity of main elements arranged from left to right in the same period and from top to bottom in the same group in the element periodic table increases with their atom number (Figure 7-1). Therefore, the argument that the effect of an element on the biological eukaryotic cells closely relate to the periodic law of the elements is proposed. The study on the subgroup elements shows that the change of their optimal concentration stimulated cells growth with their abundance in the seawater is established (Figure7-2). Figure 7-1 shows that all the 11 main elements have atom number less than 20 and they are mainly distributed on the top of the S and P areas in the element periodic table. The essential trace elements have the tendency to be at the position of the fourth period, especially at the d and ds areas. Most toxic elements except Be and Ba lie in the bottom of the P area. The majority of essential trace elements are in the top of the element periodic table and their atom numbers are less than 35 except I and Mo. Almost all the toxic elements are located in the lower part of the element periodic table, especially in the fifth and sixth period.

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The distribution of minerals in the environment and human body is displayed in Figure 72. Living organisms have evolved regulatory measures to maintain essential mineral contents in normal ranges. That is to say, a living organism can maintain a mineral in a constant range when the element intake is insufficient or excessive.

Figure 7-1. Distribution of macro and trace elements in the element periodic table [8].

Figure7-2. Distribution of elements in the seawater (upper), human body (middle) and crust (lower). (Data in graph are log c; the unit of c is mg/kg) [5].

Table 7-1. Functions of main minerals [4] Mineral B F Fe Zn I Cu Se Mn Mo

Functions Growth promotion and essential for plant growth Closely correlated with bone growth Component of hemoglobin, myoglobin and cytochrome Involvement in enzyme activity, nucleic acid structure and protein synthesis Component of thyroxine Cofactor of many metalloenzymes Component of glutathione peroxidase and participation in liver function and muscle metabolism Activation of enzyme and participation in hematopoiesis Component of molybdoenzymes

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Mineral Cr Mg Si P Co Ca S K Na Cl

Functions Effect of insulin intensive and glucose utilization promotion Activation of enzyme and component of bone Participation in skeletogeny Component of ATP Component of VB12 Component of bone and involvement in neural transmission Component of proteins Electrochemical and messenger function and extracellular cations Electrochemical and messenger function and extracellular cations Electrochemical and messenger function and extracellular anions

1.3. Functions of Minerals in the Living Body Table 7-1 lists the functions of main minerals for living organisms. Minerals play important roles in the human body and their functions often involve complex mechanisms. The functions of the minerals are affected by their interactions (such as coordination and antagonism) with other food constituents and the compounds in human body. For example, P and Ca mutually affect their absorption and their absorption rates are the highest when the two minerals are present in molar ratio 1:1 in foods. Fe increasingly antagonizes the absorption of Zn in Fe/Zn ratios range from 1:1 to 22:1 in diets and Zn reduces the absorption of Cu. Many researches have revealed that both deficiency and imbalance of minerals cause a number of abnormalities in living organisms. In contrast to most organic constituents, minerals cannot be synthesized in the human body and must be ingested from foods and drinking water. Besides, minerals cannot be metabolized and are repelled from the body by excreting. Therefore, dietary structure significantly influences the contents of minerals and their proportions in the body.

2. THE EXISTENCE STATES OF MINERALS IN FOODS Minerals in foods can be present in the dissolved or non-dissolved, colloidal or noncolloidal, organic or inorganic state, ionic or non-ionic state, or complexed and noncomplexed state. The existence states of minerals significantly influence their bioavailability and the safety of foods. For instance, iron in heme is absorbed much easier than other iron forms and its bioavailability is much less affected by other factors in diets, including iron absorption inhibitors. The absorption of minerals in foods is also affected by the presence of other food components. For example, vitamin C enhances the absorption of iron, but phytic acid and polyphenols inhibit iron absorption. The toxicity of toxic elements, such as Cd, Pb, and Hg, is affected by the presence other constituents in foods. For example, vitamin C decreases the toxicity of Cr6+ by reducing it to Cr3+; phytic acid, polyphenols, and proteins reduce the toxicity of Pb and Cd by complexing with them. The toxicity of heavy metals can be antagonized by other minerals. Zinc is the

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metabolic antagonist of Cd, competing with Cd for the thiol of metallothionein. Pb is more poisonous in the absence of Fe and Cr in the diet; Se can complex with Hg and reduce Hg toxicity. The valence significantly influences the toxicity of metals. As3+ is much more toxic than As5+; methyl arsenic acid and dimethyl arsenic acid are moderately toxic, while arsenobetaine and arsenocholine are almost non-toxic. Cr3+ is an essential trace element, while Cr6+ is toxic and Cr2O72- is a strong carcinogen. Hence, in evaluating the bioavailability and safety of a mineral in foods, its existence state must be taken into consideration. Generally, only very few minerals are present as free ions in foods.

2.1. Combination with Monosaccharides and Amino Acids According to the Lewis acid-base theory, metals are Lewis acids which can provide empty orbitals. Small molecules that are rich in N, S, and O atoms, such as sugars, amino acids, nucleic acids, chlorophylls, and hemoglobins, contain lone pair electrons and can act as Lewis bases. Therefore, minerals can form complexes with above-mentioned biomolecules. α-Amino acids are bidentate ligands and bond to metal ions via an oxygen of the carboxylic acid group and the nitrogen of the amino group to form a five-membered ring, as shown in Figure 7-3. Under certain conditions, the side chains of amino acids also participate in the coordination. In addition to carboxyl terminal peptide and certain groups of amino acid side chains, the carboxyl in peptide and imino groups are also involved in coordination [10]. O

O C

H N

H2 C

CH2

Zn N H

O

C O

Note: Left: Zn(Gly)22H2O, Midmost Glycine tripeptide mineral complex, M = Cu (II) or Ni (II); Right: Glycine tetrapeptide cupric complex. Figure 7-3. Structures of mineral-peptide complexes.

Both plant- and animal-derived foods contain a large number of monosaccharides, carbohydrate derivatives and amino acids. As long as the adjacent hydroxyls in sugars are arranged in favorable spatial configurations, coordination with metals occur readily. For example, the three hydroxyl groups on pyranose in the axial-transverse-axial configuration and the three hydroxyl groups on furanose in the cis-cis-cis structure favor the coordination with divalent or trivalent minerals. The presence of carboxyl and imino groups in carbohydrates increases the stability of their complexes with minerals by several orders of magnitude. Yunlan Su, et al. determined the molecular structure of the lanthanum-galactose acid complex by infrared spectroscopy and found two complexes with different structures. Iron can generate multi-core complexes with glucose, fructose and other monosaccharides. The osamines generated during the Maillard reaction, such as glycosyl amines and glucosyl amines, can react with minerals to form stable complexes. Nagy L [6] et al found that the Amadori isomer formed during the Maillard reaction between glucose and amino acids at high temperatures is a more stable mineral ligand. Structural analysis of complexes formed between Cu(II) and aminosaccharide reveals that all the aminosaccharides are the binary ligands, in which -NH2 is the main coordination group

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for Cu(II) and other divalent minerals and the -OH group plays less role in coordination with Cu(II). The –OH group on 1-C of GlcNH2GalNH2 is the main coordination group for divalent minerals, but if the -OH on 1-C is not available due to steric hindrance, such as that in aminosaccharide derivatives, other –OH groups participate in the coordination. Nagy L et al. [6] studied the structural parameters of carbohydrate-mineral complexes by EXAFS (extended X-ray absorption fine structure spectroscopic) and the results are given in Table 72. It could be seen that minerals in foods can form various complexes with carbohydrates and their derivatives. Table 7-2. Structural parameters of carbohydrate-mineral complexes Ligands Fru

Rib

GlcNH2

Adenosine

Uridine

HyA

N-D-glugly

N-D-glugly N-D-glugly

PHTAc

PHTAc

Binding Interaction Fe-O Fe…C Fe…Fe Cu-O(eq) Cu-O(ax) Cu…C Cu-O(eq) Cu-O(eq) Cu-O(ax) Cu…C Cu-O(eq) Cu-O(ax) Cu…C Cu-O(eq) Cu-O(ax) Cu…C Cu-O(eq) Cu-O(ax) Cu…C Zn-O Zn…C Cu-O(eq) Cu-N(eq) Cu-O(ax) Cu…C Cu…Cu Ni-O,N Ni…C Co-O,N Co…C Co…Co Zn-O,N Zn…C Zn…O,C,S Mn-O,N Mn…C Mn…O,C,S

N 6 4 1 4 2 2 3 1 2 2 4 2 2 4 2 2 4 2 2 4 2 2 2 2 4 1 6 6 6 6 1 4 6 6 6 6 8

γ/pm 195 277 310 191 230 271 193 193 234 274 191 232 275 192 234 279 191 234 313 202 299 190 190 215 270 297 204 285 200 290 303 205 290 286 216 305 375

σ/pm 9.8 7.9 7.0 7.5 13.0 10.0 7.7 7.7 13.0 10.0 5.4 7.8 8.4 7.9 7.9 9.6 8.2 3.2 13.0 8.1 13.2 5.6 5.6 2.5 2.8 11.4 7.6 5.4 11.9 11.7 7.6 8.0 10.0 14.0 9.5 11.0 18.0

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PHTAc

LacA Mal GlcA Sacc Mal GlcA

Binding Interaction Ag-N Ag-S Ag…C Ni-O,N Ni…C Ni…O,C,S Mn(IV)-O Mn(IV)-O Mn(IV)-O Mn(III)-O Mn(III)-O Mn(III)-O

N 1 1 4 6 6 8 6 6 6 6 6 6

γ/pm 203 230 294 203 284 390 208 208 209 209 206 208

σ/pm 5.0 4.6 19.0 8.0 9.5 15.0 2.3 3.5 1.9 3.6 1.0 3.3

Note: N, coordination number; γ, atomic distance; σ, Debye-Waller factor; Fru, D- Fructose; Rib, DRhamnose; GlcNH2, 2-amino-2-deoxy-D-Glucose; HyA, Hyaluronic Acid; N-D-glugly(N-Dgluconylglycine, a pseudopeptide derivative glucono-delta-lactone and glycine); PHTAc, [2(polyhydroxyalkyl)thiazolidine-4-carboxylic acid]; LacA, D-lactobionic acid; Mal, maltitol 4-O-ζD-glucopyranosyl-D-glucitol; GlcA, D- Gluconate; Sacc, D- Sucrose; Ara, L- or D- Arabinose.

2.2. Combination with Oxalic Acid and Phytic Acid Oxalic acid in plant-derived foods is an important mineral chelator. When oxalic acid and phytic acid are present in high levels in foods, the bioavailability of some essential minerals and the toxicity of some toxic elements will be significantly reduced. Phytic acid can form insoluble complexes with Ca, Fe, Mg, Zn and many other minerals. Besides, phytic acid is also able to form ternary complexes with proteins and minerals. The reaction not only reduces the bioavailability of proteins, but also weakens the absorption of minerals. About 10% of P in vegetables cannot be absorbed by bodies because of its combination with phytic acid. In cereals, the percentage is as high as about 40% and even up to 90% in some species, as shown in Table 7-3.

2.3. Combination with Nucleotides The biological functions of certain nucleotides are closely related to minerals. For example, in the presence of Mg, ATP can be hydrolyzed to ADP and phosphate. All the three components in nucleotides have the ability to coordinate with minerals and their coordination capacities decrease in the following order: bases > phosphate groups > pentose. Bases coordinate with minerals through N-3 of pyrimidine and N-7 of purine base. Ca2+, Mg2+, Cu2+, Mn2+, Ni2+ and Zn2+ are common coordination minerals for nucleotides. In the case of coordination with ATP, Ca2+ and Mg2+ bond only to the phosphate groups, whereas Cu2+, Mn2+, Ni2+ and Zn2+ combine with both phosphate groups and N-7 of adenine. The stability constants of ATP-divalent minerals follow the following order: Cu2+ > Zn2+ > Co2+ > Mn2+ > Mg2+ > Ca2+ > Sr2+ > Ba2+ > Ni2+. NMR and Raman spectroscopy reveal that the molar ratio of the Mg2+-ATP complex is 1:1 (Figure 7-4). 1H-NMR and 31P-NMR indicate that Cu2+ binds to the N-7 of purine or the N-3 of pyrimidine on ATP. Cu2+, Co2+, Ni2+ and Cd2+ coordinate with the phosphate and N-7 of adenine in ATP in two ways, with the resultant structures termed macrochelated inner sphere (Figure 7-5A) and

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macrochelated outer sphere (Figure 7-5B) respectively. In the former structure, N-7 of adenine directly binds to the mineral, but the α-phosphate group coordinates through H2O. In the later structure, N-7 of adenine coordinates with mineral through H2O, whereas the α, β, γphosphate groups directly link with the mineral (Figure 7-5). Table 7-3. Content of phosphorus coordinated with phytic acid in plant food Food

Oat Wheat Barley Rye Rice Corn Peanut

Phosphorus coordinated with Phytic Acid mg/100g % 208~355 50~88 170~280 47~86 70~300 32~80 247 72 157~240 68 146~353 52~97 205 57

Figure 7-4. Complex of Mg with ATP [4].

a)

Figure 7-5 (continued)

Food

Potato Kidney Bean Carrot Orange Lemon Walnut Soybean

Phosphorus coordinated with Phytic Acid mg/100g % 14 35 12 10 0~4 0~1 295 91 120 81 120 24 231~575 52~68

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b)

Figure 7-5. Two simplified structural models of (ATP) 2-Chelate inner ring (A) and external ring (B) coordination sphere (M indicates a mineral ion) [7].

2.4. Combination with Ring Ligands Minerals can complex with planar ring ligands in addition to low molecular weight carbohydrates, amino acids and peptides mentioned above. Take porphyrins as example. Porphyrins are the derivatives of porphine consisting of four pyrrole rings connected by four carbon atoms (Figure 7-6a, b). When H atoms in numbered positions of the porphine ring are replaced by other groups, porphyrins are generated (Table 7-4). a)

b)

Figure 7-6. Structure of porphine. In (a) and (b), the structure is numbered in different ways.

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Substituents 1 2 3 4 5 6 7 Protoporphyrin IX M V M V M P P Mesoporphyrin IX M E M E M P P Deuteroporphyrin IX M H M H M P M Hematoporphyrin IX M B M B M P P Spirograhic porphyrin IX M F M V M P P Corproporphyrin III M P M P M P P Aetioporphyrin III M E M E M E E Uroporphyrin III A P A P A P P Note: A=-CH2COOH, B=-CH (OH) CH3, E=-C2H5, F=-CHO, M=-CH3, H=-H, P=-CH2CH2COOH, V=CH=CH2.

Porphyrins can form complexes with Fe2+, Fe3+, Zn2+, Co2+, Cu2+, Mg2+ and other mineral ions, such as Fe in hemoglobin and Mn in chlorophyll. Heme (Figure 7-7) consists of an iron atom and a planar porphyrin ring. The iron in the center is connected with the N-atoms of four pyrrole rings through coordination bonds, and the fifth coordination point is provided by a histidine residue in globin, and the remaining sixth coordination point comes from negatively charged atoms in other ligands, such as H2O. Heme can connect with globin to form myoglobin (Figure 7-8). Three types of hemes, namely heme a, heme b and heme c, have been found. Heme a is the prosthetic group of cytochrome c oxidase; heme b is an iron-protoporphyrin IX complex (Table 7-4) and it is the prosthetic group of hemoglobin, myoglobin, cytochrome b, cytochrome p-450, catalase and peroxidase; heme c is the prosthetic group of cytochrome c.

Figure 7-7. Structure of heme.

Chlorophyll is a green pigment in the higher plants and photosynthetic organisms. It has a planar molecule and consists of four pyrrole rings connected through the methylene bridge. The Mg in center is connected with the N-atoms of four pyrrole rings through coordination bonds (Figure 7-9). Many chlorophyll species have been identified, such as chlorophyll a, b, c and d, as well as bacteria chlorophyll and chlorobium chlorophyll. Chlorophyll a and b occur widely in plant-derived foods in the ratio 3:1. Chlorophyll is transformed to pheophytin during food processing. In acidic conditions, the Mg atom in chlorophyll is replaced by

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hydrogen atoms and the resultant pheophytin is dark olive-brown. Heating accelerates the transformation. In addition to hydrogen, Mg can also be replaced by other divalent minerals, such as zinc and copper.

Figure 7-8. Structure of myoglobin.

Figure 7-9. Structure of chlorophyll.

Vitamin B12 consists of a corrin ring and has four pyrrole subunits. Because cobalt is essential for its bioactivity, Vitamin B12 is also known as cobalamin. Vitamin B12 is a red crystal and its systematic name is α-(5,6-dimethylbenzimidazolyl)cobamidcyanide. In the structure, the cobalt atom combines with four inner nitrogen atoms in the corrin ring. If the sixth coordination site is replaced by cyanide, VB12 is transformed to cyanocobalamin; if the cyanogroup connected with cobalt is replaced by the hydroxyl group, VB12 is converted to hydroxocobalamin, which is the ubiquitous form of VB12 in nature. If the cyano group is replaced by the nitroso group, nitriocobalamin is generated. Nitriocobalamin is found mainly in some bacteria. The structure of Vitamin B12 is given in Figure 7-10.

2.5. Combination with Proteins Protein consists of amino acids. In addition to peptide bonds, terminal amino groups and carboxyl groups, as well as some side groups of amino acid residues, such as the hydroxyl group in serine and threonine, the phenolic hydroxyl group in tyrosine, the carboxyl group in

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acidic amino acids, the amino group in basic amino acids, the imidazolyl group in histidine, the sulfhydryl group in cysteine, and the thioether group in methionine, can coordinate with minerals. However, only the groups in suitable sites can form complexes with minerals.

Figure 7-10. Structure of vitamin B12.

Figure 7-13. schematic diagram of Zn2+ in carboxypeptidase A [3].

Figure 7-11 is the schematic diagram of carboxypeptidase A. Zn2+ coordinates with the protein constituent through two imidazole groups and a carboxyl group in the protein. Metals are involved in the activity of a large number of enzymes and such enzymes are known as metalloenzymes. Zn, Fe, Cu, Mn, Mg, Mo, Co, K, and Ba are the most important cofactors for such enzymes. Living organisms synthesize a large variety of proteins for the storage and transport of minerals.

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Ferritin Ferritin is mainly distributed in the spleen, liver and bone marrow of animals, as well as in plants chloroplasts and certain fungi. Its main physiological function is to store unused or excessively absorbed iron. Transferrin Transferrin is mainly distributed in body fluids and cells of vertebrates. A large variety of transferrins have been identified, such as serotransferrin in serum and lactotransferrin in milk and lacrimal gland secretion. Serotransferrin is a class of mineral-binding glycoproteins with molecular weight about 7.7~7.4 × 104. After the transferrin releases all Fe3+ irons, the protein constituent can combine with other divalent or trivalent ions other than iron, such as Cu2+, Zn2+, Cr3+, Mn3+, Co3+, and Ga3+. Iron-sulphur protein Iron-sulphur proteins are characterized by the presence of the Fe-S chromophore and their molecular weight is about 10 kD. All the Fe atoms in the proteins have variable valence. The proteins participate in redox reactions in organism as the electron transfer group and are involved in biological oxidation, nitrogen fixation and photosynthesis. Cuprein Cupreins are copper-containing proteins and superoxide dismutase is a typical example of such proteins. To present, more than forty kinds of cupreins have been found. Cupreins with blue color are called blue copper proteins and those without the blue color are called nonceruloplasmins. Minerallothionein Metallothionein (MT) was firstly separated from horse kidney in 1957. Later, it was found that MT is widespread in nearly all organisms. MTs are a category of induced protein with a molecular weight about 6~10 kD. MTs are characterized by their high Cys contents, reaching up to 25~35%. The functions of MTs include anti-oxidation, free radicals scavenging, heavy metal complexing, and mineral storage. The structures of MTs are highly conserved. Many minerals, such as Cd, Pb and Zn, can induce the synthesis of MTs. One of the most important functions of MTs is to the alleviate toxicity of heavy metals. The affinity of MTs to some heavy metals follow the order Cd2+ > Pb2+ > Zn2+. Because the MT level is positively correlated with contaminant level, it is widely used as an indicator of environmental pollution. Capasso, C. et al [1] determined the three-dimensional structure of MT-nc obtained from the Antarctic fish (Notothenia coriiceps). NMR analysis showed that MT-nc is composed of the α- and β- domains. The α-domain in the C-terminal consists of eleven Cys residues and four mineral atoms, and the β-domain in the N-terminal has nine Cys residues and three mineral atoms (Figure 7-12). The ninth cystine of MT-nc in the α domain is different from that of mammals.

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The terminal amino acid sequence of mammal-derived MTs is CXCC (C denotes Cys, X denotes any amino acid), while the sequence of fish MT is CXXXCC. This difference attributes to the different structures of their α domains. Except few exceptions, the N-terminals of mammal-derived MTs are acetyl Met, and the carboxyl-terminals are Ala. Each MT molecule contains 20 Cys residues and the Cys residues are in the same positions. The Cys residues can form five Cys-X-Cys units, one Cys-Cys-XCys-Cys unit and one Cys-X-Cys-Cys unit. Although the amino acid compositions of MTs in different animal are quite similar, the MTs differ in the metal binding capacity. Generally, each MT molecule can bind seven mineral ions and the affinities of some metals to MTs follow the increasing order Zn2+ < Pb2+ < Cd2+ < Cu2+ < Ag+ < Hg+. a)

b)

a b

The main chain composite structure of 31-60 residues in α domain; The main chain composite structure of 2-28 residues in β domain.

Figure 7-12. NMR diagram of MT-nc.

Phytochelatins Phytochelatins are firstly found in plants and have similar metal binding capabilities as MTs. It is estimated that more than 90% of Cd2+ in plant cells are bound with phytochelatins. The general formula of phytochelatins is (γ-Glu-Cys)n-Gly. Phytochelatins are major heavy metal binding peptides in plants and some yeast species. The biosynthesis of phytochelatins is induced rapidly after exposure to heavy metals, especially Cd2+ and Hg2+. Heavy metals in living organism are present in both the soluble and insoluble forms, in which, soluble heavy metals bind mainly to phytochelatins. Phytochelatins from different crops have the following common characteristics: (1) the molecules have similar structure, as

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shown in Figure 7-14; (2) the Glu residue locates in the N-terminal; (3) the amino acid connected with Glu-γ-oxo is Cys; (4) the γ-Glu-Cys unit occurs repeatedly in the structure.

