Chemical and Functional Properties of Food Components

Chemical and Functional Properties of Food Components Second Edition

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Chemical and Functional Properties of Food Components Series SERIES EDITOR

Zdzislaw E. Sikorski Chemical and Functional Properties of Food Proteins Edited by Zdzislaw E. Sikorski

Chemical and Functional Properties of Food Components, Second Edition Edited by Zdzislaw E. Sikorski

Chemical and Functional Properties of Food Lipids Edited by Zdzislaw E. Sikorski and Anna Kolakowska

Chemical and Functional Properties of Food Components Second Edition EDITED BY

Zdzislaw E. Sikorski, Ph.D. Professor of Food Science Department of Food Chemistry and Technology Gdan´sk University of Technology, Poland

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CRC PR E S S Boca Raton London New York Washington, D.C.

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Library of Congress Cataloging-in-Publication Data Chemical and functional properties of food components / editor, Zdzislaw E. Sikorski.-2nd ed. p. ; cm. -- (Chemical and functional properties of food components series) Includes bibliographical references and index. ISBN 1-58716-149-4 (alk. paper) 1. Food--Analysis. 2. Food--Composition. I. Sikorski, Zdzislaw E. II. Series. TX545 .C44 2002 664′.07--dc21

2002276808

This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage or retrieval system, without prior permission in writing from the publisher. All rights reserved. Authorization to photocopy items for internal or personal use, or the personal or internal use of specific clients, may be granted by CRC Press LLC, provided that $.50 per page photocopied is paid directly to Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923 USA. The fee code for users of the Transactional Reporting Service is ISBN 1-58716-1494/02/$0.00+$1.50. The fee is subject to change without notice. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. The consent of CRC Press LLC does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from CRC Press LLC for such copying. Direct all inquiries to CRC Press LLC, 2000 N.W. Corporate Blvd., Boca Raton, Florida 33431. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe.

Visit the CRC Press Web site at www.crcpress.com © 2002 by CRC Press LLC No claim to original U.S. Government works International Standard Book Number 1-58716-149-4 Library of Congress Card Number 2002276808 Printed in the United States of America 1 2 3 4 5 6 7 8 9 0 Printed on acid-free paper

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Dedication I am honored to dedicate this volume to Professor Owen R. Fennema.

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Preface Water, saccharides, lipids, proteins, and minerals — the main components — form the structure of and are responsible for the sensory and nutritional properties of foods. Other constituents, present in lower quantities, especially colorants, flavor compounds, vitamins, probiotics, and additives, also contribute to different aspects of food quality. The catabolysis that takes place in raw materials postharvest, as well as chemical and biochemical changes and interactions of components during storage and processing, affect all aspects of food quality. These processes can be effectively controlled by the food processor who knows food chemistry. The contents of this book go beyond that of a standard food chemistry text. This volume contains a concise, yet well-documented presentation of the current state of knowledge on the content, structure, chemical and biochemical reactivity, functional properties, and biological role of the components most important to food quality. The first two chapters describe in general terms the contents and role of different constituents in food quality and structure. The main components are presented in Chapters 3–7, while Chapter 8 deals with their impact on the rheological properties of foods. Chapters 9 and 10 discuss the effects of different constituents on the color and flavor of foods, while Chapters 11–14 are concerned primarily with the biological value and safety aspects of the constituents. Most chapters have the character of monographs prepared by specialists in the respective areas. They are based on the personal research and teaching experience of the contributors, as well as on critical evaluation of the present state of knowledge as reflected in the current world literature. The large lists of references in the chapters include both English papers and papers published in other languages. This volume is addressed to food scientists in industry and academia, food science graduate students, nutritionists, and all persons interested in the role and attributes of various food components. I am honored to dedicate this volume to Professor Owen R. Fennema, University of Wisconsin – Madison, whom I met in person during three IUFoST congresses. Fennema’s books, especially the excellent Food Chemistry, have been an invaluable source of information and inspiration to me, my students, and probably most food professionals in the world. Zdzislaw E. Sikorski

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Acknowledgment As the editor, I have had the privilege to work with colleagues from universities and research institutions in Australia, The Netherlands, Poland, Taiwan, and the United States, who have contributed to this volume, sharing their knowledge and experience. Their acceptance of my conception of the book and of the editorial suggestions is highly appreciated. Special thanks are due to those contributors who prepared their chapters ahead of the deadline. It was possible to publish the book without delay only because of the understanding of Dr. Eleanor Riemer and Sara Kreisman of CRC Press, who agreed to accept several chapters even after the deadline. I also want to thank several of my coworkers in the department of food chemistry and technology of the Gdan´sk University of Technology, Poland, who willingly helped me in different ways, especially in handling the computer. Last but not least my gratitude goes to my wife, Krystyna, who generously tolerated a husband heavily involved for the past 40 years in writing and editing food science books. Zdzislaw E. Sikorski Gdan´sk University of Technology