Figure 7-14. Structure diagram of phytochelatins.

2.6. Complexation with Polysaccharides Polysaccharides can complex with metals through thiol, amino, and carboxyl groups in addition to hydroxyls. Complexation with metals enhances the biological functions of polysaccharides and provides an effective way for removing harmful minerals. Polysaccharide chains contain a large number of groups for coordination with metals, such as pectin and alginate of the plated ribbon-type conformation (Figure 7-15). The conformation of polysaccharides significantly affects the metal-binding capability. In the case of alginate, Ca2+ facilitates the maintenance of the egg-box conformation, as shown in Figure 7-16.

Figure 7-15. Plated ribbon-type conformation of pectin and alginate.

Ca

Ca

Figure 7-16. Sketch map of egg box conformation.

Ca

Ca

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3. PHYSICOCHEMICAL PROPERTIES, TROPHISM AND SAFETY OF MINERALS IN FOOD 3.1. Physicochemical Properties 3.1.1. Solubility The transfer and metabolism of most minerals in all biological organisms occur in aqueous solutions. Therefore, the bioavailability and activity of minerals in foods, to a great extent, depend on their solubility in water. Mg, Ca and Ba are in the same group (II A) and their halogenides are readily soluble in water. However, their hydroxids, carbonates, phosphates, sulphates, oxalates and phytates, are sparingly soluble. Besides, the presence of phytic acid significantly reduces the bioavailability of some minerals, such as Fe, Zn, Ca, Mg and Mn. The solubility of minerals is influenced by pH and other constituents in foods. Generally, minerals are present in the dissolved state in pH below 6 and their complexation with proteins, amino acids, organic acids, nucleic acids, nucleotides, peptides and carbohydrates markedly improve their solubility and bioavailability. Harmful minerals can be removed by the formation of insoluble complexes. For example, citric acid is sometimes used as a chelator in lead poisoning treatment. 3.1.2. Oxidability and Reducibility Minerals might occur in multiple valences in foods and their valences are subject to changes due to the presence of other food components. The valence of a metal largely affects its significance for human body. For example, Fe2+ is beneficial for the biological process of living organisms, but Fe3+ are toxic; Cr2+ and Cr3+ are not toxic or only moderately toxic, but Cr6+ is a well-known strong carcinogen (LD50 6~8 g). Cr6+ in blood stream can be oxidized by oxygen to chromium oxide and transform hemoglobin to methemoglobin, leading to decreased oxygen level in blood. Inhale of a small amount of chromic salt or chromic acid (+6) cause great pathological changes in the kidney, liver, nervous and blood. 3.1.3. Activity The reactivity of an ion in biochemical reactions depends on its activity instead of its concentration. The definition of the activity of minerals is as follows: ai= fi·Ci where: ai denotes the activity of the ion, fi denotes the activity coefficient, and Ci denotes the ion concentration. The activity coefficient (fi) increases as the ion concentration decreases. No method has been developed to determine the accurate activity of minerals in foods. Because the ion activity is positively correlated with ion concentration, the ion activity can be presented by ion concentration. Indeed, all mineral-involved biochemical reactions are related to the concentrations, states, valence state, dietary patterns, and so on (Figure 7-18).

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239

Figure 7-18. Schematic relationship between biological activity and relative concentration of minerals.

3.1.4. Chelating Effect Most metal ions in foods are present as complexes by coordinating with organic molecules. According to Werner‘s coordination theory, coordination compounds are divided into two parts. Take [Cu(NH3)4]SO4 as an example. Cu(NH3)42+ is defined as the internal limiting ion, Cu as the central ion, NH3 as the ligand, and SO42- as the external ion. In foods, most internal limiting ions are transition elements, such as Fe, Co, Mg, and Cu. The central ions are connected to certain atoms on ligands, such as O, N and S, which are called coordination atoms. C, H, VA group elements, and VIA group elements are important coordination atoms. A chelate complex is characterized by the presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central atom. Chelate complexes are contrasted with coordination complexes composed of monodentate ligands, which form only one bond with the central atom. Many minerals exert their functions by chelating or coordination with organic ligands. For example, Fe in heme is essential for oxygen transport and Mg in chlorophyll enables photosynthesis. In addition, many metals, such as Fe2+, Mg2+, Co2+, Mo2+, Mn2+, Cu2+ and Ca2+, can coordinate with specific groups on the side chains of amino acids in metalloenzymes.

3.2. Nutrition and Toxicity 3.2.1. Nutrition Atomic absorption spectrophotometry is a conventional method for determining the total content of a mineral in foods. However, the bioavailability and nutritional value of minerals are affected by multiple factors and the information on the total content is insufficient for bioavailability and nutrition evaluation. The utilization of minerals is discussed below by taking Fe as an example. Fe in diets is present in two forms: heme-iron and non-heme iron. Heme-iron occurs primarily in animal-derived foods. The heme-iron is bivalent and can be absorbed directly and stored in intestinal ferritin for human utilization. Non-heme iron is the major form of Fe in plant-derived foods, such as cereals food, vegetables, fruits and legumes. Non-heme iron is trivalent and cannot be easily absorbed unless it is reduced to its bivalent form. Some plant-

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derived foods contain high contents of phosphates, oxalic acid and tannic acids and these compounds can precipitate Fe by complexing with it, which further reduces Fe utilization. The presence of reducing agents in plant-derived foods improves Fe utilization. Generally, the bioavailability of Fe in animal-derived foods is higher than that in plant-derived foods, as shown in Figure 7-19. The utilization of Fe is associated with its sources, existence state and diet structure. For example, phosphoric acid in milk can precipitate Fe and reduces its utilization; polyphenols in tea can complex with Fe and affect its utilization; diet with insufficient Cu decreases Fe absorption, because Cu is essential for hemoglobin synthesis. The absorption of Fe is also affected by individual or physiological factors. People suffering Fe deficiency or anemia absorb Fe easily and women generally have a better absorption than men. Besides, the possibility of Fe deficiency decreases with age increase.

3.2.2. Toxicity All minerals are toxic when they are present in high levels in foods. In addition to contents, the toxicity of minerals is also affected by the following factors.

Figure 7-19. Utilization of Fe element in different sources. 1 to 12 represents rice, spinach, beans, corn, lettuce, wheat, soybean, iron, protein, bovine, fish, hemoglobin, and beef, respectively.

Synergistic or antagonistic actions between minerals The presence of a metal might enhance or decrease the toxicity of another. For example, As increases the toxicity of Pb; Cu increases the toxicity of Hg; Cu decreases the toxicity of Hg; Mo significantly reduces Cu absorption; Co increases the toxicity of S; Se decreases the toxicity of Cd and Ni; Cd antagonizes the absorption of Zn and Cu. Valence The toxicity of an element is closely related its valence. The toxicity of elements depends on their coordination with biological macromolecules and the valence markedly influences the coordination. For example, Cr3+ is an essential trace element, while the Cr6+ is highly toxic. Chemical form The toxicity of an element is also related to its chemical forms. For example, the LD50s (mg/kg) of various arsenides are arsenite 14.0, arsenate 20.0, methyl arsenate 700~1800, 2-

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241

methyl arsenate 700~2600, arsenocholine 6500 and betaine arsenate complex >10,000. It could be seen that volatile inorganic arsenic is the most toxic. Arsenocholine and betaine arsenate are often considered non-toxic. Poisoning mechanism of heavy metals The poisoning mechanisms of heavy metals are rather complex and affected not only by their contents in the body, but also by the exposing pathway, metabolism, and health conditions of ingesters. In general, heavy metals exert their toxicity in one or more of the following mechanisms: Heavy metals disrupt the functional groups in biomolecules. For example, Hg(II) and Ag(II) can bind with –SH of Cys residues in enzymes and block the participation of –SH in enzymatic reactions. Heavy metals replace the essential metals in biomolecules. The activity of many enzymes is closely related to the metal cofactors. The activity of metalloenzymes is easily destroyed by the replacement of metal factors. For example, Be(II) can replace the Mg(II) in Mg(II)-containing enzymes. Heavy metals change the conformation of biomolecules. Heavy metals can bind to various biomolecules, such as proteins, nucleic acids and change their conformations. For example, Pb2+, Cd2+ and Ni2+ can bind to double stranded DNA (dsDNA) and this interaction leads to different modifications in the dsDNA structure.

3.2.3. Effect of Existence State The existence states of minerals tremendously influence their nutrition and toxicity. The existence state determines mineral solubility. Minerals must be in the dissolved state for their nutrition or toxicity. Minerals often occur as complexes with various organic ligands in foods and the ligands tremendously influence mineral solubility and bioavailability. For example, Ca in protein-Ca complexes is absorbed easily, but that in the Ca-oxalic acid complex is insoluble and possibly causes urinary calculus. The solubility of minerals also depends on the counter ion. For a same heavy metal, its oxide is less poisonous than its soluble chloride and nitrate. Generally, the counter ion influences the toxicity of a metal in the following order: Nitrate > Chloride > bromide > acetate > iodide > perchlorate > sulfate >> phosphate > carbonate > fluoride > hydroxide > oxides. The solubility of metal salts in aqueous solution decreases with the increase of atomic weight of the metals.

The existence states decide the physiological functions of minerals. The physiological functions of minerals are closely related with their valences. Cr3+ plays an important role in glucose, fat, and cholesterol metabolisms. In contrast, Cr6+ is highly toxicity and its in vivo transformation to Cr3+ is nearly negligible. When the body accumulates an excessive amount of Cr6+, poisoning symptoms appear.

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Hg, Pb and Cd are well-known toxic metals, but the intake of certain components may strengthen or weaken their toxicity. For example, lipopolysaccharides increase the accumulation of Hg in mice and augment the Hg-induced nephrotoxicity. Table 7-5. Minerals contents in some foods [2, 9] Food Fried eggs Wheat bread Graham bread Salt-free macaroni Cooked rice Instant rice Mature black soybean Red cashew Whole milk Skim milk /nonfat milk Ameican cheese Sayda cheese Farmhouse cheese Low fat yogurt Vanilla ice-cream Overbark baked potato Underbark boiling otato Palm cabbage, crude stem Palm cabbage, cooked stem Crude shattering carrot Cooked freezing carrot Fresh entire tomato Canned tomato juice Attains (defrosting) Attains Apple (skin) Banana (peeling) Roasted beef (cans) Roasted veal (cans) Roasted chicken breast Roasted chicken drumstick Cooked salmon Canned salmon with bone

Ca 57 35 20 5 10 10 24 25 291 302 261 305 63 415 88 20 10 216 249 15 21 6 17 17 52 10 7 5 6 13 10 6 203

Mg 13 6 26 13 42 42 61 40 33 28 10 12 6 10 9 55 26 114 130 8 7 14 20 18 13 6 32 21 28 25 20 26 25

P 269 30 74 38 81 81 120 126 228 247 316 219 139 326 67 115 54 297 318 24 19 30 35 30 18 10 22 176 234 194 156 234 277

Content / mg/100g Na K Fe 290 138 2.1 144 31 0.8 180 50 1.5 1 22 1.0 5 42 0.4 5 42 0.4 1 305 2.0 2 356 3.0 120 370 0.1 126 406 0.1 608 69 0.2 264 42 0.3 425 89 0.1 150 531 0.2 58 128 0.1 16 844 2.8 7 443 0.4 123 1470 4.0 141 1575 4.5 19 178 0.3 43 115 0.4 11 273 0.6 661 403 1.0 2 356 0.2 0 237 0.1 1 159 0.3 1 451 0.4 50 305 1.6 68 389 0.9 62 218 0.9 77 206 1.1 56 319 0.5 458 231 0.9

Zn 2.0 0.2 1.0 0.4 0.6 0.6 1.0 0.9 0.9 0.9 1.3 1.3 0.4 2.0 0.7 0.7 0.4 2.0 2.1 0.1 0.2 0.1 0.3 0.1 0.1 0.1 0.2 3.7 3.0 0.8 2.4 0.4 0.9

Cu 0.06 0.04 0.10 0.07 0.01 0.01 0.18 0.21 0.05 0.05 0.01 0.01 0.03 0.10 0.01 0.62 0.23 0.40 0.23 0.03 0.05 0.09 0.18 0.08 0.06 0.06 0.12 0.08 0.13 0.04 0.07 0.06 0.07

Se 8 8 16 19.0 13.0 13.0 7.9 1.9 3.0 7.6 3.8 7.0 7.3 5.5 4.7 1.8 1.2 0.9 1.1 0.8 0.9 0.6 0.4 0.4 1.2 0.6 1.1 — — — — — —

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4. CONTENTS OF MINERALS IN FOODS The contents of minerals in foods are governed by multiple factors, such as place of production, cultivation methods, and processing techniques. For example, the Cu content in rice is influenced by the Cu content in soil, climate, water source, use of fertilizer, insecticide, pesticide and fungicide, and processing equipment. The mineral contents of some foods are shown in Table 7-5. Besides, the utilization of minerals in foods is closely associated with the dietary structure due to the interactions with other food components. This section concerns only the impact of material, processing, and storage on mineral contents in foods.

4.1. Effect of Raw Material on Mineral Contents in Foods The contents of minerals in plant-derived foods are subject to the influences of soil, water and fertilizer management, antagonistic effect among minerals and climate. As shown in Table 7-6, the contents of some minerals in black glutinous rice of three different provinces in China differ greatly. The contents of minerals in animal-derived foods depend on feed, animal health status and environment, in which, the mineral content of feed is the most important determinant. As shown in Table 7-7, the supplementation of trace elements in feed markedly increases the contents of K, Ca, and P in milk.

4.2. Effect of Processing on Mineral Contents in Foods Processing method, water used, processing equipment, and food additives also change the contents of minerals in foods. Table 7-8 lists the effects of four processing methods on the mineral contents of instant fiddlehead. It could be seen that the four treatments slightly increase the contents of Ca, but more or less decrease the contents of other minerals, of which, salting-out dehydration plus blanching leads to the highest loss of Mg, Mn, and Zn, and salting-out dehydration without blanching causes most Fe and Cu losses. Table 7-9 illustrates the loss of some minerals in spinach after blanching. K and Na suffer the highest loss after blanching, but the Ca content is nearly not affected. Hence, minerals in free state, such as K and Na, are lost easily during bleaching, but minerals occuring in insoluble state (such as Ca) are not lost. Table 7-6. Contents of some minerals in black glutinous rice sampled from three provinces in China (mg/100g) producing area Hunan Zhejiang Guizhou

Zn 19.48 19.47 17.64

Cu 1.779 2.549 0.702

Fe 17.18 20.13 24.97

Mn 15.46 24.25 25.36

Ca 27.59 59.48 32.00

Mg 12.27 12.00 11.42

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Table 7-7. Effect of trace element supplementation in cattle feed on the minerals content of milk (mg/100g) Fe Cu Zn Mn K Na Ca Mg P 0.122 0.032 0.417 0.008 81.60 83.70 144.0 11.00 98.60 0.137 0.007 0.442 0.010 68.39 85.34 77.67 10.06 82.09

Experimental group Control group

Table 7-8. Effect of different processing methods on some trace elements content in instant fiddlehead (mg/100g dry weight) Processing methods Ca Mg Fe Mn Cu Zn Before processing 62.5 238.0 32.0 8.1 27.4 9.5 (1) 80.0 140.9 30.6 7.3 22.4 7.1 (2) 80.1 169.5 21.1 7.3 20.3 7.0 (3) 80.6 127.0 27.6 5.1 20.2 5.7 (4) 88.0 157.3 20.7 7.7 15.5 7.9 Note: (1) natural dehydration plus blanching; (2) natural dehydration without blanching; (3) salting-out dehydration plus blanching; (4) salting-out dehydration without blanching.

Table 7-9. Effect of steam blanching on the loss of minerals in spinach Mineral

g/100g

Loss (%)

Not steam blanching

Steam blanching

K

7.9

3.0

56

Na

0.5

0.3

43

Ca Mg P Nitrite

2.2 0.3 0.6 2.5

2.3 0.2 0.4 0.8

0 36 36 70

Table 7-10. Effect of processing method on the Cu content in potato (mg/100g fresh weight) Processing method

Cu

Material

0.21

Water boiling

0.10

Fluctuati on (%) 0.00 -52.38

Baking Potato chip

0.18 0.29

-14.29 +37.20

Processing method

Cu

Potato mesh

0.10

French frying potato crisp

0.27

Fluctuati on (%) -52.38 +28.57

Fast potato Peeling potato

0.17 0.34

-19.05 +61.90

The effect of processing method on Cu content in potato shown in Table 7-10. Cu content is slightly increased in the frying potato crisp and peeling potato.

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4.3. Effect of Storage on Mineral Contents in Foods Minerals in foods can migrate from packaging materials. The contents of some minerals in canned liquid and solid foods are given in Table 7-11. The contents of Al, Sn and Fe in solid foods are evidently increased because solid foods repeatedly collide with the packing material. Table 7-11. Contents (g/Kg) of trace elements in some vegetable cans Vegetable Mung bean

Can La

State Al Sn L 0.10 5 S 0.7 10 Bean La L 0.07 5 S 0.15 10 Small green pea La L 0.04 10 S 0.55 20 Parsley heart La L 0.13 10 S 1.50 20 Sweetcorn La L 0.04 10 S 0.30 20 Mushroom P L 0.01 15 S 0.04 55 Note: a. La = Lacquered can; P = T in plate can; b. L = Liquid, S = Solid.

Fe 2.8 4.8 9.8 26 10 12 4.0 3.4 1.0 7.4 5.1 16

REFERENCES [1] Capasso, C; Carginale, V. Solution structure of MT-nc, a Novel Metallothionein from the Antarctic Fish Notothenia coriiceps. Structure, 2003, 11, 435-443. [2] ESHA Research. The Food Processor Plus, ESHA Research, Salem, OR, 1992. [3] Ji, L; Huang, J; Mo, T. Bioinorganic Chemistry an Introduction (Second), Guangdong: Zhongshan University Press, 2001 [4] Kan, JQ. Food Chemistry. Beijing: China Agricultural University Press; 2008. [5] Liu, Y. An idea on trace elements as nutrients of organism. Journal of Peking University (Natural Science), 1986, (3): 121. [6] Nagy, L; Szorcsik, A. Equilibrium and structural studies on metal complexes of carbohydrates and their derivatives. Journal of Inorganic Biochemistry, 2002, 89, 1-12. [7] Sigel, H; Sigel, A. Metal Ions in Biological Systems. 1st edition. New York: Marcel Dekker, 1995. [8] Tang Renhuan. On the Biological Element Spectrum in Organism. Acta Scientiarum Naturalium Universitatis Pekinensis, 1996, 32, 790-803. [9] U. S. Department of Agriculture (1976-1986). Composition of Foods. Agriculture Handbook Nos.8-1 to 8-16. Human Nutrition Information Service, USDA, Hyattsville, MD. [10] Yang, P. Introduction of Bioinorganic Chemistry, Xi‘an: Xi‘an Jiaotong University Press, 1991.

In: Food Chemistry Editors: D.Wang, H. Lin, J. Kan et al.

ISBN: 978-1-61942-125-7 © 2012 Nova Science Publishers, Inc.

Chapter 8

FOOD FLAVORS Xiaoxiong Zeng1 and Guaoqing Huang2 1

College of Food Science and Technology, Nanjing Agriculture University, Nanjing, China 2 College of Food Science and Engineering, Qingdao Agriculture University, Qingdao, China

ABSTRACT The special tastes or aromas of foods are contributed by abundant small-molecular weight compounds and these compounds significantly affect the acceptability of foods by consumers. This chapter firstly concerns the major tastes and their perception by human beings. Then, the major flavor compounds in various foods, including fruits, vegetables, and meats are elucidated in detail. Besides, reactions that contribute to the formation of flavors during food processing and storage are also presented in detail in this chapter. Foods should not only meet the needs of consumers on nutrition, but also provide desirable flavors so that consumers can enjoy the foods. Flavor is an important aspect that determines food quality, and affects the intake and absorption of nutrients in foods. Flavor is an overall integrated perception of all contributing senses (smell, taste, sight, feeling, and sound) at the time of food consumption. The evaluation and preference to flavors differ significantly among individuals, areas, and ethnics. Flavor together with nutritional value and security determine consumers‘ acceptance of foods. The flavor of food is contributed by multiple compounds in addition to environmental factors. The development of modern analysis techniques, such as chromatography and mass spectrometry, provides great convenience for further study of food flavor. However, since food flavor is a physiological perception, neither qualitative nor quantitative methods can accurately measure or describe a food flavor yet.