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Editor Zdzislaw E. Sikorski received his B.S., M.S., Ph.D., and D.Sc. degrees from the Gdan´sk University of Technology (GUT) and his doctor honoris causa from the Agricultural University in Szczecin, Poland. He served as head of the department of food chemistry and technology and dean of the faculty of chemistry at GUT and was visiting researcher and professor at the Ohio State University, Columbus, Ohio; CSIRO, Hobart, Australia; DSIR, in Auckland, New Zealand; and National Taiwan Ocean University, Keelung. He is currently professor at GUT and, since 1996, chairman of the Committee of Food Technology and Chemistry of the Polish Academy of Sciences. He has published 200 journal articles, 11 books (in Polish, English, Russian, and Spanish), and 8 book chapters in marine food science and food chemistry. He holds seven patents.

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Contributors Agnieszka Bartoszek, Ph.D. Department of Pharmaceutical Technology and Biochemistry Gdan´sk University of Technology Gdan´sk , Poland Maria Bielecka, Ph.D. Professor Division of Food Science Institute of Animal Reproduction and Food Research of the Polish Academy of Sciences Olsztyn, Poland Yan-Hwa Chu, Ph.D. Food Industry Research and Development Institute Taiwan, Republic of China Barbara E. Cybulska, Ph.D. Department of Pharmaceutical Technology and Biochemistry Gdan´sk University of Technology Gdan´sk , Poland Peter E. Doe, Ph.D. Professor Department of Engineering University of Tasmania, Hobart Tasmania, Australia Lucy Sun Hwang, Ph.D. Professor Graduate Institute of Food Science and Technology National Taiwan University Taiwan, Republic of China

Jen-Min Kuo, Ph.D. Professor Department of Food Health Chai-Nan University of Pharmacy and Science Taiwan, Republic of China Tadeusz S. Matuszek, Ph.D. Department of Mechanical Engineering Gdan´sk University of Technology Gdan´sk , Poland Julie Miller Jones, Ph.D. Department of Home Economics College of St. Catherine St. Paul, Minnesota Michal Nabrzyski, Ph.D. Professor Emeritus Department of Bromatology Medical Academy of Gdan´sk Gdan´sk , Poland Krystyna Palka, Ph.D. Department of Animal Food Products Agricultural Academy Kraków, Poland Bonnie Sun Pan, Ph.D. Professor Department of Marine Food Science National Taiwan Ocean University Taiwan, Republic of China

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Adriaan Ruiter, Ph.D. Professor Emeritus Department of the Science of Food of Animal Origin Utrecht University The Netherlands

Alphons G.J. Voragen, Ph.D. Professor Department of Agrotechnology and Food Sciences Wageningen University The Netherlands

Zdzislaw E. Sikorski, Ph.D. Professor Department of Food Chemistry and Technology Gdan´sk University of Technology Gdan´sk , Poland

Jadwiga Wilska-Jeszka, Ph.D. Professor Emeritus Institute of Technical Biochemistry Technical University of £ód´z ´ Poland Lódz,

Piotr Tomasik, Ph.D. Professor Department of Chemistry Academy of Agriculture Kraków, Poland

Chung-May Wu, Ph.D. Professor Department of Food Science and Nutrition Hungkuang Institute of Technology Taiwan, Republic of China

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Table of Contents Chapter 1 Food Components and Their Role in Food Quality ..................................................1 Zdzislaw E. Sikorski Chapter 2 Chemical Composition and Structure of Foods......................................................11 Krystyna Palka Chapter 3 Water and Food Quality ..........................................................................................25 Barbara Cybulska and Peter Edward Doe Chapter 4 Mineral Components ...............................................................................................51 Michal Nabrzyski Chapter 5 Saccharides ..............................................................................................................81 Piotr Tomasik Chapter 6 Food Lipids............................................................................................................115 Yan-Hwa Chu and Lucy Sun Hwang Chapter 7 Proteins ..................................................................................................................133 Zdzislaw E. Sikorski Chapter 8 Rheological Properties of Food Systems ..............................................................179 Tadeusz Matuszek Chapter 9 Food Colorants.......................................................................................................205 Jadwiga Wilska-Jeszka

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Chapter 10 Flavor Compounds.................................................................................................231 Chung-May Wu, Jen-Min Kuo, and Bonnie Sun Pan Chapter 11 Probiotics in Food..................................................................................................259 Maria Bielecka Chapter 12 Major Food Additives............................................................................................273 Adriaan Ruiter and Alphons G.J. Voragen Chapter 13 Food Safety............................................................................................................291 Julie Miller Jones Chapter 14 Mutagenic, Carcinogenic, and Chemopreventive Compounds in Foods.............................................................................................307 Agnieszka Bartoszek Index ......................................................................................................................337