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TASTE COMPOUNDS IN FOODS Taste and Taste Compounds Food Taste A taste is a feeling caused by the stimulation of food compounds on taste receptors in the mouth. Taste buds are the most important taste receptors in the mouth, followed by free nerve endings. Taste buds locate on the surface of the tongue and the back of the oral cavity. Each taste bud is the cluster of approximately 30-50 taste cells and taste receptors are distributed on the membranes of these cells. Each taste bud is connected to the oral cavity through a hole on the top. When foods are ingested, the taste compounds get contact with the corresponding receptors in the taste cells through the hole and consequently a taste is perceived. Free nerve ending acts as a micro-receiver and can identify different chemical substances. It is scattered in the entire oral cavity and surrounded by sacs. Different parts of the tongue are sensitive to different tastes. Each of the primary tastes is associated with a specific area in the tongue. The tip of the tongue is most sensitive to sweet and salty tastes, while sour seems to register more strongly on the sides of the tongue. Far to the rear of the tongue are most of the receptors for bitter tastes. The sensitivity to a taste is often described by threshold, which is the lowest concentration required for eliciting a sensation. Threshold values are usually determined using individuals representative of the general populations, and each panelist indicates whether or not the compound can be detected. The concentration range where at least half (sometimes greater) of the panelists can detect the compound is designated as the flavor threshold. Table 8-1 lists the threshold values of some taste compounds. Three kinds of threshold value are proposed based on measurement methods. Absolute threshold value, also called sensory threshold value, is the lowest level of a taste compound that can be perceived. It is determined by tasting a series of diluted taste compound solutions. Differential threshold value is the smallest change in taste compound concentration that consumers can detect. Terminal threshold value is the minimum concentration of a taste compound, whose further increase does not enhance the perception. In this chapter, all thresholds value mentioned refer to the absolute threshold value, except special instructions. Taste classification. Different taste classifications have been proposed due to the differences in culture and dietary habits. In Japan, the primary tastes include sweet, sour, bitter, salty and pungent. In European and American countries, six primary food tastes are identified, including sweet, sour, bitter, salty, pungent, and astringent. In India, up to eight tastes are recognized as primary tastes and they are sweet, sour, bitter, salty, pungent, insipid, astringent and abnormal. In China, the original standard specifies only five primary tastes, including sweet, sour, bitter, salty and pungent, but the astringent and delicious tastes are added later. From the physiological point of view, only the sweet, sour, bitter and salty tastes can be perceived by taste receptors. The pungent taste is the pain feeling when pungent substances stimulate oral mucosa, nasal mucosa, skin and trigeminal nerve. The astringent taste is the response of tactile nerves to protein aggregation. Both the tastes affect the flavor of foods independently. The presence of delicious compounds enhances the sensation of other tastes. In European and American countries, the delicious taste is excluded from the primary tastes.

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Food Flavors Factors affecting taste perception.

The perception of tastes is affected by many factors. In addition to dietary habits, health status, age and other individual factors, taste perception is also affected by the following: Temperature: the optimum temperature for taste sensation is between 10-40 °C and taste receptors are the most sensitive at 30 °C. When the temperature is lower than 10 °C or higher than 50 °C, the sensation becomes dull. It can be seen from Table 8-1 that the salty taste is the most sensitive to temperature variation and the sour taste of citric acid is the least sensitive. Solubility: the intensity of a taste is related to the solubility of the taste compound, because taste receptors or free nerve endings are stimulated only by dissolved compounds. Table 8-1. Threshold values of several taste compounds Taste compounds

Taste

Quinine sulfate Sucrose Sodium chloride Citrate

Bitter Sweet Salty Sour

Threshold (%) 25 °C 0 °C 1.0×10-4 3.0×10-4 0.1 0.4 0.05 0.25 2.5×10-3 3.0×10-3

Generally, taste compounds that dissolve quickly are perceived in a very short time and the sensation disappears very fast. For example, sucrose is readily soluble. Its sweetness is perceived rapidly after ingestion, but the sensation lasts only a short time. In contrast, the sweetness of saccharin is perceived slowly, but the sensation can last for a relative long time. Presence of other taste compounds: Interactions might occur between different taste compounds. a.

Enhancing: the presence of a taste compound could enhance the intensity of another taste. For example, table salt in concentration 0.017% enhances the sweetness of sucrose solution (15 %). b. Modification: the presence of a taste compound might change the taste of another compound. For example, water tastes sweet if table salt or quinine has been perceived before. c. Elimination: the presence of a taste compound reduces or eliminates the intensity of another taste. For example, any one of sucrose, citric acid, table salt, and quinine can reduce the taste intensity of the other three compounds. In wines or beverages, the sweetness of sugars weakens the sour taste and vice versa. d. Multiplication: when two taste compounds are present at the same time, the intensity of the tastes increases significantly. For example, the mixture of sodium glutamate and 5‘-inosinic acid has markedly improved delicious taste than alone and maltol significantly enhances the sweet taste in beverages and candies. e. Adaptation: adaptation refers to the gradual decline of taste intensity with prolonged stimulation. The time required for the recovery of taste sensation varies with tastes. The time needed for recovery of sour taste sensation after adaptation is 1.5~3 min and that for sweetness, bitterness, and salty is 1~5 min, 1.5~2.5 min and 0.3-2 min respectively.

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The interactions between tastes are very complicated and are affected by both psychological and physicochemical factors. The mechanisms involved are not well understood yet to present.

Sweet Taste Sweetness is the most popular taste for consumers. It improves the palatability and certain properties of foods. The intensity of sweetness is expressed in relative sweetness by using 10 % sucrose solution as reference. Table 8-2 lists the relative sweetness of several sweeteners. The mechanism of sweet taste can be explained by the AH/B through proposed by Robert Shallenberger and Terry Acree in 1963. It is supposed that the initial event in the perception of sweetness is the hydrogen bonding of a stimulant to receptors on the tongue. Each sweet compound possesses an electronegative atom A (usually N or O) covalently connected with a hydrogen atom (AH). Hence, the AH can be hydroxyl (-OH), imino (-NH) or amino (-NH2) group. AH functions as the proton donor. The sweet compound contains another electronegative atom B (usually N, O, S, or Cl), which is 0.25-0.4 nm away from the AH group and acts as the proton receptor. The sweetness receptor contains the AH/B unit. When the AH/B unit of a sweet compound binds to the AH/B unit in the receptor through hydrogen bonding, the taste nerve is stimulated and the sweet taste is perceived. Figure 8-1 shows the AH/B structure of chloroform, saccharin and glucose. This theory has been successfully used to explain the sweet sensation of many compounds. However, this theory has some limitations. It can not explain why the sweet intensities of sugars and D-amino acids that contain the AH/B structure differ significantly and why the optical isomers of amino acids have different tastes. Table 8-2. Relative sweetness of several sweeteners (with sucrose as 1.0)

β-D-fructose α-D-glucose α-D-galactose β-D-mannose

Relative sweetness 1.0-1.75 0.40-0.79 0.27 0.59

Xylitol

0.9-1.4

Sweetener

Chloroform

Sweetener

Relative sweetness

D-tryptophan glycyrrhizic acid saccharin Naringin dihydrochalcone

35 200-250 200-700 100

Neohesperidin dihydrochalcone

1500-2000

Saccharin

Figure 8-1. AH/B relationships of chloroform, saccharin, and glucose [1].

Glucose

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251

Sweet receptor

Figure 8-2. Schematic showing the relationship between AH/B and γ sites in the saporous sweet unit for β-D-fructopyranose [1].

To solve these problems, Kier extended the AH/B theory. He proposed that each sweet compound also contains a lipophilic region γ in addition to the AH/B structure. The lipophilic region might be methylene (-CH2-), methyl (-CH3) or phenyl (-C6H5) groups. All the three active units (AH, B and region γ) must be arranged correctly in space so that the units can contact with receptor molecules. The relationship between the three groups can be illustrated in Figure 8-2.

Bitter Taste The bitter taste is unpleasant on its own, but its combination with other tastes can provide foods with special flavor, such as in tea, coffee, beer, and balsam pear. Strychnine is the bitterest substance (threshold 0.0016 %) ever found and quinine is often used as reference in the assessment of bitter intensity of other substance. The sensation of the bitter taste is similar to that of the sweet taste. Bitter compounds posses the AH/B entity and can interact with receptors through hydrogen bonding. The difference is that the proton donor (AH) in bitter compounds are –OH, –C(OH)COCH3, – CHCOOCH3, or –NH and the proton donor (B) is –CHO, –COOH, or –COOCH3. In addition, the distance between AH and B is 0.15 nm, which is much less than that in sweet compounds. Naturally occurring bitter compounds are alkaloids, terpenoids, and glycosides in plants and bile in animals. Caffeine, theobromine, theophylline: caffeine, theobromine and theophylline are derivatives of purine and are important bitter substances in foods. Caffeine occurs in tea, coffee and cacao, and theophylline is found in cacao and tea (Figure 8-3). All the three alkaloids can stimulate the central nervous system. Naringin and neohesperidin: naringin and neohesperidin are main bitter taste compounds in citrus fruits and are mainly distributed in the peel of the fruits. Both the two compounds are flavanone glycosides and are water soluble. The bitterness of naringin is related to the l→2 glycosidic bond between rhamnose and glucose. Enzymatic hydrolysis of the linkage yields products without bitterness (Figure 8-4).

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Xiaoxiong Zeng and Guaoqing Huang O

R1

N

O

N

N

N

R3

R1=R2=R3=CH3 caffeine R1=H R2=R3=CH3 theobromine R1=R2= CH3 R3= H theophylline

R2

Figure 8-3. Structure of caffeine, theobromine and theophylline.

Enzyme

Figure 8-4. Debittering site of naringin by naringinase.

humulone

cis-isohumulone

trans-isohumulone

Figure 8-5. Thermal isomerization of humulone to isohumulone [2]. R3 COOH

R1=R2=OH R3=H chenocholic acid R1=R3=OH R2=H deoxycholic acid R1=R2=R3= OH

R1

cholic acid

R2

Figure 8-6. Structures chenocholic acid, deoxycholic acid and cholic acid.

Bitter substance in beer: the bitter taste of beer is contributed by the bitter substances originally contained in hops and those generated during brewing. The major bitter substances in beer are α-acids, such as humulone, cohumulone, adhumulone. Hops are often added as the mashed malted barely (or malt extract) is boiled in water. During this boiling process the modestly bitter α-acids undergo thermal isomerization to form extremely bitter iso-α-acids. (Figure 8-5). Bile acid: bile acid is an extremely bitter fluid secreted by the liver of most vertebrates. It is involved in the digestion of lipids in the small intestine and the adsorption of fat-soluble vitamins. The major bitter compounds in bile acid are chenocholic acid, deoxycholic acid, and cholic acid (Figure 8-6). If bile acid leaks from gallbladder during the processing of livestock, poultry and aquatic products, the bitter taste cannot be removed even after repeated washing.

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Sour Taste The sour taste is produced by hydrogen ion of organic acid, inorganic acids and acidic salts. Moderate sour intensity yields pleasant sensation and enhances appetite. Generally, the intensity of the sour taste is positively related to the concentration of hydrogen ion in solutions. When the H+ concentration is too high (pH formic acid > lactic acid > oxalate acid > hydrochloride yield. Anions decide the characteristics of the sour taste, which is why the sour taste of sour compounds differs. Acidulants are important additives in food processing. The compounds not only impart foods with desired sour taste, but also inhibit the growth of microorganisms. The most commonly used acidulant in food industry is acetic acid, followed by citric acid, lactic acid, tartaric acid, gluconic acid, malic acid, fumaric acid and phosphoric acid. Acetic acid is the major active component of vinegar. Citric acid is the most widely used acidulant in food industry. Malic acid is used to mask the lingering bitterness of synthetic sweeteners. Gluconic acid-δ-lactone can produce gluconic acid upon heating and is hence a slow-acting acidulant. It can be used as leavening agent in biscuit processing and curing agent in tofu preparation. Phosphoric acid is mainly used in the production of cola-type beverages. Salty Taste The salty taste is contributed by neutral salts and it is an indispensable and the most fundamental taste in foods. This taste is the compromise of the effects of dissociated anions and cations. Cations produce salty taste, while anions suppress the sensation of salty taste and elicit other undesirable tastes. Whether an inorganic salt is bitter or salty depends on the diameters of cations and anions. If the sum of the diameters of cation and anion of a compound is less than 0.65 nm, the salt tastes salty; otherwise, the salt tastes bitter. For example, the sum of the diameters of Mg2+ and Cl- is 0.85 nm and MgCl2 tastes quite bitter.

L-sodium glutamate L-sodium glutamate (MSG)

(MSG)

Inosine-5‘-monophate (5‘-IMP). Inosine-5'monophosphate (5′-IMP)

Sodium chloride is the only substance known to evoke a purely salty taste in any concentration that is suprathreshold. The optimum concentration of NaCl in liquid foods is 0.8-1.2 %. Because excessive intake of NaCl can cause negative impacts on health, the development of NaCl substitutes has attracted much attention. For example, a mixture of 20 % of KCl and 80 % NaCl has been developed as a low-sodium salt. Delicious Flavor The delicious flavor is a very complex sensation. The flavor enhances the appetite of consumers and makes foods tasted more delicious. When the concentration of a delicious

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compound in foods is higher than threshold value, it evidently increases the delicious taste of the food, and enhances the original flavor of the food even its concentration is lower than its threshold value. Hence, delicious compounds are also known as flavor enhancers. Delicious compounds can be divided into amino acid, nucleotide and organic acid types and their typical representatives are L-sodium glutamate (MSG), 5‘-inosine monophosphate (5‘-IMP), and sodium succinate respectively. MSG is the first discovered and commercially available flavor enhancer. It occurs widely in nature and is abundant in kelps. 5‘-IMP is widely found in chickens, fishes and broths and 5‘-IMP in animals is mainly generated by the degradation of ATP in muscles. 5‘-Guanosine monophosphate (5‘-GMP) is the major delicious component in mushrooms. Sodium succinate has been identified in poultry, livestock and mollusks and the highest content is detected in shellfishes. It is also present in low levels in fermented products, such as soy sauce, sauce, and yellow rice wine. Aspartic acid and its sodium salt are the main delicious substances in bamboo shoots, though their intensities are lower than that of MSG. The combination of IMP, GMP and MSG significantly enhance the delicious taste of MSG. For instance, the delicious intensity of the mixture containing 1 % IMP, 1 % GMP, and 98 % MSG is four times of that of MSG alone.

Pungency Certain compounds found in some spices and vegetables cause characteristic hot, sharp and stinging sensations that are known collectively as pungency. Pungent compounds enhance the appetite and increase the secretion of digestive juice of consumers and they are indispensable condiments in daily life. Based on sensation, natural pungent compounds are divided into hot, aromatic and irritative types. Hot compounds: pungent compounds of this type are odorless and can cause the burning feeling in mouth. The pungent compounds in chili, black pepper and zanthoxylum belong to this type. Chili. Capsaicinoids are the vanillyl amides of monocarboxylic acids with varying chain length (C8~C11) and capsaicin is the active component in chili pepper. The contents of capsaicin in chili peppers vary significantly with species. The contents of capsaicin in red chili, horn red chili, Sam chili, and Uganda chili are 0.06 %, 0.2 %, 0.3 %, and 0.85 %, respectively.

Capsaicin

Black pepper. Piperine is the major pungent compound in black pepper. Pepperine is an amide compound and has three isomers, of which the isomers with more cis double bonds possess

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high pungency intensity. The liability of black pepper to light and storage is due to the isomerization of piperine isomers.

Pipirine

Chinese prickly ash. The main pungent compound in Chinese prickly ash is suberosin and it also an amide compound. Aromatic compounds: pungent compounds of this type are volatile and aromatic. Ginger. Zingiberols are the main active components responsible for the pungency of fresh ginger. The carbon chain length on the lateral side of hydroxyl in the side chain on the ring is different (n=5-9). When fresh ginger is dried, zingiberols are converted to more pungent gingerols through dehydration. When ginger is heated, the side chain of zingiberols breaks to produce zingerone, which is a moderate pungent compound.

Zingiberols

Zingerols

Zingerone

Cloves and nutmeg. The main pungent components in the two species are eugenol and isoeugenol respectively.

Eugenol

Isoeugenol

Irritative compounds: in addition to tongue and oral mucosa, compounds of this type are also irritative to nose and eyes.

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Xiaoxiong Zeng and Guaoqing Huang Mustard, radish, horseradish.

The pungency and irritability of these materials are contributed by mustard oil that is generated through the hydrolysis of sinigrin. It is the collective of isothiocyanate. The following active substances have been identified in mustard, radish, and horseradish: CH2=CH CH2- NCS CH3CH=CH-NCS allyl isothiocyanate propylene isothiocyanate. CH3(CH2)3- NCS C6H5CH2-NCS butyl isothiocyanate benzyl isothiocyanate. Welsh onion, garlic, leek, and onion.

Disulfides are the major pungent and irritative substances in welsh onion, garlic, leek, and onion. The pungent components of garlic are generated by the decomposition of alliin and include diallyl disulfide and allyl propyl disulfide. The pungent components in leek and onion are also sulfur-containing compounds. These substances are converted to sweet thiol compounds when heated. This is why cooked onion and garlic taste sweet other than pungent.

Astringency Astringency is the sensation of the aggregation of proteins in oral mucosa. Tannins and polyphenols are major components in foods that cause astringency. Besides, some salts (such as alum), aldehydes, organic acids (such as oxalic acid), and quinine acid are sometimes also involved in astringency sensation. Mature fruits taste less astringent than immature fruits, because polyphenols are decomposed, oxidized, or polymerized during maturation. Tea also contains polyphenols, but their contents vary with processing methods. Red tea is less astringent than green tea, because polyphenols are oxidized during fermentation. Astringency is a characteristic taste of red wine. To obtain an acceptable astringency, measures must be taken to reduce the contents of polyphenols in wines.

FLAVOR COMPONENTS IN FOODS Aroma Compounds in Plant-Derived Foods Fruits The characteristic aromas of fruits are mainly contributed by esters, aldehydes and terpenes, followed by alcohols, ethers and volatile acids. The contents of these compounds increase gradually as fruits get mature. Table 8-3 lists the aromatic compounds in some fruits. Vegetables Generally speaking, the intensity of the aroma of vegetables is weaker than that of fruits. However, some vegetable, such as onion, garlic, leek, and onion, possess unique and strong aromas.

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Food Flavors Table 8-3. Main aroma components in some fruits Fruit Apple

Major aroma components Isoamyl acetate

Pear Banana Muskmelon Peach

Isoamyl formate Isoamyl acetate, isoamyl isovalerate Diethyl sebacate Ethyl acetate, agarolactone

Apricot Grape

Amyl butyrate Methyl o-aminobenzoate

Citrus

Butyraldehyde, octyl aldehyde, decanal, linalool Formic acid, acetaldehyde, alcohol, acetone Phenylethyl alcohol, formic acid, ethyl acetate

Peel Juice

Minor aroma components Volatile acids, ethanol, acetaldehyde, geranium alcohol Volatile acids Hexanol, hexenal Volatile acids, acetaldehyde, advanced aldehyde C4~C12 fatty acid esters, volatile acids

Fresh vegetables. Many fresh vegetables have the smell of soil. This aroma is generated by methoxy alkyl pyrazines, such as 2-methoxy-3-isopropyl pyrazine in fresh tomato and pee, 2-methoxy-3isobutyl pyrazine in green pepper, and 2-methoxy-3-sec-butyl pyrazine in red beet root. These compounds are synthesized from the precursor leucine. The biosynthesis pathway of pyrazine compounds in plants is shown in Figure 8-7. enzyme

leucine 2-methoxy-3-isobutyl pyrazine

Figure 8-7. Biosynthesis pathway of methoxy alkyl pyrazines in plant tissues [3].

Unsaturated fatty acids in vegetables can be oxidized by endogenous lipoxygenase to produce peroxides, which are further decomposed to produce aromatic aldehydes, ketones and alcohols. Vegetables of the Liliaceae family. The aroma of these vegetables is generated by sulfur compounds, such as dialkyl thioethers, dialkyl disulfides, dialkyl trisulfides, and dialkyl tetrasulfides. In addition, thio-

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propanal, thiocyanate, thiocyanate esters, mercaptan, dimethylthiophene, and thiosulfinate are also involved in aroma formation. These compounds are transformed by enzymes from their precursors when plant tissues are damaged. The aroma precursor of onion is S-(1-propenyl)-L-cysteine sulfoxide, which is converted from cysteine. In aroma formation, the precursor undergoes rearrangement with the help of allinase and generates 1-propenyl sulfenic acid and pyruvate. 1-Propenyl sulfenic acid is unstable.

acetone alkyl garlic alcohol ammonia

1-allyl-1-subsurfuracid

S-propionaldehyde-S-oxidate

Figure 8-8. Reactions involved in the formation of onion aroma [4].

Figure 8-9. Reactions involved in the formation of garlic flavor compounds [5].

Part of the compound is rearranged to lacrimatory sulfoxide thio-propanal and part is converted to mercaptan, disulfide compounds, tri-sulfur compounds and thiophene (Figure 88). All these compounds are involved in the characteristic aroma of onion. 2-Amino-3-[(S)-prop-2-enylsulfinyl]propanoic acid (alliin) is the aroma precursor of garlic and it is degraded in a similar way to that of S-(1-propenyl)-L-cysteine sulfoxide in onion (Figure 8-9). The generated 2-propene-1-sulfinothioic acid S-2-propenyl ester (allicin) has a strong irritating smell. Allicin also undergoes rearrangement to produce mercaptan, disulfide compounds and other aromatic compounds. Allicin together with diallyl disulfide and methyl allyl disulfide produce the characteristic aroma of garlic. The characteristic smell of chives flavor is contributed by dimethyl disulfide, dipropyl disulfide and propyl propenyl disulfide and that of asparagus is generated by 1,2-disulfide-3cyclopentene and 3-hydroxy-butanone. 5-Methyl-2-hexyl-3-dihydro-furanone and propylmercaptanare contribute to the characteristic smell of leek. Vegetables of the Cruciferous family. Cruciferous vegetables, such as mustard, radish, and horseradish, have strong pungent smells. These smells are caused by isothiocyanate esters, such as 2-vinyl isothiocyanate, 3-

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propenyl isothiocyanate and 2-styryl isothiocyanate. Isothiocyanate esters are generated by the enzymatic hydrolysis of glucosinolates. In addition to isothiocyanate esters, allyl isothiocyanate (R-S-C=N) and nitrides are also formed during the hydrolysis (Figure 8-10). Mushroom.

Figure 8-10. Reactions involved in the formation of Cruciferae flavors [6].

Figure 8-11. Reactions involved in the formation of lenthionine [7].

The aroma precursor of mushrooms is lenthionine acid. The precursor can be hydrolyzed by S-alkyl-L-cysteine sulfoxide lyase to produce the aromatic lenthionine (Figure 8-11). In addition, benzyl isothiocyanate, phenethyl isothiocyanate, and benzaldehyde cyanohydrin are also involved in the aroma formation of mushroom. Other vegetables. The major aromatic compounds in cucumber are carbonyl substances and alcohols and its characteristic aroma is contributed by 2-trans-6-cis-nonadienal, trans-2-nonene aldehyde and 2-trans-6-cis-nonadienol. Besides, 3-cis-hexenal, 2-trans-hexenal, and 2-trans-nonnenal also affect the aroma of cucumber. These flavor compounds are synthesized from linoleic acid and linolenic acid. More than 80 kinds of volatile compounds have been identified in tomato, in which, 3cis-hexenal, 2-trans-hexenal, β-ionone, hexanal, β-damascenone, 1-penten-3-one, 3-methyl butyraldehyde, are major active compounds for tomato aroma. In heated products such as

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ketchup, the aroma changes due to the formation of dimethyl sulfide, the increase of βionone, β-damascenone and the decrease of 3-cis-hexenal and hexanal. Potato contains only trace amounts of aromatic compounds. Pyrazines, including 2isopropyl-3-methoxy-pyrazine, 3-ethyl-2-methoxy-pyrazine and 2, 5-dimethoxy-pyrazine, are the major active components in fresh potato. Volatile compounds in cooked potato are carbonyl compounds (including saturated and unsaturated aldehydes, ketones and aromatic aldehydes), alcohols (C3~C8 alcohols, linalool, neroli and geraniol), sulfur compounds (mercaptan, sulfide and thiazole) and furan compounds. A large variety of terpenes have been identified in the volatile oil of carrot, mainly including γ-bisabolene, caryophyllene, and terpinolene. cis-γ-Bisabolene, trans-γ-bisabolene and hypoxanthine contribute to the characteristic smell of carrot.

Aromatic Components in Tea Aroma is an important factor that determines the quality of tea. The aroma type and characteristic aroma compounds of teas are associated with tea varieties, growing conditions, harvesting time, maturity and processing methods. Only tens of aroma compounds have been identified in fresh tea leaves, but up to more than 500 aromatic compounds are found in processed teas. Aroma components in green tea. Green tea does not undergo fermentation during processing and exhibits the typical roasted and fresh smell. The first step of green tea processing is water removing and enzymes are deactivated in this step. Hence, most aromatic components of green tea are original from fresh leaves and only a few are formed during processing. The main volatile components in fresh tea leaves are leaf alcohols (3-cis-hexenol, 2-cishexenol) and leaf aldehydes (3-cis-hexenal and 2-cis-hexenal), which have a strong flavor of grass. During processing, part low boiling point substances, such as leaf alcohols and leaf aldehydes, are evaporated, while part leaf alcohols and leaf aldehydes are isomerized to transleaf alcohols and trans-leaf aldehydes. These products have the faint scent and are the main body of the aroma of green tea. The smells of high boiling point aromatic compounds, such as linalool, benzyl alcohol, phenylethanol, and acetophenone, get exposed with the volatilization of low boiling point substances. Linalool is an important active component and accounts for 10 % of total aromatic components in green tea. These high boiling point compounds have pleasant smells and are important aromatic substances of green tea. The characteristic aroma of teas harvested right before or after Ching Ming Festival is contributed by disulfide ether and leaf alcohols and the aroma disappear gradually during prolonged storage. Semi-fermented tea. Oolong tea is the representative of semi-fermented tea and its major aromatic components include leaf alcohol, cis-jasmone, jasmine lactone, methyl jasmonate, nerolidol, benzyl alcohol cyanohydrin, and ethyl acetate.

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Food Flavors HO

CH2COOCH 3

CH CN

CH 2CH=CHCH 2CH3 O

OH

O

O

O

Cis-jasmone

Jasmine lactone

Methyl jasmonate

Nerolidol

Benzyl alcohol cyanohydrin

Red tea Red tea undergoes fermentation during processing and has strong aroma. During processing, tremendous reactions occur to produce hundreds of aromatic components. Hence, red tea has obviously different aroma from green tea. Alcohols, aldehydes, acids and esters are major constituents of the aroma of red tea and violet ketone plays important role in the formation of the characteristic aroma. O

O

O

+

O2

Cis-theaspirane

β-carotene

β-ionone

β-damascenone

Figure 8-12. Oxidation and decomposition of β-carotene in red tea.

Carotenoids, amino acids, and unsaturated fatty acids are the aroma precursors for red tea. During processing, β-carotene is oxidized and decomposed to ionone (Figure 8-12), which is further oxidized dihydroaclinidiolide and theaspirone. Unsaturated fatty acids in tea, especially linolenic acid and linoleic acid, undergo enzymatic oxidation during processing to produce C6-C10 aldehydes and alcohols. Meanwhile, the fatty acids are also esterified by alcohols to yield esters with different aromas, such as benzyl acetate, ethyl phenylacetate, methyl benzoate, and methyl salicylate. These compounds have important effects on the aroma of tea. Amino acids are also decomposed by enzymes to produce aldehydes, alcohols, and acids. These compounds also contribute to the aroma of red tea.

Aromatic Compounds in Animal-Derived Foods Livestock and Poultry Meat Raw pork contains more than 300 kinds of volatile compounds, of which, most are hydrocarbons, aldehydes, ketones, alcohols, esters, furan compounds, nitrogenous compounds and sulfur compounds. The aroma of different raw meats varies and depends mainly on lipid composition. Raw beef or pork has no special odor, but raw mutton and dog meat possess special smells. The goaty flavor of mutton is contributed by methyl fatty acids, such as 4methyl-capryli acid, 4-methyl-pelargonic acid and 4-methyl-capric acid, while the fishy smell of dog meat is closely related to the presence of trimethylamine and lower fatty acids. The special smell of sexually maturated male livestock is due to the secretion of gonad. For example, two compounds, namely 5α-male-16-en-3-one and 5α-male-16-ene-3α-ol (Figure 813), are responsible for the strong odor of the meat of emasculated male pig.

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enzyme

Pregnenolone

5α-male-16-en-3-one

Figure 8-13. Formation of characteristic aromatic components in boar [8].

Aromatic compounds in cooked meats are generated in three ways: lipid oxidation and hydrolysis; Maillard reaction between amino acids or proteins and reducing sugars; and further decomposition or recombination of flavor compounds. The aroma compositions of cooked meats vary with the cooking temperature and processing methods. Cooked pork contains reduced volatile compounds (mainly aldehydes, ketones, carboxylic acids and sulfur compounds). Some non-volatile compounds, including free amino acids, peptides, carbohydrates, vitamins and nucleotides, are important aroma precursors of livestock and poultry meats. When heated, the precursors undergo various chemical reactions to yield characteristic aromas. When fat-containing beef is cooked, abundant volatile compounds are produced, including fatty acids, aldehydes, ketones, alcohols, ethers, furan, pyrrole, lactones, aromatic hydrocarbons, sulfur compounds (thiazole, thiophene, alkyl sulfur, sulfide, disulfide compounds) and nitrogenous compounds (oxazole, pyrazine). To present, more than 600 volatile compounds have been identified in cooked beef, of which acidic compounds have only minor effect on the aroma. Instead, thiophenes, furan, pyrazine compounds and pyridine compounds contribute largely to the characteristic aroma of cooked beef. The composition of the characteristic aroma of cooked pork is similar to that of beef, except that the contents of γor δ-lactones converted from 4 (or 5)-hydroxyl fatty acid precursors are higher than in cooked beef. Besides, cooked pork contains more unsaturated carbonyl and furan compounds than cooked beef. Because mutton contains less free fatty acids and unsaturated fatty acids than beef or pork, cooked mutton with less carbonyl compounds. The characteristic aroma of cooked chicken meat is provided by sulfide and carbonyl compounds, in which, carbonyl compounds, such as 2-trans-4-cis-decadien-1-al, are the most important active components. The aroma of boiled meats is provided mainly by neutral compounds, such as sulfides, furan-type and benzene-type compounds, while that of roasted meats is contributed mainly by basic compounds, such as pyrazine, pyrrole, pyridine, and carbonyl compounds. However, regardless of the processing method, sulfur compounds are the most important active components for meat aroma. If sulfur compounds are removed, cooked meats will lose their aroma. The content of hydrogen sulfide significantly affects the aroma of cooked meat. When the content is too high, the odor of rotten egg is perceived; however, when the content is too low, the aroma intensity is reduced. Smoked meats have unique aroma and tastes. The smoke used contains phenols, formaldehyde, acetaldehyde, acetone, cresol, fatty acids, alcohols, pyromucic aldehydes, and guaiacol, in which, fatty acids, phenols, and alcohols play important role in the formation of the unique taste and aroma of smoked meats. Lipids play an important role in the formation of the aroma of livestock meats. When tallow is heated, it is decomposed and generates abundant compounds, including esters, hydrocarbons, alcohols, carbonyl compounds, lactones, pyrazine and furan compounds, which

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are important active components of beef aroma. The same decomposition products have also been detected in heated lard. When pork is cooked at temperatures lower than 100 °C, flavor compounds derived from fat accounts for more than 50 % of total aroma compounds.

Aquatic Products Volatile compounds in fresh aquatic products. Generally, fresh marine fishes and freshwater fishes have only light odor and the odor is mainly provided by volatile carbonyl compounds and alcohols, including aldehyde (C6, C8, and C9), ketones and alcohols (such as 1-octene-3-one, 2-trans-nonyl aldehyde, cis-1-5octadiene-3-one and 1-octene-3-ol). These compounds are generated from the oxidation of unsaturated fatty acids by lipoxygenase. As the freshness decreases, the compositions of the odor change gradually and a characteristic fishy smell is perceived. This smell is caused by δamino-valeraldehyde, δ-amino-valeric acid and hexahydropyridine compounds in fish skin mucus, which are synthesized from basic amino acids. δ-Amino-valeraldehyde and δ-aminovaleric acid have strong fishy smell. Because fish blood also contains δ-amino-valeraldehyde, fish blood also smells fishy.

Volatile compounds in rotting fish. Rotted aquatic products have disgusted odor. Ammonia, dimethylamine (DMA), trimethylamine (TMA), methyl mercaptan, indole, skatole and fatty acid oxidation products are major constituents of the odor. All these compounds are alkaline and can be neutralized by acetic acid to eliminate the unpleasant odor. In fresh fish, ammonia can also be generated in the formation of inosine monophosphate (IMP) through the hydrolysis of adenine nucleotide (AMP) by AMP deaminase (Figure 8-14). As the rot proceeds, free amino acids, urea and proteins are decomposed to produce large amount of ammonia, such as the muscle of cartilaginous fishes contains high content of urea, which can be decomposed by microbial urease to produce ammonia and carbon dioxide (Figure 8-15). Hence, the fishes have strong smell of ammonia. TMA is a main representative compound that causes the unpleasant odor of rotten fish and its threshold is as low as 300-600 μg/kg. Fresh fishes contain trimethylamine oxide (TMAO) instead of TMA. TMAO is odorless and is important in maintaining the osmotic equilibrium between blood and tissues. It is found only in marine fishes. TMAO can be reduced to TMA under the action of enzymes or microbes (Figure 8-16). TMA has been used as an indicator of the degradation of unfrozen fishes. DMA and formaldehyde are also the decomposition products of TMAO (Figure 8-16). The odor intensity of DMA is lower than that of TMA.

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Figure 8-14. Generation of ammonia in AMP hydrolysis.

Figure 8-15. Formation of ammonia from urea.

Figure 8-16. Formation of volatile amines in marine fishes [9].

Volatile sulfur compounds are always detected in degraded marine fishes, such as hydrogen sulfide, methyl mercaptan, dimethyl sulfide and diethyl sulfide. These compounds are also involved in the unpleasant odor of marine fishes. Marine fishes might exhibit the smell of oxidized fish oil or cod liver oil during storage due to the oxidation of polyunsaturated fatty acids. Linolenic acid, arachidonic acid and docosahexaenoic acid are the main unsaturated fatty acids in fish oil, their auto-oxidation decomposition products can cause unpleasant odor. The smell of the oxidation products depends on the degree of oxidation. In the early phase of oxidation, the products have the smell of cucumber. As the oxidation proceeds, the smell of cod liver oil appears.

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FLAVOR COMPOUNDS FORMATION PATHWAYS Lipoxygenase-Catalyzed Reactions Lipid Oxidation Lipoxygenase occurs widely in plants and catalyzes the oxidation of polyunsaturated fatty acids. The produced peroxides are then decomposed by lyase to yield aldehydes, ketones, alcohols and many other flavor compounds. Hexanal, synthesized by linoleic acid as precursor (Figure 8-18.), is the flavor compound of apple, strawberry, pineapple, and bananas. Lipid oxidation can also generate undesirable flavors. For example, the oxidation of linoleic acid leads to the formation of the characteristic beany flavor of soybean. 2-trans-Hexenal and 2-trans-6-cis-nonadienol are the characteristic aroma compounds of tomato and cucumber, and both the compounds are synthesized with linolenic acid as precursor (Figure 8-19). Among the flavor compounds generated in the lipoxygenase-catalyzed pathway, C6 compounds has the fragrance of grass, C9 compounds generate aroma similar to that of cucumber and watermelon, and C8 compounds possess the smell of mushroom or violet. C6 and C9 compounds are generally aldehydes and primary alcohols and C8 compounds are often ketones and secondary alcohols. The pleasant aroma of mature pear, peach, apricot and other fruits is generally contributed by the β-oxidation products of long-chain fatty acid (C8-C12).

linoleic acid

lipoxygenase

peroxidelyase

hexanal

Figure 8-18. Formation of hexanal by the oxidation of linoleic acid.

+lipoxygenase +aldehyde lyase ++--yde lyase

oxygen acid

2-trans-hexenal

2-trans-6-cis-nonadienal aldehyde

Figure 8-19. Lipoxygenase catalyzed formation of aldehydes from long-chain polyunsaturated fatty acids [10].

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Figure 8-20 β-oxidation of linoleic acid [11].

Figure 8-21. Pathways of aromatic compounds formation from leucine [11].

For example, 2-trans-4-cis-decadienoic acid ethyl ester is generated by the β-oxidation of linoleic acid (Figure 8-20) and it is the characteristic aroma compound of pear. The βoxidation of lipids also generates C8-C12 hydroxyl acids. These compounds can be cyclized to γ-lactones or δ-ketones by enzymes, of which, C8-C12 lactones have aroma similar to that of coconut and peach.

Degradation of Branched-Chain Amino Acids Branched-chain amino acids are important flavor precursors of mature fruits. The characteristic branched-chain carboxylic acid esters formed in the post-ripening process of banana, pear, kiwi fruit, and apples, such as isoamyl acetate and 3-methyl ethyl butyrate, are derived from branched-chain amino acids (Figure 8-21). Shikimic Acid Pathway Shikimic acid is the precursor of the three essential aromatic acids. In addition to its involvement in aromatic amino acids synthesis, it can also produce various volatile compounds, as shown in Figure 8-22:

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enzyme

eugenol

enzyme enzyme

p-cresol

cinnamyl alcohol lignin polymer 3-methoxy-4-hydroxybenzaldehyd(vanillin)

Figure 8-22. Volatile compounds produced in the synthesis of shikimic acid [12].

Figure 8-23. Structures of several important aromatic terpenoids.

In the synthetic pathway of shikimic acid, aromatic compounds (phenylalanine and other aromatic amino acid) can be produced from intermediate product in the pathway. Besides aromatic amino acid, the pathway can also form other volatile compounds related to essential oil. Aromatic components which are formed by fumigation of food, some of them are also formed from compounds in the shikimic acid pathway as precursors, such as vanillin. Cinnamyl alcohol is an important aroma component in cinnamon perfume. Eugenol is the main flavor and pungent ingredients in clove. Some important flavor compounds in shikimic acid pathway are showed in Figure 8-22.

Terpenoids Synthesis Terpenoids are important aromatic compounds of citrus fruits. Terpenoids containing two or more isoprene units are nonvolatile and do not directly involve in aroma sensation. Sesquiterpenes neral and ngcuka ketone are the characteristic aroma components of orange and grapefruit respectively. Citral and limonene are monoterpenes and possesses the unique smell of lemon and sour orange respectively. The enantiomers of terpenes may exhibit quite different smells. For example, l-carvone [4(R)-(-) carvone] has a strong aroma of spearmint, while d-carvone has the characteristic aroma of sweet wormwood (Figure 8-23).

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Heterolactic Fermentation Pathway Microbial fermentation can yield abundant favor compounds and these compounds significantly influence the taste and aroma of dietary products and alcoholic beverages. Figure 8-24 shows the production of various flavor compounds in the heterolactic fermentation of glucose and citric acid. The products of microbial fermentation constitute the main body of the flavor of alcoholic beverages. The flavor of beer is influenced by the presence of alcohols, esters, aldehydes, ketones and sulfides, in which, isoamyl alcohol, α-phenyl ethanol, ethyl acetate, isoamyl acetate, and ethyl benzene are the most important active components. Acetaldehyde, diacetyl and hydrogen sulfide impart beer with the flavor of green apple and their contents must be reduced to allowed ranges in post-fermentation. The flavor of Chinese white spirit is affected by alcohols, esters, carbonyl compounds, phenols, and ethers. Aldehydes (mainly aldehyde) are predominant aroma compounds in newly distilled wines and these components make the wines tasted pungent. Furfural is usually not good for the flavor of wine, but it is an important component which constitutes the sauce flavor of Maotai wine and its content reaches up to 29.4 mg/L. Esters, especially the ethyl esters and iso-amyl esters of C2-C12 fatty acid, ethyl phenylacetate, ethyl lactate, and phenethyl acetate, are determinants for the flavor of Chinese white spirits.

Non-Enzymatic Reactions Maillard Reaction The Maillard reaction can yield a large number of aromatic compounds and their contents and proportion vary with substrate type, heating duration, and temperature. When the reaction occurs in a low temperature for a short time, aromatic l actones, pyran compounds and furan compounds as well as Strecker aldehydes are produced. When the reaction proceeds in high temperatures for a long time, pyrazine, pyrrole and pyridine compounds with baking aroma are formed. Pyrazine compounds are important flavor compounds in all bakery foods and thermally processed foods. It is generally believed that pyrazine compounds are generated through the Strecker degradation between amino acids and α-dicarbonyl compounds, which are the intermediates of the Maillard reaction (Figure 8-25). Small sulfides formed in the Maillard reaction also influenced the flavor of foods. For example, methionic aldehyde is the characteristic aroma compound of boiled potatoes and cheese biscuits. Methionic aldehyde is unstable and can be easily decomposed into methane thiol and dimethyl disulfide, resulting in the increase of low molecular weight sulfides. The thermal degradation produces H2S and NH3 are also involve in the flavor formation through Maillard reaction. For example, H2S, NH3, and acetaldehyde are the thermal degradation products of cysteine. They can react with hydroxy ketones formed in the Maillard reaction to generate thiazoline, which has the flavor of cooked beef (Figure 8-26).

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Figure 8-24. Formation of volatile compounds in the heterolactic fermentation of citric acid and glucose.

Figure 8-25. Formation of an alkyl pyrazine and small sulfur compounds through the Maillard reaction [13].

cysteine

3-hydroxyl-2-butanone Fig. 9-24. Methionine reacts with carbonyl compounds to form thiazoline

2,4,5-trimethyl-3-thiazoline

Figure 8-26. Formation of a thiazoline through the reaction between the thermal degradation products of cysteine and carbonyl compounds [13].

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Thermal Degradation Reaction Thermal decomposition of carbohydrates, proteins and fats. Carbohydrates can be decomposed by heat in the absence of amines to produce a serious of flavor compounds. The thermal decomposition of monosaccharides and disaccharides produce mainly furan compounds, accompanied by a small proportion of lactones, cyclodiones and other substances. The compounds can be further decomposed to methylglyoxal, glyceraldehyde and other low molecular weight volatile compounds. Starch, cellulose and other polysaccharides are decomposed to furan compounds, furfural compounds, maltol, as well as organic acids at temperatures below 400 °C. The thermal degradation products of proteins and amino acids include hydrogen sulfide, ammonia, pyrrole compounds, pyridine compounds, thiazole, thiophene, and many other sulfur-containing compounds. Most of the products have strong smells. For the thermal degradation of lipids, please refer to related contents in Chapter 4. Degradation of vitamin. When vitamin Bl is heated, it is decomposed and produces a large number of sulfurcontaining compounds, furan and thiophene and some of the products have meat flavor. Ascorbic acid is unstable. When it is heated in the presence of oxygen, it is degraded to yield furfural, glyoxal, glycerol aldehyde and other low-molecular aldehydes, of which furfural compounds are important constituents of the aroma of cured tea, peanut and cooked beef. Fat oxidation. The non-enzymatic oxidation of lipids can cause rancidity. Nevertheless, moderate oxidation of lipids imparts foods with desired flavor, such as in breads. Please refer to Chapter 4 for the mechanism of lipid oxidation.

REFERENCES [1] Shallenberer, RS; Acree, TE. Molecular theory of sweet taste. Nature, 1967, 216, 480482. [2] DeTaeye, L; DeKeukeleire D; Siaeno E; Verzele M. Recent developments in hop chemistry. In European Brewery Convention Proceedings. Amsterdam: European Brewing Congress, 1977; 153-156. [3] Morgan, ME; Libbey, LM; Scanlan, RA. Identity of the musty-potato aroma compound in milk cultures of Pseudomonas taetrolens. Journal of Dairy Science, 1972, 55, 666 [4] Whitfield, FB; JH. Last Vegetables. In: Maarse, H. Volatile Compounds in Foods and Beverages. New York: Marcel Dekker, 1991; 203-269. [5] Shankaranarayana, ML; Raghaven, B; Abraham, KO; Natarajan, CP. Sulphur compounds in flavours. In: Morton, ID; Macleod, AJ. Food Flavours, Part A, Introduction. Amsterdam: Elsevier Scientific, 1982; 169-281.