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Food Components and Their Role in Food Quality Zdzislaw E. Sikorski

CONTENTS 1.1

Main Food Components...................................................................................1 1.1.1 Introduction ..........................................................................................1 1.1.2 Contents and Role in Food Raw Materials .........................................2 1.1.3 Factors Affecting Food Composition...................................................4 1.2 Quality of Foods ..............................................................................................5 1.3 Functional Properties of Food Components....................................................5 1.4 Role of Chemistry and Processing Factors .....................................................6 1.4.1 Introduction ..........................................................................................6 1.4.2 Effect on Safety and Nutritional Value................................................7 1.4.3 Effect on Sensory Quality....................................................................7 References..................................................................................................................8

1.1 MAIN FOOD COMPONENTS 1.1.1 INTRODUCTION Foods are derived from plant material, carcasses of animals, and single-cell organisms. They are composed mainly of water, saccharides, proteins, lipids, and minerals (Table 1.1). These main components serve as nutrients by supplying the human body with the necessary building materials and source of energy, as well as elements and compounds indispensable for the metabolism. Some plant polysaccharides are only partly utilized for energy. However, as dietary fiber, they affect various processes in the gastrointestinal tract in different ways (Kritchevsky and Bonfield, 1995). Foods also contain a host of other constituents present in smaller quantities, especially nonprotein nitrogenous compounds, vitamins, colorants, flavor compounds, and functional additives. Many of the minor components originally present in foods are nutritionally essential, e.g., vitamins (some can be utilized by the body) and amino acids. Numerous groups, including tocopherols, ubiquinone, carotenoids, ascorbic acid, thiols, amines, and several other nonprotein 1-5871-6149-4/02/$0.00+$1.50 © 2002 by CRC Press LLC

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Chemical and Functional Properties of Food Components

http://avibert.blogspot.com TABLE 1.1 Main Components in Typical Foods Water

Saccharides

Proteins

Lipids

Minerals

Vitamins

Juices Fruits Milk Vegetables Jellies Lean fish Lean meat

Saccharose Honey Cereals Chocolate Potato Cassava Fruits

Soybean Beans Meat Fish Wheat Cheese Eggs

Oils Lard Butter Chocolate Nuts Egg yolk Pork

Vegetables Fruits Meat Fish products Dairy products Cereals Nuts

Vegetables Fruits Fish liver Meat Cereals Milk Yeast

nitrogenous compounds, serve as endogenous muscle antioxidants, playing an essential role in postmortem changes in meat (Decker et al., 2000). Other minor components are useless or even harmful if present in excessive amounts. Most food raw materials are infected with different microorganisms — putrefactive and often pathogenic — and some contain parasites. A variety of compounds are added intentionally during processing to serve as preservatives, antioxidants, colorants, flavorings, sweeteners, and emulsifying agents and to fulfill different other technological purposes. The chemical nature and role of functional food additives are presented in detail in Chapter 12.

1.1.2 CONTENTS

AND

ROLE

IN

FOOD RAW MATERIALS

Polysaccharides, proteins, and lipids are involved in different structures of the plant and animal tissues used for food. The structures built from these materials are responsible for the form and tensile strength of the tissues and create the necessary conditions for the metabolic processes to occur. Compartmentation resulting from these structures plays a crucial biological role in the organisms. Some other saccharides, proteins, and lipids are stored for reserve purposes. Other constituents are either bound to different cell structures or distributed in soluble form in the tissue fluids. The content of water in various foods ranges from a few percent in dried commodities, e.g., milk powder, through about 15% in grains, 16–18% in butter, 20% in honey, 35% in bread, 65% in manioc, and 75% in meat — to about 90% in many fruits and vegetables. Most of the water is immobilized in the plant and animal tissues by the structural elements and various solutes, contributes to buttressing the conformation of the polymers, and interacts in metabolic processes. Saccharides are present in food raw materials in quantities ranging from about 1% in meats and fish, to about 4.5% in milk, 18% in potatoes, and 15–20% in sugar beets, to about 70% in cereal grains. Polysaccharides participate in the formation of structures in plants. They are also stored in plants as starch and in muscles as glycogen. Other saccharides are dissolved in tissue fluids or perform different biological functions: in free nucleotides, as components of nucleic acids, or bound to proteins and lipids.