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[6] Govindarajan, VS. Pungency: the stimuli and their evaluation. In: Boudreau, JC. Food Taste Chemistry. Washington DC: American Chemical Society, 1979; 52-97. [7] Hiraider, M; Miyazaki, Y; Shibata, Y. The smell and odorous components of dried shiitake mushroom, Lentinula edodes I: relationship between sensory evaluations and amounts of odorous components. Journal of Wood Science, 2004, 50, 358-364. [8] Gower, DB; Hancock, M Bannister, LH. Biochemical studies on the boar pheromones, 5a-androst-16-en-3-one and 5a-androst-16-en-3a-ol, and their metabolism by olfactory tissue. In: Cagan, RH; Kare, MR. Biochemistry of Taste and Olfaction. New York: Academic Press, 1981; 7-31 [9] Hebard, CE; Flick, GJ; Martin. Occurrenceand significance of trimethylamine oxide and its derivatives in fish and shellfish. In: Martin, RE; Flick, GJ; Ward, DR. Chemistry and Bilchemistry of Marine Products. Westport: AVI Publishing, 1982; 149-304. [10] Blank, I; Lin, J; Vera, FA; Weli, DH; Fay, LB. Identification of potent odorants formed by autoxidation of arachidonic acid: structure elucidation and synthesis of (EZZ)-2,4,7tridecatrienal. Journal of Agricultural and Food Chemistry, 2001, 49, 2959-2965. [11] Tressl, R; Holzer, D; Apetz, M. Biogenesis of volatiles in fruit and vegetables. In: Maarse, H; Groenen, PJ. Aroma Research: Proceedings of the International Symposium on Aroma Research, 1975 in Zeist, the Netherlands. Wageningen: Centre for Agricultural Publishing and Documentation, 1975; 41-62. [12] Wittkowski, R; Ruter, J; Drinda, H; Rafiei-Taghanaki, F. Formation of smoke flavor compounds by thermal lignin degradation. In: Teranishi, R; Takeoka, GR; Guntert, M. Flavor Precursors: Thermal and Enzymatic Conversions. Washington DC: American Chemical Society, 1992; 232-243. [13] Mussinan, CJ; Wilson, RA; Katz, I; Hruza, A; Vock, MH. Identification and some flavor properties of some 3-oxazolines and 3-oxazolines and 3-thiazolines isolated from cooked beef. In: Charalambous, G; Katz, I. Phenolic, Sulfur, and Nitrogen Compounds in Food Flavors. Washington DC: American Chemical Society, 1976; 133-145.

In: Food Chemistry Editors: D.Wang, H. Lin, J. Kan et al.

ISBN: 978-1-61942-125-7 © 2012 Nova Science Publishers, Inc.

Chapter 9

FOOD ADDITIVES 1

Linwei Liu1 and Shiyuan Dong2

Northwest AandF University, Yangling, Shaanxi, China College of Food Science and Engineering, Ocean University of China, Qingdao, China

2

ABSTRACT Food additives are substances intentionally added by manufacturers to foods to preserve flavor or enhance taste and appearance. A substantial amount of food additives with different functions have been widely used in the food industry and these compounds contribute largely to the development of the industry. Due to the increased concern on food safety and the rapid development of analysis techniques, the definition of food additive has evolved greatly. This chapter firstly describes the evolution of the definition of food additives. Then, various food additives are introduced briefly according to their classification and the functions, properties and related aspects of important additives are described one by one.

1. INTRODUCTION 1.1. Definition Food additives are regulated substances and therefore defined in law. The definition of food additives was firstly proposed by the FAO/WHO Joint Expert Committee for Food Additives (JECFA) in 1955 as ―non-nutritive substances added intentionally to food, generally in small quantities, to improve its appearance, flavor, texture, or storage properties‖. This definition covered a rather narrow range and did not include flavorings and nutrients. Since then, the definition evolves gradually and various definitions have been proposed by different countries or organizations. According to the Chinese Hygienic Standards for the Use of Food Additives (GB27602007), food additives refer to ―artificially chemosynthetic or natural substances to be added to foods in order to improve food quality and color, flavor and taste, and meet the need of

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preservation and processing technology. Nutritional fortification substances, flavoring agents and processing aids, are also included in this definition as well‖. The official definition of food additives in the European Economic Community (EEC) is ―any substance does not normally consumed as a food in itself and not normally used as a characteristic ingredient of food, whether or not it has nutritive value, the intentional addition of which to a food for a technological purpose in the manufacture, processing, preparation, treatment, packaging, transport or storage of such food results, or may be reasonably expected to result, in it or its by-products becoming directly or indirectly a component of such foods‖. Federal Food Drug and Cosmetic Act, US (FFDCA) defines food additives as ―any substance the intended use of which results or may reasonably be expected to result, directly or indirectly, in its becoming a component or otherwise affecting the characteristics of any food, including any substance intended for use in producing, manufacturing, packing, processing, preparing, treating, packaging, transporting, or holding food, if such substance is Generally Recognized As Safe (GRAS)‖. Though the definitions vary, they specify three common functions for food additives. Firstly, food additives improve food quality and meet consumers‘ requirements on flavor, color, and taste. Secondly, food additives make food processing more reasonable, more hygiene, more convenient and enhance the mechanized, automated, and scaled production of foods. Thirdly, food additives contribute to saving resources, reducing cost, and providing significantly social and economic benefits.

1.2. Classification According to origin or source, food additives are divided into natural and artificial ones. Food additives are also classified by functions in most cases. In China, food additives are subdivided into 21 categories, including acidulant, anticaking agent, antifoaming agent, antioxidant, bleaching agent, bulking agent, chewing gum base, coloring agent, color fixative, emulsifier, enzyme preparation, flavor enhancer, flour treatment agent, coating agent, water retention agent, nutrition enhancer, preservative, stabilizing and coagulating agent, sweeter, thickener and others. Among the 1500 kinds of food additives approved for use in China, more than 700 kinds are spices and essence oils.

1.3. Regulation of Food Additives To guarantee food safety, use of food additives is strictly regulated by national and international laws. According to the Chinese Hygiene Standards for Use of Food Additives (GB2760-2007), the use of food additives must comply with the following principles: 1. Food additives must be nontoxic, do not generate toxic compounds upon decomposition and do not cause chronic poisoning symptoms after long-term intake of dose allowed. 2. Food additives do not destroy the nutritional components of foods, reduce food quality, or generate toxic compounds upon decomposition. 3. Food additives can not be applied for the purpose of adulteration or for shielding the facts of food spoilage and food deterioration.

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4. Food additives cannot be applied for the purpose of concealing the quality deficiency caused by the food itself or by processing. 5. The application ranges, dosages, and residues of food additives must comply with related national standards and regulations, and try to minimize the using amount to bring about the desired result. 6. Processing aids must be removed after processing, except those with allowed residues.

2. ACIDULANTS 2.1. Functions Acidulants or acidity regulators are some acids and their salts, which serve a variety of functions shown as the following: (1) Flavoring to provide a desired taste and serve to intensify, enhance, blend of modify the overall flavor of the product. (2) Reduction of the pH to prevent or retard the growth of microorganisms and the germination of spores, and to increase the lethality of the process. (3) Maintainer or establishment of pH by serving as buffering agents. Usually a combination of free acids and salts are used. (4) Chelation of metal ions (Cu, Fe) to assist in minimizing lipid oxidation, reducing color changes and controlling texture in some fruits and vegetables. (5) Alteration of the structure of foods including gels made from gums (pectin, carrageenan), and proteins. (6) Interaction with proteins and emulsifiers to modify the structure of foods such as doughs, alter the heat stability of proteins, and to serve as an emulsifier in processed cheese. (7) Modification of sugar crystallization in hard candy manufacturing.

2.2. Acidulants The usual use of acidulants in food are acidic, phosphoric, citric, malic, succnic, tartaric, lactic, gluconic, glycolic, fumaric and adipic acid. The pKa or pKa1 of them are 4.75, 2.1, 3.08, 3.4, 4.2, 3.2, 3.86, 3.60, 3.03 and 4.43, respectively. Their structures are shown in Figure 9-1: The differences between acidulates are flavor, acidity, metal chelating and antimicrobial activity, solubility, hydroscopicity and cost. Citric, malic, tartaric and gluconic acids have similar taste which compliant with making beverage and candy. But for cola drink, using phosphoric acid is more suitable. Fumaric and acitic acid have good antimicrobial activity, and acetic acid (vinegar) is an important condiment and functional agent for Chinese food, mayonnaise and pickles. Tartaric acid is often used to make leavening (bulking agent) owing to its lower solubility.

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Figure 9-1. Sketch of structures of some acidulants added in food.

Gluconolactone can be used to make Doufu and also used as a ingredient for making the leavening because it can change to gluconic acid gradually in hot water. Lactic acid has special taste, which compliant with many dairy products and fermented food, such as yoghourt and alcohol. Phosphoric, citric, malice, succinct and tartaric acids are metal chelating agents, they can be used as antioxidant synergist.

3. SWEETENERS 3.1. Type and Functions There are two types of sweeteners: caloric (nutritive) and noncaloric (non-nutritive). The caloric sweeteners provide about 4 calories per gram. The noncaloric varieties provide zero calories. Caloric sweeteners provide sweet flavor and bulk when added to food. They also maintain freshness and contribute to product quality. Caloric sweeteners act as a preservative in jams and jellies, and a flavor enhancer in processed meat. They provide fermentation for breads and pickles, bulk to ice cream, and body to carbonated beverage. Some caloric sweeteners are rich in natural resources, e.g., sucrose and fructose. Some are mainly made by processing sugar or other small molecule compounds, e.g., molasses and sugar alcohols. Sugar alcohols are low glycemic sweetener that is safe for diabetics, and helps manage healthy glucose levels, and does not promote tooth decay. Non-caloric sweeteners are used in place of caloric sweeteners in some foods. They do not provide calories, but they do provide the sweet taste. Most non-caloric sweeteners are chemically sythenic. Some of them, e.g., saccharin, acesulfame K and sucralose are more stable than sugar to high temperature and other conditions of food processing, e.g., they do not go in for non enzyme browning. Almost all of them can use in the diet for diabetics and obesity patients.

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3.2. High Fructose Corn Syrup (Hfcs) This sweet enhancer is derived from cornstarch and is a mixture of 55% fructose and 45% sucrose. Starches are treated with enzymes that convert glucose to fructose, which results in a sweeter product. The sweeters are used in many mass-produced foods, including soft drinks, baked goods, jelly, syrups, condiments (like ketchup), fruits and desserts.

3.3. Sugar Alcohols Sugar alcohols belong to polyols family which has many functions in food, such as increasing viscosity, modifying sugar crystallization, being water binder, plasticizer, cryoprotectant, and non-cariogenic sweetener. In China and many other countries, xylitol, maltitol, sorbitol and mannitol are allowed to add into food. Xylitol is odorless and a white crystalline powder with a pleasant and sweet taste, and its energy value is 16.72 kJ/g. It has the same sweetness and bulk as sucrose with one-third fewer calories and no unpleasant aftertaste. It quickly dissolves, produces a cooling sensation in the mouth and reduces the development of cavities. Xylitol occurs naturally in many fruits and vegetables and is even produced by the human body during normal metabolism. It is produced commercially from plants such as birch and other hard wood trees and fibrous vegetation, or from xylose hydrogenation. It can be used into any food with the amount as the required of food making because its safety level is high. Maltitol (4-O-α-glucopyranosyl-D-sorbitol) has 75-90% of the sweetness of sucrose and its food energy value is 8.8 kJ/g. It is used to replace table sugar because it has fewer calories, and does not promote tooth decay and has a somewhat lesser effect on blood glucose. Maltitol is made by hydrogenation of maltose obtained from starch. It is especially used in production of sweets: sugarless hard candies, chewing gum, chocolates, baked goods, and ice cream. Its similarity to sucrose allows it to be used in syrups with the advantage that crystallization is less likely. Maltitol may also be used as a plasticiser in gelatine capsulesand as a humectant.http://en.wikipedia.org/wiki/Maltitol - cite_note-1#cite_note-1 Maltitol does not brown and caramelize after liquifying by exposing to intense heat. It is somewhat more slowly absorbed than sucrose which makes it somewhat more suitable for people with diabetes than sucrose. It can use with quantity stipulated by use standard for different food categories. Sorbitol, also known as glucitol, is a sugar alcohol that the human body metabolises slowly. It is obtained by the reduction of glucose changing the aldehyde group to an additional hydroxyl group. It also occurs naturally in many stone fruits and berries from trees of the genus sorbus. It provides dietary energy 2.6 kJ per gram. and is often used in diet foods including diet drinks and ice cream, mints, cough syrup, and sugar-free chewing gum. Sorbital sometimes is used as a sweetener and humectant in cookies. Ingesting large amounts of sorbitol can lead to abdominal pain, gas, and mild to severe diarrhea. In China, the maximum level of use to each food is 5.0 g/kg. Mannitol is also used as a sweetener for diabete patient. Since mannitol has a positive heat of solution, it is used as a sweetener in "breath-freshening" candies, the cooling effect contributing to the fresh feel. The pleasant taste and mouth feel of mannitol also make it as a popular excipient for chewable tablets. It can be used a little in food coating for flavor.

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3.4. ISO-Maltulose Iso-maltulose, also known by the trade name Palatinose, is a disaccharide that is commercially manufactured from sucrose hydrolysed via bacterial fermentation. It is a natural constituent of honey and sugar cane and has a very natural sweet taste. It is particularly suitable as a non-cariogenic sucrose replacement. Iso-maltulose is fully absorbed in the small intestine as glucose and fructose. Like sucrose, it is fully digested and provides the same caloric value of approximately 4 kcal/g. However, it is low-glycemic and low-insulinemia. Because isomaltulose is released to the blood slowly, this sweetener avoids the sudden increase of drop of blood glucose level. This leads to a more balanced and prolonged energy supply in the form of glucose. In China, iso-maltulose is allowed to use in many foods and the quantity of use is decided by the needing of food manufacture.

3.5. Noncaloric Sweeteners There are several noncaloric sweeteners, such as saccharin, aspartame, acesulfame K, and sucralose and their chemical structures are shown in Figure 9-2: (1) Saccharinhttp://en.wikipedia.org/wiki/Saccharin - cite_note-1#cite_note-1 is a colorless, crystal and artificial sweetener. The basic substance, benzoic sulfimide, has effectively no calories and is many times sweeter than sucrose (about 300-700 times as sucrose) [1], but has an unpleasant bitter or metallic aftertaste, especially at high concentrations. In countries where saccharin is allowed as a FD, it is used to sweeten products such as drinks, candies, medicines, and toothpaste. In China, the maxium usage quantity of saccharin for food is 0.15 g/kg. Many studies have been performed on saccharin, some showing a correlation between saccharin consumption and increased frequency of cancer in rats (especially bladder cancer) and others finding no such correlation. No study has ever shown a clear causal relationship between saccharin consumption and health risks in humans at normal doses, though some studies have shown a correlation between consumption and cancer incidence. (2) Aspartame is 200 times sweeter than sugar in typical concentrations. Aspartame has a caloric value of 17 kJ per gram, so the minimum quantity of aspartame which needs to produce a sweet tast is so small that its caloric contribution is negligible. Therefore, it is a popular sweetener for those trying to avoid calories from sugar. The taste of aspartame is not identical to that of sugar. Blends of aspartame with acesulfame K taste more like sugar, and to be sweeter than either substitute used alone. Like many other peptides, aspartame could be hydrolyzed into its constituent amino acids under conditions of high temperature or pH. This makes aspartame undesirable as a baking sweetener.. At room temperature, it is the most stable at pH 4.3, so that it is reasonably stable in soft-drinks. In the powdered beverages, the amine in aspartame can undergo Maillard reaction with the aldehyde groups present in certain aroma compound. The safety of aspartame has been studied thoroughly, and the scientific evidence indicates it is safe at current level of consumption as a non-nutritive sweetener. (3) Acesulfame K, also known as cesulfame potassium, is a white crystalline powder. It is 180-200 times sweeter than sucrose with a slightly bitter aftertaste, especially at high concentrations.

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Figure 9-2. Structures of some noncaloric sweeteners.

Acesulfame K is often blended with other sweeteners (usually sucralose or aspartame) which give a more sugar-like taste, and each of them masks the other aftertaste. It is stable under heat even under moderately acid or basic conditions, allowing it to be used in baking, and it has been also used in beverages, tabletop sweeteners, desserts, puddings, baked goods, soft drinks, dairy products, candies and canned foods to replace part sucrose. Research shows that acesulfame K is safe to consume if the concentration in any edible form is less than 3%. (4) Sucralose is approximately 600 times as sweet as sucrose (table sugar), twice as sweet as saccharin, and 3.3 times as sweet as aspartame. Unlike aspartame, it is stable under heat and over a broad range of pH conditions. Therefore, it can be used in baking or products that require a longer shelf life. The commercial success of sucralose-based products stem from its favorable comparison to other low-calorie sweeteners in terms of taste, stability, and safety. In China, the maxium usage quantity of sucralose for many foods is 0.25 g/kg or less. (5)There are other noncaloric sweeteners being used in different countries. Sodium cyclamate is a water soluble chemical powder. It is 30 times as sweet as sucrose and stable in most conditions of food processing. It is allowed to use in cold drink and pickle with the ML equal to 0.65 g/kg. Alitame, made from amino acids, is an artificial sweetener about 2000 times sweeter than sucrose and has no aftertaste. It is relatively stable. Alitame was approved for use in Mexico, Australia, New Zealand and China. Leaves of Stevia rebaudiana are a source of several sweet glycosides of steviol. The major glycoside, stevioside, diterpenoid glycoside are used in oriental countries as food sweetener. Its medical use is also reported as a heart tonic. Besides, it is used against obesity, hypertension, and stomach burn and to lower uric acid levels.

4. PRESERVATIVES 4.1. Definition Preservatives or antimicrobial agents play an important role in today's supply of safe and stable foods. Preservative means any substance which is capable of inhibiting, retarding or arresting the process of fermentation, retarding acidification or other deterioration of food or masking any of the evidence of putrefaction. Preservatives work by preventing the growth of

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microbes (fungi, bacteria) or by killing the microbes. Food preservatives, low toxicity, are commonly used in food processing. They should not pose significant health effects on consumers during normal consumption.

4.2. Use and Limitation To prolong the shelf-life of food against deterioration caused by microorganisms, the preservatives are indispensable in the food industry. In several decades, increasing demand for convenient foods and reasonably long shelf life of processed foods makes the use of chemical food preservatives imperative because they are more effective and cheaper than biopreservative. Traditional preservatives, such as wood smoke or vinegar, have been used for centuries. However, they are not always safe. Smoking may introduce carcinogenic materials into the food. Some of the commonly used preservatives, belong to inorganic chemicals, such as sulfites, nitrate, and salt have also been used for centuries in processed meats and wine. The use quantity of them are limited strictly to avoid their harmful affects on human health. Many bio-preservatives are produced by microorganism; a few others are made from plant. They also have some unhealthy effects on people. Since there are not any preservatives being using in food without any danger, some countries have set up their mandatory use standard of food preservatives. The using levels must be obeyed the items of the standards to minimize risk.

4.3. The Choice of Preservative The choice of antimicrobial agent has to be based on the antimicrobial spectrum of the preservative, the chemical and physical properties of both food and preservative, the conditions of storage and handling, and the assurance of a high initial quality of the food to be preserved. Many preservatives have broad bacteriostasis spectrum, but some others can only affect on some species of microorganism. Salts of acids, such as sodium acetate, calcium citrate, sodium benzoate, calcium propionate, and sodium nitrite all have antimicrobial properties. Sulfur dioxide can be derived from a various sulfur additives such as bisulfite, and produce a multitude of effects in food. Most preservatives only have antimicrobial effects at certain range of pH because their states can be changed with the change of pH. Some preservatives are unstable in high temperature, and some are dissoluble in water or oil. Therefore, the user must know those properties before using them. Table 9-1 shows properties of some common preservatives.

4.4. Preservatives in Common Use 4.4.1. Benzoic Acid Benzoic acid naturally occurs in many types of berries, plums, prunes, and some spices. As an additive, it is used as benzoic acid or as benzoate.

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Food Additives Table 9-1. Properties of some preservatives [2,3] Compound Benzoic acid (benzoate) Sorbic acid (sorbate) Parabens Dehydroacetic acid Sodium propionate Sulfites Nitrites

Effective against Yeast, mold, bacteria Yeast, mold, bacteria, Bacteria,mold, yeast Mold,yeast, bacteria Mold

Effective pH range 2.5-4.0

Bacteria,mold, yeast Bacteria,mold, yeast

2.5-5.0

Water easily, Oil slightly Water easily, Oil slightly Ethnol easily, Water slightly Ethnol easily, Water slightly Water easily, Ethnol easily, Water easily

4.0-5.5

Water easily

3.0-6.5 3.0-9.0 3-10 6-8

Solubility

Some food applications Jam,juice,sauce Meat, juice, bread Sauce, beverage, baking product Pickles, orange pulp Baking product, meat Wine, dried fruits Meat

The latter is used more often because benzoic acid is sparsely soluble in water, and sodium benzoate is more soluble. The undissociated form on benzoic acid is the most effective antimicrobial agent. The optimum pH range ranges from 2.5 to 4.0, which makes it an effective antimicrobial in highly acidic foods. Benzoic acid and sodium benzoate are widely used in fruit drinks, cider, carbonated beverages, sauce, jams and pickles with 1.0 g/kg or less as the ML for different foods.

4.4.2. Sorbic Acid and Potassium Sorbate Sorbic acid is a straight-chain, trans-trans unsaturated fatty acid, 2, 4-hexadienoic acid, and it has a low solubility in water at room temperature. The salts of sorbic acid, such as sodium, or potassium are more soluble in water. Sorbates are stable in the dry form; they are unstable in aqueous solutions because they decompose through oxidation. The rate of oxidation increases at low pH, high temp, light exposure. Sorbic acid and other sorbates are so effective against yeasts and molds that they can be used in many foods, such as wine, fruit juice, dried fruit, cottage cheese, meat, and fish products. Sorbates are most effective in products with low pH including salad dressings, tomato products, carbonated beverages. In China, its ML in most foods is 1.0 g/kg. 4.4.3. Propionic Acid, Sodium Propionate and Calcium Propionate Propionic acid and its salt are effective against molds, but no effect on yeast. These additives are mainly used in baked foods. The calcium salt is the most popular form used. In China, its ML in many foods is 2.5 g/kg [4]. 4.4.4. Sulfites Sulfur dioxide and sulfites have been used as preservatives serving both as antimicrobial substance and antioxidant. Instead of sulfur dioxide solutions, a number of sulfites, such as sodium bisulfite, sodium metabisulfite, sodium sulfite, potassium bisulfite, sodium hydrogen sulfite, sodium hyposulfite and potassium metabisulfite can be used. When dissolved in water, they all yield active SO2 (see the following equations)[3]. The most widely used of these sulfites is potassium metabisulfite.

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All of these forms of sulfur are known as free sulfur dioxide. Meanwhile, the bisulfite ion (HSO3-) can react with aldehydes, dextrins, pectic substances, proteins, ketones, and certain sugars to form addition compounds, which are known as bound sulfur dioxide. Sulfurous acid inhibits molds and bacteria to a lesser extent yeasts. For this reason, SO2 can be used to control undesirable bacteria and wild yeasts in fermentations without affecting the SO2- tolerant cultured yeasts. The antiseptic activity of SO2 highly depend on the pH. The lower the pH, the greater the antiseptic action of SO2. The amount of SO2 added to foods is self-limiting because the product may develop an unpleasant off-flavor at concentration from 200 to 500 ppm. The use of SO2 is not permitted in foods that contain significant quantities of thiamine, because this vitamin is destroyed by SO2. Because SO2 is volatile and easily lost to the atmosphere, the residual levels may be much lower than the amounts originally applied. In China, the ML of sulfites are calculated by means of the residual levels in the finished food. Different food has different maxium residual quantity. For example, the maximum residual quantities for fresh and dried fruits processed are 0.05 g/kg and 0.2 g/kg, respectively.

4.4.5. Natamycin Natamycin (Figure 9-3) is a naturally occurring antifungal agent produced by Streptomyces natalensis. Natamycin has a very low solubility in water; however, it is effective at very low levels. Natamycin has been used for decades in the food industry as a hurdle to fungal outgrowth in dairy products, meats, and other foods. It has a neutral flavor impact, and less dependence on pH for efficacy. It may be applied by spraying a liquid suspension, by dipping the product in an aqueous suspension, or by mixing it into the product in a powdered form along with cellulose on whole, shredded, or soft cheeses. 4.4.6. Nisin Nisin is an antimicrobial polypeptide, with a molecular weight of 3500, produced by some strains of Lactococcus lactis. Nisin-like substances are wide products from lactic acid bacteria.

http://upload.wikimedia.org/wikipedia/commons/a/a3/Natamycin.svg Figure 9-3. Structure of natamycin [5].

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These inhibitory substances are known as becteriocins. Nisin can be used as a processing aid against gram-positive organisms. It has been effectively used in preservation of processed cheese. It is also used in the heat treatment of nonacid foods and in extending the shelf life of sterilized milk [6].

5. ANTIOXIDANTS 5.1. Definition and Classification Food antioxidants in the broadest sense are all of the substances that have some effects on preventing or retarding oxidative deterioration that leads to rancidity, loss of flavor, color and nutritive value of foodstuffs in foods. Antioxidants can be classified into a number of groups: (1) Primary antioxidants which can terminate free radical chains reactions and function as electron donors including the phnolic antioxidants, butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tertiary butyl hydroquinone (TBHQ), propylgallate (PG), natural and synthetic tocopherols and so on. (2) Oxygen scavengers which can remove oxygen in a closed food system. Most widely used compounds are Vitamin C, and related substances, ascorbyl palmitate, and erythorbic acid (the D-isomer of ascorbic acid). (3) Chelating agents or sequestrants. They remove metallic ions, especially copper and iron, that are powerful pro-oxidants. Citric acid is widely used for this purpose. Amino acids and ethylene diamine tetraacetic acid (EDTA) are examples of chelating agents. (4) Enzymatic antioxidants which can remove head space oxygen dissolved, such as glucose oxidase. Superoxide dismutase can be used to highly remove oxidative compounds from food systems. (5) Natural antioxidants which are present in many spices and herbs. Rosemary and sage are the most potent antioxidant spices.

5.2. Mechanism of Antioxidation Primary antioxidants are mostly phenolic compounds. They can react with free radicals, which scavenge the active radicals and change themselves into lower active radicals. The lower active radicals can react each other or with other radicals until becoming stable molecules. That is why they can terminate free radical chains reaction, which is the main oxidation process of lipids. Phenolic compounds are a mixture and in combination with a chelating agent because different antioxidants have the synergistic effect. This kind antioxidant should be added into food before serious oxidation has occurred and used with small quantity. Otherwise, it may act as a pro-oxidant. For example, phenolic radicals can promote ROOH decomposition to produce ROOO. ROO

AH 2

ROO AH AH AH

ROOH ROOH A AH 2

AH A

Chelating agents which have ability to bind metal ions have contributed significantly to stabilization of food color, aroma and texture because traces of heavy metal ions can act as

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catalysts for lipid oxidation. Many natural constituents of food can act as chelating agents, e. g., carboxylic acids (oxalic, succinic), hydroxy acids (lactic, malic, tartaric, citric), polyphosphoric acids (ATP, pyrophosphates), amino acids, peptides, proteins and porphyrins. Both ascorbic acid and glucose plus glucose oxidase and catalase can deplete O2 in food system. When they react with oxygen, ascorbic acid becomes to dehydroascorbic acid, and glucose becomes to gluconic acid, and H2O2, another product of the oxidation of glucose, can be broken down to water by catalase. When the oxygen has been depleted completely, the food oxidation will stop. However, owing to VitC, oxidation is based on a complex mechanism with some active substance giving birth to the midway of the reaction, adding less quantity of VitC will prooxidant.

5.3. Common Antioxidants The common used antioxidants in food are tocopherols, ascorbic acid, ascorbyl palmitate, sodium isoascorbate, tert-butylhydroxyanisole (BHA), di-tert-butylhydroxytoluene (BHT), tertiary butyl hydroquinone (TBHQ), propylgallate (PG), tea polyphenols, antioxidant of glycyrrhiza, and so on. The structures of some of the antioxidants are shown in Figure 9-4. Tocopherols and ascorbic acid are two vitamins, and their use in food have no danger but expensive. Tocopherols and ascorbic acid, which are supplyed either natural or synthetic equivalents, are less restricted in food using. In China, they can be used in food with any quantities as the needs for food process. Ascorbyl palmitate, a derivative of ascorbic acid, has good solubility in oil, so as to be a wide use as antioxidant which contains abundant oil or fat. Sodium isoascorbate has almost the same function as ascorbic acid except it is not a vitamin and cheaper than ascorbic acid. The use standard of FDs of China permit those antioxidant to be added into many food with ML being 0.2-0.5 g/kg. BHA, BHT and PG have been used, with powerful antioxidant function, for many years in most countries. The first two are typical lipophilic artificial antioxidants and the last one has some polarity. The first two are very stable during food process, but PG is unstable at high temperature and discolour in presence of iron and metal complexes. The use of these synthetic antioxidants is very restricted. The ML of them in most food categories is 0.1 g/kg according to hygienic standards for uses of FDs in China. TBHQ is more stable and less volatile than BHA at high temperature. TBHQ gives excellent antioxidant potency to oils and fats.

Figure 9-4. Structure of some commercial antioxidants.

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It can maintain freshness and quality of crude oils during long distance transportation, and can offer protection to fried foods, thus enhancing their storage life and freshness. The highest level of TBHQ permitted in the GSFA (USA) is 1 g/kg for frozen fish, fish fillet, and fish products, and in the FDs use standard (China) is 0.2 g/kg for edible oils and fats, fried food, curing meat class, and dried fishes. It is often used in conjunction with BHA, BHT and PG to provide a synergistic antioxidant effect. Tea polyphenols, antioxidant of glycyrrhiza and antioxidant of bamboo leaves are natural antioxidants being extracted from tea, liquorice and bamboo. Tea polyphenols are water soluble. Antioxidant of glycyrrhiza is unsoluble in water but in ethanol and ethyl acetate. The antioxidant of bamboo leaves is soluble in ethanol. They are phenols substance, such as catechins, flavonoids and phenolic acids. The ML of TP, AG and ABL in the FDs use standard (China) are 0.3, 0.2 and 0.5 g/kg, respectively. Rosemary extract is another natural antioxidant. The active ingredients are ursolic acid, rosmarinic acid, conorsic acid, and conorsol. The ML of it in the FDs use standard (China) is 0.3 g/kg. EDTA is abbreviation of disodium ethylene-diamine-tetraacetate, a famous chelating agent in analysis chemistry. Phytic acid (inositol hexaphosphoric acid), orthophosphoric acid, pyrophosphoric acid, triphosphoric acid, hexameta-phosphoric acid, tartaric acid, citric acid, gluconic acid, and their salts are also good chelating agents. All of those chelating agent have certain ability of antioxidant. In the use standard of FDs of China, the ML of EDTA and phytic acid are 0.07 and 0.2 g/kg repectivesly. The orthophosphoric acid, pyrophosphoric acid, triphosphoric acid, hexameta-phosphoric acid are classified also as water retention reagents which can be use with high levels as 3-5 g/kg. The other chelating agents meationed above can be used in food without level limits.

6. EMULSIFIERS AND STABILIZERS 6.1. General Description The purpose of emulsifiers and stabilizers is mix together of different ingredients that normally would not mix, namely fat and water. This mixture of the aqueous and lipid phases is then maintained by stabilizers. For example, these additives are essential in the production of mayonnaise, chocolate products and fat spreads. The manufacture of fat spreads (reducedfat substitutes for butter and margarine), has made a significant contribution to consumer choice and dietary change, which would be impossible without the use of emulsifiers and stabilizers. Other reduced and low-fat versions of a number of products are similarly dependent on this technology. In addition to this function, the term stabilizers is also used for substances that can stabilise, retain or intensify an existing colour of a foodstuff and substances that increase the binding capacity of the food to allow the binding of food pieces into reconstituted food. Some emulsifier and a few stabilizers used in food are synthetic compounds, while many of them are natural substances. Hygeian use of these additives need to be known and controlled their negative effects on people health. Although almost each of them is generally recognized as safe (GRAS), some of them still strict the level of use. Moreover, the increasing awareness of problems with food allergy and intolerance has led to the requirement to state the source of certain emulsifiers on food labelling. For example, lecithin derived from soya is

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not suitable for an individual with an allergy to soya, so labelling of the source of the ingredient is vital to aid in consumer choice in products safe for individuals with specific dietary requirements.

6.2. Principle of Emulsifying and Stabilization When two immiscible liquids, e.g., oil and water are mixed by stirrer throughly together, there are great interface appearing between them. Because the surface tension of the interface send up the free energy of the system greatly, the system, named as disperse system, is high unstable and will separate water and oil quickly. Emulsifier, an amphiphilic substance, has usually a hydrophilic head and a hydrophobic tail. When emulsifier moleculars are adsorbed to the interface, the head and the tail will point into the water and oil respectively, which will decrease the surface tension and increase stability of the disperse system. If water, oil and emulsifier are mixed together, the disperse system will formed much easier than before. That is why emulsifier has the funtion to make and stablize emulsion and other disperse system. Stabilisers may have a little emulsifier's function or not, but they can enhance the stability of disperse system by other effects. It may increase the viscosity of the system, which decrease the thermal motion speed of the droplets so as to retard their aggregation. It may be adsorptive as the second layer on the first adsorptive layer on the surface of the droplets, which increase the steric restriction to aggregation. Addition from the fundamental function, emulsifier has other useful functions for food. Formation or deformation of foaming also need emulsifier to be an accelerant for forming or deforming the airtight film on the bubble. Another function is wetting effect, which means moisten the solid surfaces. Solid material is mixed with an emulsifier or its surface is spread with emulsifier, the surface then becomes hydrophilic. For example, chewing gum is apt to stick to teeth. We can prevent adhesion by wetting the surface of chewing gum by adding emulsifier. Emulsifier can act on starch and protein, which is important also. When emulsifier combine with the amylose helix structure, the α-starch can not change to β-starch, which prevent degradation of starch in bread and other millet cake, so that extending shelf life of bread products. When making sponge dough, emulsifier can modify gluten molecules and enhance its film-forming power, resulting in good spreadability and improvement of working efficiency. Thus, easy-rising bread can be obtained. Emulsifiers stabilise the emulsion in lowfat spreads providing the right stability and mouthfeel and reduce spattering in frying margarine also.

6.3. Kinds of Emulsifiers 6.3.1. Kinds of Emulsifiers Only food emulsifiers defined as food additives are usable to food by law. Those emulsifiers are shown in Table 9-2. Some of the emulsifiers have been used ordinarily.

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Table 9-2. Food emulsifiers Name Glycerin Fatty Acid Esters Acetic Acid Esters of Monoglycerides Lactic Acid Esters of Monoglycerides Citric Acid Esters of Monoglycerides Succinic Acid Esters of Monoglycerides Polyglycerol Esters of Fatty Acids Sorbitan Esters of Fatty Acids Polyoxyethylene sorbitan esters of fatty acids Propylene Glycol Esters of Fatty Acids Sucrose Esters of Fatty Acids Lecithin

Common Name Monoglyceride (MG) Acetylated Monoglyceride (AMG) Lactylated Monoglyceride (LMG) CMG SMG PolyGlycerol Ester (PGE) Sorbitan Ester (SOE) Tween PG Ester (PGME) Sugar Ester (SE) Lecithin (LC)

When an emulsifier will be used for a purpose, knowing its classification is important because different classes of emulsifier have different properties, function and safety. For the student to learn the basical knowledge, the concept of HLB (hydrophile-lipophile balance) should be understood. The Hydrophilic-lipophilic balance of a surfactant is a measure of the degree to decide whether it is hydrophilic or lipophilic, determining by calculating values for the different regions of the molecule, as described by Griffin in 1949 and 1954. Griffin's method for non-ionic surfactants as described in 1954 works as follows: HLB = 20Mh / M Where, Mh is the molecular mass of the hydrophilic portion of the Molecule, and M is the molecular mass of the whole molecule, giving a result on an arbitrary scale of 0 to 20. An HLB value of 0 corresponds to a completely hydrophobic molecule, and a value of 20 would correspond to a molecule made up completely of hydrophilic components. The HLB value can be used to predict the surfactant properties of a molecule: Value from 0 to 3 indicates an anti-foaming agent. Value from 4 to 6 indicates a W/O emulsifier. Value from 7 to 9 indicates a wetting agent. Value from 8 to 18 indicates an O/W emulsifier. Value from 13 to 15 is typical of detergents. Value of 10 to 18 indicates a solubiliser or hydrotrope. Some of emulsifies' HLB values are listed in Table 9-3.

6.3.2. Common Used Emulsifiers (1) Glycerine fatty acid esters. Glycerin fatty acid esters (mono[di,tri] glycerides of fatty acids) are made from glycerin and animal and plant oils/fats or their fatty acids. Those are generally produced by interesterification method. Monoglycerides have various characteristics depending on the kind and the content of its fatty acid. They are applied to many different fields being as an emulsifier, foaming agent, anti-foaming agent, anti-tack agent, starch-modifying agent and anti-bacterial agent, so you need to select the most appropriate type of monoglycerides for respective

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purposes. In China, those emulsifiers are extensively used in dairy, pasta, butter, and coffee drinks, with ML from 5 g/kg to production need. (2) Acetic acid esters of monoglycerides. Acetic acid esters of monoglyceride, also called as acetylated monoglyceride, are an emulsifier in which acetic acid is bound with monoglyceride. It has little emulsifying activity but there are many characteristics and application fields as follows: Soft acetylated monoglyceride is able to expand by more than 8 times with tension. It is an extremely stable oil of which peroxide value does not increase even when heated at 97.7 °C for 1000 hours. The combination of liquid acetylated monoglyceride and hydrogenated fats can improve the quality of fats, for example, margarine characterized with small temperature changes and wide plasticizing range, can be produced with them. It is a liquid characterized by being less oily even at low temperatures and available as a solvent, lubricant, etc. Although it has no function as a good emulsifier, it is usable for foaming fats and oils by itself or in combination with other emulsifiers because of its stable alpha-crystal structure. Practically, it is used as powdered foaming agents, solvents, plasticizers for gums and coating agents for food. Table 9-3. HLB values of some emulsifiers [7] Emulsifier Acetylated monoglycerides Sorbitan trioleate Glycerol dioleate Sorbitan tristearate Propylene glycol monostearate Glycerol Monoleate Glycerol monostearate Acetylated monoglycerides (stearate) Sorbitan monooleate Propylene glycol monolaurate Sorbitan monostearate Calcium stearoxyl-2-lactylate Glycerol monolaurate Sorbitan monopalmitate Soy lecithin Diacetylated tartaric acid esters of monoglycerides Sodium Stearoyl lactylate Sorbitan monolaurate Polyoxyethylene (20) sorbitan tristearate Polyoxyethylene (20) sorbitan trioleate Polyoxyethylene (20) sorbitan monostearate Sucrose monolaurate Polyoxyethylene (20) sorbitan monooleate Polyoxyethylene (20) sorbitan monopalmitate

HLB value 1.5 1.8 1.8 2.1 3.4 3.4 3.8 3.8 4.3 4.5 4.7 5.1 5.2 6.7 8.0 8.0 8.3 8.6 10.5 11.0 14.9 15.0 15.0 15.6

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(3) Lactic acid esters of monoglycerides. Lactic acid esters of monoglyceride are called lactylated monoglyceride in which lactic acid is bound with monoglyceride (Figure 9-4). Its foaming ability is stronger than emulsifying ability. It is used in shortening for cakes, desserts and in foaming for cream by itself or in combination with monoglyceride. Lactic acid esters of monoglyceride are a mixture of isomers even only one kind of fatty acid being its fatty acid constituents (see the Figure below this paragraph). That is why these kind emulsifiers have wide HLB from 2.0 to 15. Lactic acid esters of monoglyceride are a very safe emulsifier which can be used in appropriate level based on the need of food production according to the Chinese GB2760. (4) Citric acid esters of monoglycerides. Citric acid esters of monoglyceride are called citrated monoglyceride, in which citric acid is bound with monoglyceride and the obtainable products are mixtures containing a few monoglycerides. It is a highly hydrophilic emulsifier and a stable alpha-crystal structure used for margarine, dairy products, such as coffee whitener and cream. It also used as an emulsion stabilizer for mayonnaise and dressing by utilizing its strong acid-resistance. (5) Succinic acid esters of monoglycerides. Succinic acid ester of monoglyceride is called succinylated monoglyceride, in which succinic acid is bound with monoglyceride. It is insoluble in cold water, dispersible in hot water, and soluble in hot alcohol, fats and oils. Succinylated monoglyceride forms a complex with starch which is able to react with protein. It is used as a dough modifying agent and an emulsifier for shortening. (6) Polyglycerol esters of fatty acids. Polyglycerol esters of fatty acids are called polyglyceryl ester, an emulsifier in which fatty acid is bound by esterification with polyglycerine, and generally it is dispersible in water and soluble in oil. Its hydrophilicity and lipophilicity greatly change with the degree of its polymerization and the type of fatty acid. Its HLB ranges from 3 to 13. It has a variety of functions depending on these conditions, and is usable for various purposes. It is used in many types of food as an O/W and W/O emulsifier for milk products containing acid and salt and a modifier to control the crystallization of fats. In China, tripolyglyceryl monostearate is used in ice-cream, bread and pastry with ML being 3.0, 0.1 and 0.1 g/kg. (7) Sorbitan esters of fatty acids. Sorbitan esters of fatty acid are called sorbitan ester, which is produced by esterification of sorbitol and fatty acid. It is a mixture of sorbitol ester and sorbide ester, which are simultaneously produced as well as sorbitan ester. There are many types of sorbitan esters with the kinds of fatty acids and various degrees of esterification. Those are generally used as emulsifier for cream, W/O emulsion, etc. It is applied in limited fields by itself because its special characteristics other than emulsifying capability are few; however, it is widely used as a major emulsifier in combination with other emulsifiers with different functions.

Figure 9-4. Sketch of two isomers of lactic acid esters of monoglyceride.

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In China, sorbitan monolaurate, monopalmitate, monostearate, tristearate, and monooleate (brand name are Sorbitan 20,40,60,65 and 80) are widely used in many foods such as hydrogenated vegetable oil, dairy, bean products, wheaten food, and drinks with ML from 0.5-10.0 g/kg according to the Chinese GB 2760. (8) Polyoxyethylene sorbitan esters of fatty acids. Polyoxyethylene sorbitan esters of fatty acids are called polyoxyethylene sorbitan ester, which is produced by the combination of polyoxyethylene and sorbitan ester. Polyoxyethylene sorbitan monolaurate, monopalmitate, monostearate, monooleate are called Tween 20, 40, 60, and 80 (brand name) respectively. They are generally used as emulsifier and stabilizer for O/W emulsion system, such as flavored milk and other liquid milks that use non-diary ingredients. According to the Chinese FDs standard, they can be used in dairy, drinks, bean products and bread and its ML is about 0.5-2.5 g/kg. (9) Propylene Glycol Esters of Fatty Acids (PG ester). Propylene glycol esters of fatty acids is called PG ester, in which propylene glycol and fatty acid are linked by ester bonding and the product obtained from inter-esterification, is a mixture of monoester and diester. To isolate the monoester with surface effects, high purity product of monoester is produced by molecular distillation and distilled monoglyceride. PG ester has little emulsifying action, but has a tendency to keep its alpha-crystal structure. Since it is usable as a foaming agent when combined with monoglyceride, it is used as a powder-foaming agent for cakes and desserts, liquid shortening, etc, with the exception of whole or partially skimmed milk. The ML is 5.0 g/kg according to the Chinese GB2760. (10) Sucrose Esters of Fatty Acids. Sucrose esters of fatty acids are called sucrose ester, prepared from sucrose and methyl and ethyl esters of food fatty acids or extracted from sucroglycerides. They occur as white to yellow-brown powdery or massive substances, or as colorless to red-brown, viscous resinous or liquid substances. They are odorless or have a slight, characteristic odor, sparingly soluble in water, soluble in ethanol. The solubility in oil increases with the increase of the degree of esterification. Because sucrose esters have many sub-categories and most of them are complex sucrose and different fatty acids, the products of sucrose esters, generally being mixtures of different components, have HLB ranging from 1 to 16. Owing to the wide ranging HLB, sucrose esters have many different functions. It is used as an emulsifying and dispersing agent for foods, such as cream, and bactericidal agents for canned coffee. The ML is from 1.5 g/kg for fats and oils essentially free from water to 10.0 g/kg for vegetable oils and fats according to the Chinese GB2760. (11) Calcium Stearoyl-Lactate and sodium stearoyl lactylate. Calcium (or sodium) stearoyl-2 lactate is called CSL. It is a product obtained by bonding two lactic acids and stearic acid and partially neutralized with calcium (or sodium). It is a mixture including the original stearic acid and salt. It is an anionic emulsifier having a strong ability to bind protein and is used as a dough modifier for flour foods like bread. In China, it also used in wheaten foods, meat enema and dairy drinks with ML being 2.0 g/kg. Sodium stearoyl lactylate can be used as emulsifier or stabilizer for setting up water-inoil food emulsions containing at least 80% fat. The maximum level of use is 1.5 g/kg according to the Chinese GB2760. Sodium stearoyl lactylate can be used as emulsifier or stabilizer for making breads, rolls and pastries. The ML is 2.0 g/kg according to the Chinese GB2760.

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(12) Lecithin (LC). Lecithin is a mixture containing phospholipids as the major component and widely found in animals and plants such as soybeans, corn, rapeseed and egg yolk. It has long been used as a natural emulsifier. The products in the market are paste lecithin or powdered lecithin of high purity. In the food industry, lecithin has multiple uses: in confectionery it reduces viscosity, to replace more expensive ingredients, control sugar crystallization and the flow properties of chocolate, improve the homogeneous mixing of ingredients and shelf life for some products, and be used as a coating. In emulsions and fat spreads, it stabilizes emulsions, reduces spattering during frying, and improves texture of spreads and flavor release. In doughs and bakery, it reduces fat and egg requirements, helps even distribution of ingredients in dough, stabilizes fermentation, increases volume, protects yeast cells in dough when frozen, and acts as a releasing agent to prevent sticking and simplify cleaning. It improves wetting properties of hydrophilic powders (e.g. low-fat proteins) and lipophilic powders (e.g. cocoa powder), controls dust, and helps complete dispersion in water. It can be used as a component of cooking sprays to prevent sticking and as a releasing agent. For example, lecithin is the emulsifier that keeps cocoa and cocoa butter in a candy bar from separating. In margarines, especially those containing high levels of fat (>75%), lecithin is added as an 'anti-spattering' agent for shallow frying. Most major religions have no dietary restrictions on the use of lecithin. In China, this emulsifier can be used in appropriate level based on the need of food production according to the Chinese GB2760.

7. STABILIZERS, THICKENERS AND GELLING AGENTS 7.1. Definition All the stabilizers, thickeners and gelling agents are known and described as food hydrocolloids implying that functional properties are obtained by mixing them with water. They are macromolecules, such as carbohydrate polymers, proteins and their modified forms, which are water soluble. These additives include traditional materials such as starch, cellulose, gums, obtained from many land plants, and gelatin, an animal by-product, and microbial polysaccharides of xanthan, gellan and pullulan, and their modified forms. Even at low concentrations, they are able to increase a system's viscosity, to form gels and to stabilize emulsions, suspensions or foams. These compounds are also active as crystallization inhibitors and are suitable for aroma encapsulation, which is often needed for dehydrated food.

7.2. Classification Raw material origin has been used to classify hydrocolloids, for example as seaweed extracts, seed gums, fermentation products, plant exudates, animal gelatin or microorganic gums. Their general functional properties may also be used to classify them as thickeners, stabilizers or gelling agents. The resourceful plant polysaccharoses, like starch and cellulose, are derived by some chemical methods are classified modify polysaccharoses. Natural

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stabilizers, thickeners and gelling agents coming from plants can be classified based on their sources shown in Table 9-4.

7.3. Functional Properties The following is a brief overview of the key functional properties for gelling agents, stabilizers or thickeners used. These ingredients need to be added to commercially-produced food to maintain their structure and physical properties during processing. For example, fruits such as strawberries, raspberries and cherries need the addition of a small amount of pectin to form a jam. Nutritional properties are relatively new and nutraceutical or health-enhancing properties are even more recent. Further work is sure to advance the use of hydrocolloids beyond modification of the rheology of foods.

7.3.1. Viscosity Adjustment Viscosity is probably one of the most widely used properties. In this respect, hydrocolloids are widely used to thicken sauces, soups, stews and other dishes with a high liquid content, and the thickened water simply adds body, texture and mouth feel to a food such as table syrups, particularly low-calorie syrups. In other cases, hydrocolloids are often used also in systems where the oil or fat content has been reduced or eliminated through substitution with water. The hydrocolloid thickens water, which, in turn, replaces the fat or oil to give a product with similar properties to the full-fat food. A typical application for this function is reduced-fat salad dressings. 7.3.2. Emulsion Stabilization If oil or fat is partially removed from a formulation or replaced with thickened water, an emulsion is usually formed. Often the function of the hydrocolloid is to stabilize the emulsion, to prevent separation and, in the case of frozen foods, to control ice crystal formation. New technology and new ingredients have been developed specifically to address the problem of ice crystals in frozen foods, but hydrocolloids will continue to play a role. Virtually every ice cream product sold in retail outlets is stabilized with carrageenan, locust bean gum and/or guar gum. Low-fat salad dressings also benefit from emulsion-stabilizing properties. Table 9-4. Groups of plant-derived hydrocolloids [8] Group Seeds gum Plant exudates Fruits extract Plant materials Seaweed extract

Compounds examples Guar gum, Locust bean gum, Tara gum Acacia gum, Tragacanth gum Pectin Cellulose, Starches Agar, Alginates, Carrageenan

7.3.3. Suspension Stabilization If insoluble particles are included in the thickened product, then separation and settling should be eliminated or at least minimized. Some hydrocolloids create solutions with a ''yield point'' that will keep particles immobilized in suspension. Salad dressing is a good example of

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this and xanthan gum is the typical hydrocolloid to supply this functionality. For beverage with pulp suspended, like orange juice with pulp particle, precipitation can be prevented by adding suitable amount agar to crate a small yield point which is big enough to stop the pulp settling during the shelf-life and small enough to keep the beverage's flow property like normal drink.

7.3.4. Gelation One of the key texturising aspects of hydrocolloids is the ability to gel and solidify fluid products. For example, in gelled milk desserts, even low levels of carrageenan will form a solid milk gel. Other classic gelling agents are pectin, gelatin and agar. However, others will form a gel under specific conditions. Certain grades of alginates form gels with calcium ions. Xanthan and locust bean gum alone do not gel, but their combination display synergy and form a strong cohesive gel. Methyl cellulose and hydroxypropylmethyl cellulose are unusual in forming solutions that reversibly thicken or gel when heated. The food industry has a myriad of gelling applications ranging from soft, elastic gels to hard and brittle gels. 7.3.5. Nutritional and Nutraceutical Function There is already a wide use of some hydrocolloids. For example, Arabic and guar gum are sources of soluble dietary fiber. Much research has been conducted in the nutraceutical benefits of hydrocolloids. Potential benefits range from cholesterol reduction to cancer risk prevention.

7.4. Common Used Stabilisers, Thickeners and Gelling Agents The following agents are widely used in food industry. Almost every one is permitted to be used in any food categories, except those special mentioned in appropriate level, based on the need of production according to the Chinese GB 2760.

7.4.1. Acacia Gum Acacia gum, also known as gum arabic, is a natural gum exudate obtained from acacia trees in the 'African sub-Sahelian zone'. The gum is a complex mixture of polysaccharides and glycoproteins that have a highly branched compact arabinogalactan structure which gives a low-viscosity solution together with a central protein fraction that provides good emulsification properties. The powder hydrates readily in water and concentrations up to 4050% can be handled easily. Its applications include a range of confectionery products, flavored oil emulsions and capsules and health foods as a stabilizer and a source of soluble fiber with prebiotic properties. 7.4.2. Agar Agar is extracted from red seaweed (Rhodophyceae) and has been used in foods for more than 350 years. Agar consists of a mixture of agarose and agaropectin. Agarose is a linear polymer, made up of the repeating monomeric unit of agarobiose. Agarobiose is a disaccharide including D-galactose and 3, 6-anhydro-L-galactopyranose. Agaropectin is a heterogeneous mixture of smaller molecules that occur in lesser amounts. Agar is insoluble in cold water and hydrates when boiled. Cooling solutions below about 40 °C produce very firm brittle gels which can be melted by heating above 85 °C. Food applications include water

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dessert gels, aspics, confectionery jellies, canned meats gels, a vegetarian gelatin substitute, a thickener for soups, and as a clarifying agent in brewing.

7.4.3. Alginates Alginates are derived from various species of brown seaweed found off the coasts of the North Atlantic, South America and Asia. They are produced as a range of salts, but sodium and potassium alginate are predominantly used in foods. It is a polysaccharide, consisting primarily of D- and L-galactose units. About every tenth D-galactopyranose unit contains a sulphate ester group. Calcium, magnesium, potassium or sodium cations are also associated with the polysaccharide. It is insoluble in cold water and soluble in boiling water. Sodium or potassium alginate hydrates in cold or hot water to give viscous solutions. The controlled interaction between sodium or potassium alginate and calcium salts gives cold-setting gels that are shear irreversible and heat stable. Control is affected using citrate or phosphate sequestrants, or by processing at temperatures above about 70 °C and cooling. Typical food applications include reformed foods such as onion rings and olive fillings, cold-setting bakery cream fillings and heat-stable bakery and fruit fillings. 7.4.4. Carrageenan Carrageenan is obtained from red seaweeds. Different species provide a number of carrageenan extracts. Carrageenan is a hydrocolloid consisting mainly of the ammonium, calcium, magnesium, potassium and sodium sulphate esters of galactose and 3, 6anhydrogalactose polysaccharides. These hexoses are alternatively linked -1, 3 and -1, 4 in the copolymer. The relative proportions of cations existing in carrageenan may be changed during processing to the extent that may become predominant. Depending on the source of seaweed, the functional components are kappa-carrageenan, iota-carrageenan or lambdacarrageenan, each of which is characterized by the content of sulphate groups and gelling properties. Carrageenan is soluble in water of about 80 °C, forming a viscous, clear or slightly opalescent solution that flows readily. It is insoluble in ethanol. Kappa carrageenan and furcellaran form thermally reversible firm, brittle gels. Iota carrageenan gives soft, elastic gels. Kappa carrageenan interacts synergistically with polymannans, such as locust bean gum and konjac, to give strong cohesive gels. Blends of kappa with iota carrageenan or polymannans are used to give a range of gel textures used in injected meats, canned meats and water dessert gels.. Hybrid (kappa/iota) carrageenan is particularly suitable for firm, creamy textures in dairy desserts. Lambda carrageenan is non-gelling but thickens in drinks and dairy desserts. A specific interaction between kappa carrageenan and kappa casein is widely used to stabilise dairy products including milk beverages, ice cream mixes and processed cheese products. 7.4.5. Microcrystalline Cellulose Microcrystalline cellulose is produced by converting fibrous cellulose to a fine-particle form crystalline cellulose using acid hydrolysis. This material can be readily dispersed in water using high shear. This dispersion will reconstitute to deliver a colloidal form which is unique when compared to other soluble food hydrocolloids. It exhibits a variety of desirable characteristics including suspension of solids, heat stability, ice crystal control, emulsion stabilization, foam stability, texture modification and fat replacement. Food applications include frozen desserts and ice cream, whipped toppings, low-fat mayonnaise and salad

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dressings, ambient-stable dairy and non-dairy beverages, nutritional drinks, ready-to-use bake-stable bakery fillings and fruit fillings and dairy and non-dairy creams.

7.4.6. Cellulose Derivatives Cellulose derivatives are substances commonly made from cellulose by chemical modification. The common used cellulose gum in food is carboxymethyl cellulose and hydroxypropylmethyl cellulose. In food applications, cellulose gum is an effective thickener and moisture binder used for clear beverages, dairy products, such as ice cream, bakery and other prepared foods. Hydroxypropylmethyl cellulose exhibits thermogelation, utilized for bake-stable sauces, fillings and formed foods. Hydroxypropylmethyl cellulose is also a surface-active agent to be used to stabilize foams and emulsions. 7.4.7. Starch Derivatives Starch derivatives are wide substances commonly made from starch by chemical or phsical modification. The common used starch derivatives in food are starch acetate, distarch phosphate, hydroxypropyl distarch phosphate, acid treated starch, oxidized starch, oxidized hydroxypropyl starch, acetylated distarch phosphate and acetylated distarch adipate. Starch derivatives are easier gelatination and more anti-aging than common strarch. The viscosity of their solution is also more stable than the common starch. Starch derivatives can be used in any food to substitute starch to make the food more stable. 7.4.8. Gelatine Gelatine is a protein material obtained from animal connective tissue using hydrolysis in acidic (type A) or basic (type B) solution followed by hot water extraction. Gelatine hydrates readily in warm or hot water to give low-viscosity solutions that have good whipping and foaming properties. Concentrated solutions containing up to 40 % gelation are made for use in confectionery. After cooling, the network of polypeptide chains associates slowly to form clear, elastic gels that are syneresis free. The thermoreversibility of gelatine gels gives them unique properties: the melting point is below 37 °C so they melt in the mouth to give smooth textures with excellent flavour releasing. Gelatine is used in a wide range of food, pharmaceutical and photographic applications. Food products include jelly candies and aerated confectionery, yogurt and other cultured desserts, dairy desserts and creams, low-fat spreads, canned meat products, water dessert gels and decorative aspics. 7.4.9. Seed Gums The origin of seed gums, such as guar gum and carob or locust bean gum, are produced by removing the outer coating of the seed and grinding the endosperm. The composition and structure of the galactomannans are described and linked to their functional properties. Seed gums are very effective thickening agents in water. In addition, guar gum interacts synergistically with xanthan to increase viscosity. Locust bean gum consists mainly of high molecular weight (approximately 50,000-3,000,000) polysaccharide composed of galactomannans. It is soluble in hot water and insoluble in ethanol. Carob gum forms cohesive, elastic gels with xanthan gum and it increases the strength and elasticity of kappa carrageenan and agar gels. The main applications are for thickening convenience foods, dairy products, including frozen products such as ice cream, soft drinks and fruit juices, bread and pastry, fruit preserves, baby food, instant products including puddings, flans and pudding powder, and for dietary fibre in baked goods and pet foods.

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7.4.10. Pectin Pectin is a polysaccharide that is naturally present in most land plants, although commercial pectin is primarily extracted from citrus peel and apple pomace. Two forms of commercial pectin are available: high methyl- and low methyl-esterified pectin; and two versions of the latter exist: a conventional and an amidated form. High methyl-esterified pectin forms gels in high soluble solids and acidic systems, whereas low methyl-esterified pectin forms gels in a much broader pH and soluble-solids range, but requires the presence of divalent cations for gelling. As a consequence, each type has its own particular function. Nevertheless, general attractive features include excellent flavor release, good processing characteristics and stability at low pH. Its traditional and major function is to act as a gelling agent in foods, but, nowadays, it also serves as a thickening and stabilizing agent. The application of pectin is diverse and covers fruit-based products, dairy products, acidified milk drinks and other beverages, confectionery, bakery products, various fine foods and spreads. Additionally, pectin is used in the pharmaceutical industry. Finally, increasing consumer awareness of healthy life-style habits and the emerging trend to produce functional foods increases the significance of the status of pectin as a water-soluble dietary fiber. 7.4.11. Xanthan Gum Xanthan gum is a high-molecular-weight polysaccharide of which D-glucose and Dmannose are the dominant hexose units, along with D-glocuronic acid and pyruvic acid, secreted by the microorganism Xanthomonas campestris and produced commercially in a batch fermentation process. It exists as the sodium, potassium or calcium salts. Its solutions are neutral. It is soluble in water, and insoluble in cold water. The solution is a neutral and viscous one with pseudoplastic flow behaviour. This gives excellent suspension and cling at low shear and excellent mouthfeel and pouring properties at high shear. The xanthan gum molecule has a cellulosic backbone with side chains that wrap around the backbone protecting it and conferring excellent stability across a wide pH range and tolerance of high salt concentrations and ingredients, such as glycerol and alcohol. The rigid backbone helps to maintain viscosity during heating. Xanthan gum shows synergistic thickening with guar gum and forms very elastic cohesive gels with locust bean gum and konjac mannan. Typical applications include sauces and dressings, baked goods, beverages, desserts and ice creams. 7.4.12. Gellan Gun Gellan gum is a fermentation polysaccharide produced by the microorganism Sphingomonas elodea. It has a straight chain structure based on repeating glucose, rhamnose and glucuronic acid units with side groups of acryl groups. Gellan gum hydrates in hot water and the low-acyl form also hydrates in cold water with sequestrants. On cooling, native highacyl gellan gum gives gels that are soft and elastic. Low-acyl gellan gum gels at very low concentrations using both monovalent and divalent cations gives firm, brittle textures with excellent thermal stability. Combinations of the two kinds of gellan gum can be used to control syneresis and form a range of textures from soft and elastic to firm and brittle. A major food application is water dessert gels, particularly for Asian desserts. Other significant applications include confectionery, dairy desserts and bakery fillings. At levels too low to form a demoldable gel, gellan gum can form fluid gels that can suspend particulates in sauces and dressings and fruit pulp in beverages.

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7.4.13. Konjac Glucomannan Konjac flour yields a high molecular weight, viscous polysaccharide: konjac glucomannan. Glucomannan is a highly soluble, neutral plant polysaccharide that has been used as a gelling agent for 2000 years in China. It gels alone or in association with other polysaccharides where it shows strong synergistic effects in viscosity, gel strength and elasticity. Konjac has a greasy mouthfeel and chewy texture that resemble fat. It is resistant to digestion and has a very low calorific value. As well as being a healthy food, it is a preferred texturing ingredient in Asia. Glucomannan has generally recognised as safe (GRAS) status in the USA and it is well established as an additive in the EU food industry. Cost-effective as a thickener, the effects on post-prandial sugar and lipid levels, low digestibility and prebiotic fermentation have placed glucomannan amongst the most exciting hydrocolloids on the 'emerging' world nutraceutical market. 7.4.14 Tragacanth Gum Tragacanth is a dried exudation obtained from the stems and branches of Astragalus gummifer Labilliardiere and other Asiatic species of Astragalus (family Leguminiosae).Tragacanth consists mainly of high molecular weight polysaccharides (approximately 800.000) i.e. galactoarabans and acidic polysaccharides, which are hydrolyzed to galacturonic acid, galactose, arabinose, xylose, and fucose. Small amounts of rhamnose and glucose (derived from traces of starch and/or cellulose) may be present. Gum tragacanth is a viscous, odorless, tasteless, water-soluble mixture of polysaccharides. It is used in pharmaceuticals and foods as an emulsifier, thickener, stabilizer, and texturant additive. 7.4.15. Chitin and Chitosan Chitin and chitosan have the similar chemical structure. Chitin is made up of a linear chain of acethylglucosamine groups. Chitosan is obtained by removing enough acethyl groups (CH3-CO-) for the molecule to be soluble in most diluted acids. This process, called deacetylation, releases amine groups (-NH) and gives the chitosan a cationic characteristic. This is especially interesting in an acid environment where the majority of polysaccharides are usually neutral or negatively charged. Chitin can use in non-dairy creamer, ice creams, jams, marmalades, peanut butter, sesame paste, mayonnaise, lactobacillus drink, and beer with ML 0.4-5 g/kg based on GB2760. Chitosan can be an effective complement to help lose weight during diet period or to stabilise ones weight, and used as clarifier in wine industry. 7.4.16. Others Polydextrose, propylene glycol alginate, fenugreek gum, sesbania gum, tamarind seed polysaccharide gum, sodium carboxymethyl starch, ablmoschus manihot gum, artemisia gum, gleditsia sinensis lam gum, linseed gum, polyglycerol esters of fatty acid, hydroxypropyl starch phosphated distarch phosphate, methyl cellulose and ethyl cellulose are other thickeners, stabilisers and gelling agents used in food in China. They are used in some appointed food categories with limited amount as ML in GB2760.

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8. PHOSPHATE AND POLYPHOSPHATES Phosphates' buffering capacity, chelating metal ions, water-holding capacity, and interacting with long chain polyelectrolytes such as protein, make them widely used as FDs. Phosphates are one of the most widely used functional food ingredients. Applications of the various salt forms of phosphates, such as leavening agents in baked goods, moisture loss inhibitors in frozen and processed meats, emulsifiers in dairy products, and buffering agents for many different food formulations, are included. For example, monocalcium (MCP) and dicalcium phosphate dihydrate act as leavening agents in baked goods. MCP is also useful as a dough conditioner, and can be used to strengthen the gel formation in instant pudding products. Tricalcium phosphate is very useful in dry powder mixtures, to prevent the adsorption of moisture and allow the powders to flow properly. When these phosphates are used as functional food ingredients, they also provide the fortification of calcium and phosphorus. Phosphates are used in pasteurized cheese products, ice cream, frozen custard, breads, rolls, buns, flour, macaroni products, fruit jellies, preserves and jams, frozen eggs, vanilla powder, as well as many other foods. Polyphophates are added to flesh foods, such as meat, fish and poultty, in order to increase the retention of water and the solubility of proteins to improve texture. A range of phosphoric acids and their sodium potassium, and calcium salts, are used as FDs, from the simple acid orthophosphoric acid, H3PO4, to complex polyphosphates are permitted for use in food. Because too much of them in diet leads to loss of calcium in bones and onset of osteoporosis, almost every kind of those FDs are used with special limited ML for different food catogries according to the GB2760, and is banned in organic foods and drinks in some countries.

8.1. Phosphoric Acid Phosphoric acid (other names: orthophosphoric acid.) can only be obtained pure in the crystalline state and slowly undergoes dehydration to diphosphoric acid. Phosphoric acid is added to food to enhance the antioxidant effects of other compounds present, and also as an acidity regulator. Typical products include carbonated beverages, processed meat, chocolate, fats and oils, beer, jam and sweets. Phosphoric acid is a highly acidic ingredient in cola drinks.

8.2. Sodium Phosphate (1) Monosodium phosphate is a sodium salt of phosphoric acid. It is added to food to act as an antioxidant synergist, a stabiliser and a buffer. Typical products include processed meat products, processed cheese products. It is also used in bread, rolls, buns, artificial sweetened fruit jelly, canned potatoes, canned sweet peppers, and canned tomatoes and as a jelling agent. No food safety problems have been shown to occur with this chemical at the levels commonly used in foods, but high intakes may upset the calcium/phosphorus equilibrium. (2) Disodium phosphate is a sodium salt of orthophosphoric acid and is used as an antioxidant synergist, stabiliser and buffering agent in food. It is also used as an emulsifier in

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the manufacture of pasteurised processed cheese. Disodium phosphate is added to powdered milk to prevent gelation. Typical products include processed meat products, processed cheese products, powdered milk. (3) Trisodium phosphate is the sodium salt of orthophosphoric acid and used as an antioxidant synergist, stabiliser and buffering agent in food. Typical products include processed meat products, processed cheese products.

8.3. Potassium Phosphates (1) Monopotassium phosphate is a potassium salt of phosphoric acid used as an antioxidant synergist, buffer and emulsifier in food. Typical products include sauce and dessert mixes, jelly products. (2) Dipotassium phosphate is a potassium salt of phosphoric acid used as an antioxidant synergist, buffer and emulsifier in food. Typical products include cooked and other cured meats, milk and cream powders, drinking chocolate. (3) Tripotassium phosphate is a potassium salt of phosphoric acid used as an antioxidant synergist, buffer and emulsifier in food. Typical products include cooked and other cured meats, milk and cream powders, drinking chocolate.

8.4. Calcium Phosphates (1) Monocalcium phosphate is commercially available in the anhydrous or monohydrate form. Both are used as a leavening acid to replace cream of tartar in foods. Mono-calcium phosphate is used extensively in the fertiliser industry. Typical products include self-raising flour, baking powder, cake and pastry mixes, cakes and other pastry products as a baking agent. (2) Dicalcium phosphate - Manufactured from phophoric acid, dicalcium phosphate is used as an antioxidant in food, as well as being a firming agent. It is available in the anhydrous or dihydrate forms. Typical products include tinned and packaged fruit deserts, granular food products. In the canned products it provides calcium which has been shown to maintain the firmness of fruits and vegetables during the canning process. (3) Tricalcium phosphate is added to table salt, sugar, baking powder and fertilisers to give a 'free-flowing' quality. Typical products include salt, sugar and other granular foods, packet sauce mixes, cake mixes etc.

8.5. Sodium Triphosphate Sodium triphosphate is beside potassium triphosphate, one of the polyphosphates that are allowed as FDs. It is used in food for soften water, as emulsifying salt for process cheese or for the preparation of cooked sausages, surimi or fish fingers. It has antimicrobial function also.

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9. COLORANTS Dyes and pigments used in the food are known collectively as food colours, or more tersely, simply as colours. They are added to foods to make it more attractive to the purchaser or consumers or to replace natural colours lost during the food processing. Canned strawberries, for example, would be grayish-brown if colours are not added and canned peas would be brownish green without colouring with green colorant. In China, only 56 colorants are permitted for use as FDs. Most of them are natural, and include saffron, cochineal, carotenes (and the closely related xanthophylls) and anthocyanins. Other permitted colorants include synthetic or semi-synthytic coumpound, such as carotene structure identical, sodium copper chlorophyllin and erythrosin, and inorganic substances, such as the white pigment titanium dioxide and finely divided aluminum, silver and gold which are used in cake decoration. The list of permitted colours includes 9 synthesis dyes and 47 natural pigments or extracts. Most of them are used in foods with strictly limits. For example, colouring matter must not be added to raw meat, fish, poultry, fruit, cream, milk, honey, vegetables, wine, coffee, tea and condensed or dried milk in most countries. Also, by agreement with the manufacturers, colours are not used in foods made for babies and infants. The 9 synthesis colours use in China are amaranth, ponceau 4R, erythrosin, new red, lemon yellow, indigo blue, sunset yellow, brilliant blue and crimson. Sodium copper chlorophyllin is a semisynthesis colour which is also permitted in China. The 47 natural colours are β-carotene (zymotechnics), bect-root red, turmeric yellow, curcumin, safflower yellow, http://www.chinaadditives.com/Carthamus-Red.htm lac dye, cowberry red, papika extract include capsorubin and capsanthin, chilli orange, caramel, carthamus yellow, gardenia yellow, coreopsis yellow extract, wild groundnut red, broomcorn red, maize yellow include zeaxanthin and crtotixabthin, radish red, cacao husk pigment, ang-kak, monascorubin, vinespinach red, black currant red, neutral mustard red, gardenia blue, hippophae rhamnoides(sea backthern) yellow, hibiscus sabaariffa(roselle) red, acorn husk brown, NP red (NP), tanoak brown, mulberry red, natural amaranth, cherokee rose brown, wide jujube skin extract, peanut underwear red, grape skin red, uguisukagnra color, spirulina blue, vegetable carbon black, pale butterflybush flower yellow, gromwell red, theaflavin, tea green pigment, citrus yellow, bixin, cochineal red and iron oxide red or black. All of the 9 synthesis colours and most of natural colours used in China are water soluble and purchased in a powder format that exhibits coloring solutions when they are dissolved. Therefore, their using are very easy. Aluminum lake pigments, made of aluminum and colorants, are water insoluble material but oil dispersible (but generally not oil soluble) and thus can be mixed with oils and fats. They can also be dispersed in other carriers such as propylene glycol, glycerin and sucrose (water and sugar). Aluminum lake pigments are used in a variety of applications: (1) To color a fat based product, such as chocolate or compound coatings. For these, we produce a concentrated dispersion in a high quality and very stable vegetable oil. The dispersion is added directly to the chocolate to dye it accordingly. (2) For "hard panning" (to dye the outside of a product such as a gum ball or a pill). In this case, we produce a dispersion usually using sucrose (sugar and water) that is applied to the candy or food as it is being tumbled and dried. Multiple layers are applied to produce the desired shade.(3) Lakes tend to resist bleeding. Dyes have a tendency to "bleed", or migrate from one part of the product to another. This can be a problem in candy canes or any product where there are defined borders such as

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the food with desired stripes. While Dyes are normally used in hard candy, lakes are sometimes substituted if bleeding is a problem.

10. FOOD FLAVORANTS AND FLAVOUR MODIFIERS 10.1. Food Flavorants Flavourings, or flavors, spices and essence, are substances or food flavor additives used to give taste and/or smell to food. There are different types of flavourings, such as natural, natural-identical or artificial flavouring substances, flavouring preparations of plant or animal origin, processing flavourings which evolve flavour after heating and smoke flavourings. The expression "natural flavouring" may only be used for flavouring substances or flavouring preparations which are extracted from vegetable or animal materials. Generally, different flavors are specially formulated for various food items to make them more delightful. Flavorants contains many flavour compounds. Only the Generally Recognized As Safe flavour compounds can be used as food flavor additives. Flavour compounds often classify into 18 groups, shown in Table 9-5. Each of the 18 groups contains substances that have similar chemical structure at least partly, but have not similar odors mostly. Table 9-5. Groups of flavouring substances Group 1

Chemical substances Isothiocyantaes (except for those generally recognized as toxic).

2

Indole and its derivatives.

3 4 5

Ethers. Esters. Ketones.

6 7 8 9

Fatty Acids. Aliphatic Higher Alcohols. Aliphatic Higher Aldehydes (except for those generally recognized as toxic) Aliphatic Higher Hydrocarbons (except for those generally recognized as toxic)

10 11 12 13

Thioethers (except for those generally recognized as toxic) Thiols(Thioalcohols) (except for those generally recognized as toxic) Terpens Phenol Ethers (except for those generally recognized as toxic)

14 15 16 17

Phenols (except for those generally recognized as toxic) Furfural and its derivatives (except for those generally recognized as toxic) Aromatic Alcohols Aromatic Aldehydes (except for those generally recognized as toxic)

18

Lactones (except for those generally recognized as toxic)

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10.2. Flavour Modifiers A flavour modifier is a substance which is capable of enhancing or modifying the taste or odour, or both, of a food. Flour modifiers or enhancers are themselves practically tasteless but they intensify the flavour of soups, meat and other savoury foods. The most widely used flavour enhancers are monosodium Glutamate (MSG) and sodium 5-ribonucleotide. MSG is present in most dehydrated soups, stock bouillon cubes and other meaty prepared foods. Some people have an alergy to MSG, which has come to be known as the "Chinese restaurant syndrome", if they eat food containing it. This manifests itself in a variety of ways including palpitations, chest or neck pain and dizziness. The cause is not known and the ill effects soon disappear. It has also been reported in potter, N.N. (1986) of brain damage resulting when MSG was injected under the skin of young mice. Though MSG is not thought to be harmful according to Fox B.A. and Cameron, A.G (1989), precautionary measures are still been taken for not adding it to babies and infants food. Sodium 5'-ribonucleotide is another powerful permitted flavour enhancer. Ribonucleotides are compounds formed from the sugar ribose, phosphoric acid and an organic base such as guanine. They occur in all animal tissues and are present in yeast extracts and contribute substantially to their characteristic meaty taste. Ribonucleotide flour enhancers are used in soups, meat and fish pastes, all types of canned meat products, sausages, meat pies and other processed food products of which the main ingredients is meat or fish. Their flavour enhancing power is 10 times greater than that of MSG. Hydrolyzed proteins, broken apart into amino acids, are used by the food industry to enhance flavor. The chemical breakdown of proteins may result in the formation of free glutamate that joins with free sodium to form MSG. In this case, the presence of MSG does not need to be disclosed on labeling. However, labeling is required when MSG is added as a direct ingredient.

REFERENCES [1] [2] [3] [4] [5]

[6] [7]

Salant, A. Nonnutritive sweeteners. In: Furia, TE. Handbook of Food Additives. Cleveland: CRC Press, 1972. Luo, AS; Chun, Z; Luo, AX; Fan, YJ; Ge, SR. The survey and progress of the food preservative. China Food Additives, 2005, 55-57. Wedzicha, RL. Chemistry of Sulphur Dioxide in Foods. 1st edition. London: Elsevier, 1984. Chichester, DF, et al. Antimicrobial food additives. In: Furia, TE. Handbook of Food Additives. Cleveland: CRC Press, 1972. Zsolt, F, et al. A synthetic and in silico study on the highly regioselective Diels-Alder reaction of the polyenic antifungal antibiotics natamycin and flavofungin. Tetrahedron Letters, 2010, 51, 4968-4971. Ma, LR; Zhang, ZL; Li, H. New type food preservative-NISIN. Journal of Hebei Academy of Sciences, 2001, 18, 180-182. Griffin, WC. Classification of Surface-Active Agents by HLB. Journal of the American Oil Chemists' Society, 1949, 311.

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Ren, P; R, YW; Wang, LD. Properties of Plant Gum and Its Application in Food Industry. Food Research and Development, 2004, 39-44.

In: Food Chemistry Editors: D.Wang, H. Lin, J. Kan et al.

ISBN: 978-1-61942-125-7 © 2012 Nova Science Publishers, Inc.

Chapter 10

TOXICANTS IN FOODS Wang Dongfeng1, Guoqing Huang2 and Shuhui Wang3 1

College of Food Science and Engineering, Ocean University of China, Qingdao, China 2 College of Food Science and Engineering, Qingdao Agricultural University, Qingdao, China 3 College of Agriculture - Ginn College of Engineering, Aubum University, Auburn, AL, US

ABSTRACT Microorganisms, plants and animals have evolved multiple defense systems against predators. Among the systems, the chemical defense system is of special interest for the food industry. Some of these compounds are toxic to specific microorganisms, insects, birds, and mammals, but most of those are toxic to humans and can give rise to chronic or acute symptoms of poisoning. Though some of the compounds occur in very low concentrations, the compounds have a cumulative harmful effect if they constitute a part of the normal diet over a prolonged period. Besides, food materials might be contaminated by exogenous toxins. Intrinsic toxins and contaminants have been recognized as the major components that affect food safety. This chapter introduces the occurrence and properties of the most important toxicants in foods, including allergens, toxic glycosides, toxic amino acids, lectins, saponins, toxins in aquatic organisms, various antinutrients, heavy metals, pesticide residues, dioxins, veterinary and fish drug residues, benzopyrenes as well as heterocyclic amines. Besides, measures for preventing the generation of some toxicants are proposed.

1. INTRODUCTION According to structure and biological effects on human body, toxicants in foods are divided into toxic substances, harmful substances, and antinutrients. A toxic compound shows its toxicity even in a very low dose, and a harmful substance is toxic only when its content exceeds a certain threshold. An antinutrient may interfere in or inhibit the absorption of other nutrients in foods. The definitions of these terms, however, are subject to changes due to the

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improvement of analysis techniques and the availability of more data on their biological effects. It has been found that some toxicants are beneficial in certain contents and some substances are toxic or hazardous only in certain cases. For example, phenols in foods are often recognized as antinutrients since they inhibit the adsorption of proteins. However, phenols have the anti-oxidation and free radical scavenging capabilities and can be used as natural antioxidants in foods. In addition, some well-known nutrients may be risky to certain individuals, such as lactose, which may be deleterious to lactose-intolerant individuals. According to origin, hazards in foods can be divided into inherent toxicants (or endogenous toxicants) and contaminants (or exogenous toxicants). Endogenous toxicants are metabolites produced via biosynthesis by food organisms under normal growth or metabolites produced via biosynthesis by food organisms that are stressed. Contaminants refer to toxicants that directly contaminate foods, toxicants that are absorbed from the environment by food-producing organisms, toxic metabolites produced by food organisms from substances that are absorbed from the environment, and toxicants that are formed during food preparation [1]. Table 10-1. Endogenous toxicants in foods Toxicants Glucosinolates (goitrogens)

Cyanogenic glycosides Lectins

Source Cruciferae seeds, oilseeds, mustard seed, kale, radish, cabbage, peanut, soybean, cassava, onion, et al. Cassava, sweet potato, nuts, kidney bean, limabean, millet, broomcorn millet, et al Papilionaceae, cereals, soybean, and other beans

Glycoalkaloids

Potato, tomato, and other immature fruits

Gossypol

Cottonseed

Tetrodotoxin

Globefish

Shellfish toxins Histamine

Mussels, Mytilus edulis, scallops, Meretrix meretrix, et al. Scomber japonicus, tunny, sardine, et al. Toxic mushrooms

Mycotoxins

Effects on human upon ingestion Thyroid enlargement, thyroxine synthesis decrease, metabolism impairment, reduced iodine adsorption and protein digestion Cellular inspiration block, stomach upset, sugar and calcium transfer block, and iodine uptake interference Intestinal epithelial cells damage, nutrient adsorption retardation, enzyme inhibition, B12 and lipid adsorption impairment, et al. Cholinesterase inhibition, stomach upset, hematolysis, kidney function impairment Metal binding, iron adsorption decrease, enzyme inhibition Neuron paralysis, dyspnoea and even death Nerve paralysis and liver poisoning Allergy, including blush, dizziness, and tachypnea Gastroenteritis, nerve system injury, hematolysis, visceral parenchymal injury

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Toxicants in Foods Table 10-2. Endogenous antinutrients in foods Antinutrient

Source

Effect on human upon ingestion

Oxalic acid

Beet root, spinach, celeriac, Rhubarb, amaranth, tomato

Interference in the adsorption of Ca, Fe, Zn and other metals and Ca metabolism

Phenols

Vegetables, fruits, grape wines, cereals, soybeans, potato, tea, et al.

Block of thiamine adsorption and decrease of metal bioavailability

Phytates

Papilionaceous plants, cereals, and the seeds of all plants

Decrease of the bioavailability of Ca, Mg, Fe, Zn, Cu, and other metals and that of proteins and starch

Protease inhibitors

Papilionaceous seeds, peanut, cereals, rice, maize, potato, apple, sweet potato, et al.

Inhibition of trypsinase, chymotrypsin, glycopeptidase, and amylase

Saponins

Papilionaceous plants, spinach, lettuce, sugarbeet, soybean, tea, peanut

Complexation with proteins and lipoids, hematolysis, and gastroenteritis Note that most saponins are nontoxic.

Tannins

Plant-derived foods, such as most fruits, tea, and coffee

Inhibition of pancreatic enzymes, bioavailability decrease of thiamine, proteins, cobalamin, and Fe

Non-protein amino acids

Legume, seaweed

Interference with protein metabolism, bone and nerve poisoning

Table 10-3. Exogenous toxicants in foods Hazards

Source

Effect on human upon ingestion

Heavy metals

Plants and animals cultivated in polluted rivers, estuaries, and soils Foods polluted by organophosphorus pesticides

Inappetency, gastroenteritis, insomnia, dizziness, muscle soreness, anemia, and many other symptoms

Veterinary drugs

Diary and meat products

Microbial toxins

Microorganism-polluted foods and materials

Allergy, drug-resistance development, microecological environment change, early maturity, physiological disorder, chronic disease symptom exacerbation Multiple adverse effects

Organophosphoru s pesticides

Enzyme inhibition, muscle vibration, spasm, myosis, blood pressure elevation, heartbeat acceleration, dyspnea, pneumonedema, and coma

It should be noted that all chemicals are potentially toxic, and that the dose alone determines whether or not a toxic effect occurs. Indeed, toxicant (or toxin) is recently defined as a ―substance that has been shown to present some significant degree of possible risk when consumed in sufficient quantity by humans or animals‖ [2]. For example, pure water drunk to excessive can induce an electrolyte imbalance and lead to death, which is called the ―water toxicity‖ that is due in part to decreased renal capacity to excrete water [3]. The innumerable

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naturally occurring chemicals in food are potentially capable of inducing toxic effects, but few are ordinarily present in sufficient concentrations to actually show toxic effect [2, 4]. Hence, we should distinguish between ―toxic substance‖ and ―toxic effect‖. The most frequently occurring toxicants in foods are listed in Table 10-1~10-3. A substantial number of toxic substances have been isolated and identified This chapter concerns only the most important toxicants that have been identified in foods.

2. ENDOGENEOUS TOXICANTS Endogeneous toxicants include any substances produced by food materials (either plants or animals) that are harmful to human beings. Lectins, saponins, and toxic peptides mentioned above are endogenous toxicants.

2.1. Allergen An allergen refers to any substance that can cause an allergy. Most immune responses of caused by food components are IgE-mediated immediate allergic reactions. Upon exposure to allergens, the lymphatic system of human body acts to avoid or alleviate the adverse effects. Most antigens can be metabolized to monosaccharides, amino acids and lower fatty acids after antigen-antibody reactions. However, complete-antigens, which are characteristic of both immunogenicity and immunoreactivity, can cause immune reactions. The intake of allergen-containing foods can cause allergic symptoms, such as pruritus, functional gastrointestinal disorder and fever. Many foods have been found to contain allergens, but the sensitivity to these foods varies with local diet, environmental factors and possibly genetics. Epidemiological investigations reveal that the incidence of food-induced allergy in infants and children is higher than that in adults and the incidence decreases as age increases. Food allergens share the following characteristics: 1. Allergens are present in most foods, such as milk, eggs, and soybean, of which, milk and egg are the most important source of allergens. Though allergens occur widely in foods, 90% of food allergies are induced by only a small number of foods. 2. Only few food components are allergenic. For example, up to 23 types of glycoproteins have been identified in white egg, but only ovalbumin, conalbumin, and ovomucin are allergic. 3. The allergenicity of foods is subject to change after processing. For example, heat treatment inactivates most allergens and the increase of acidity or the presence of digestive enzymes reduces the allergenicity of foods. 4. Sensitive consumers could suffer allergic cross reactions. Some proteins contain the same antigenic determinant and these proteins can cause allergic cross reactions. For instance, at least 50% of patients allergic to cow‘s milk also react to goat‘s milk and patients allergic to chicken eggs might be allergic to the eggs of other birds.

Toxicants in Foods

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2.2. Toxic Glycosides 2.2.1. Properties of Toxic Glycosides Toxic glycosides are also known as cyanogenic glycosides. These compounds often contain glucose or rhamnose as their sugar components and are widely present in cassava, sweet potato, nuts, kidney bean, limabean, millet, and many other cereals. Excessive intake of toxic glycosides may lead to gastrointestinal malaise and affect the transfer of sugars and calcium. The contents of toxic glycosides in plants vary with species and cultivation technologies. Tables 10-4 and 10-5 list the distribution of toxic glycosides and their contents in various cereals. Glycosides can be hydrolyzed by enzymes to produce thiocyanates, isothiocyanates, and perthiocyanates. All these products are toxic and can cause goiter. Table 10-4. Distribution and hydrolytic products of some cyanogenic glycosides in food plants [5] Glycoside Amygdalin Dhurrin Linamarin Lotaustralin Prunasin Cicianin

Source Almond, cherry, peach, plums, apples Sorghums Linseed, clovers, cassava, lima beans Linseed, cloves, cassava, lima beans Cherry, almond Vetches

Hydrolytic products HCN, gentobiose, and benzaldehyde HCN, glucose, and hydroxybenzaldehyde HCN, glucose, and acetone HCN, glucose, and 2-butanone HCN, glucose, and benzaldehyde HCN, vicianose, and benzaldehyde

Table 10-5. Thiocyanate contents in typical vegetables (mg/100g edible part of fresh leaf) Plant Cabbage Savoy Brussels sprout Cauliflower Kidney bean, turnip Kohlrabi Colza Swedish turnip Lettuce, Spanish, onion

Content 3~6 18~31 10 4~10