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The protein content in foods is given mainly as crude protein, i.e., as N × 6.25. The 6.25 nitrogen-to-protein (N:P) conversion factor has been recommended for most plant and animal food products under the assumption that the N content in their proteins is 16% and they do not contain nonprotein N. The N content in the proteins in various foods, however, is different, since it depends on the amino acid composition. Furthermore, the total N consists of protein N and of N contained in numerous nonprotein compounds, e.g., free peptides and amino acids, nucleic acids and their degradation products, amines, betains, urea, vitamins, and alkaloids. In some foods the nonprotein N may constitute up to 30% of total N. In many of these compounds the C:N ratio is similar to the average in amino acids. However, the N content in urea (47%) is exceptionally high. Most of the nonprotein nitrogen compounds can be utilized by the organism as a source of nitrogen. The average conversion factor for estimation of true protein, based on the ratios of total amino acid residues to amino acid N, determined for 23 various food products is 5.68 and for different classes of foods, 5.14–6.61 (Table 1.2). The N:P factor of 4.39, based on analysis of 20 different vegetables, has been proposed by Fujihara et al. (2001) for estimating the true protein content in vegetables. A common N:P factor of 5.70 for blended foods or diets has been recommended by Sosulski and Imafidon (1990). Proteins make up about 1% of the weight of fruits, 2% of potatoes, 3.2% of bovine milk, 12% of eggs, 12–22% of wheat grain, about 20% of meat, and 25–40% of different beans. They serve as the building material of muscles and other animal tissues and, in plants and animals, play crucial metabolic roles as enzymes and enzyme inhibitors, participate in the transport and binding of oxygen and metal ions, and perform immunological functions. During their development cereal grain and legume seeds deposit large quantities of storage proteins in granules known also as protein bodies. In soybeans these proteins constitute 60–70% of the total protein content, and the granules in 80% are made of proteins. Lipids constitute below 1% of the weight of fruits, vegetables, and lean fish; 3.5% of milk; 6% of beef; 32% of egg yolk; and 85% of butter. The lipids contained in the food raw materials in low quantities serve mainly as components of proteinphospholipid membranes and perform metabolic functions. In fatty commodities the majority of the lipids are stored as depot fat in the form of triacylglycerols. The lipids of numerous food fishes, such as orange roughy, mullets, codfish, and sharks,

TABLE 1.2 N:P Conversion Factors in Foods Product

Factor

Product

Factor

Dairy products Egg Meat and fish Cereals and legumes

6.02–6.15 5.73 5.72–5.82 5.40–5.93

Potato Leafy vegetables Fruits Microbial biomass

5.18 5.14–5.30 5.18 5.78–6.61

Source: From Sosulski, F.W. and Imafidon, G.I., J. Agric. Food Chem., 38, 1351, 1990.

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as well as some crustaceans and mollusks, also comprise wax esters. Some shark oils are very rich in hydrocarbons, particularly in squalene (Sikorski et al., 1990). Furthermore, the lipid fraction of food raw materials harbors different sterols, vitamins, and pigments that are crucial for metabolism.

1.1.3 FACTORS AFFECTING FOOD COMPOSITION The content of different components in food raw materials depends on the species and variety of the animal and plant crop, the conditions of cultivation and harvesting of the plants, the feeding and age of the farm animals or the season in which fish and marine invertebrates are caught, and postharvest changes taking place in the crop during storage. The food industry, by establishing quality requirements for raw materials, can encourage producers to control within limits the contents of the main components in their crops, e.g., saccharose in sugar beets, starch in potatoes, fat in various meat cuts, pigments in fruits and vegetables and in the flesh of fish from aquaculture, or protein in wheat and barley, as well as the fatty acid composition of lipids in oilseeds and meats. The contents of desirable minor components such as natural antioxidants can also be effectively controlled to retard the oxidation of pigments and lipids in beef meat (Matsumoto, 2000). Contamination of the raw material with organic and inorganic pollutants can be controlled by observing recommended agricultural procedures in using fertilizers, herbicides, and insecticides and by restricting certain fishing areas seasonally to avoid marine toxins. The size of predatory fish like swordfish, tuna, or sharks that are fished commercially can be limited to reduce the risk of too high a content of mercury and arsenic in the flesh. The composition of processed foods depends on the applied recipe and on changes taking place due to processing and storage. These changes are mainly brought about by endogenous and microbial enzymes, active forms of oxygen, heating, chemical treatment, and processing at low or high pH (Haard, 2001). Examples of such changes are: • Leaching of soluble components, e.g., vitamins and minerals during washing, blanching, and cooking • Drip formation after thawing or due to cooking • Loss of moisture and volatiles due to evaporation and sublimation • Absorption of desirable or harmful compounds during salting, pickling, or smoking • Formation of desirable or harmful compounds due to enzyme activity, e.g., development of a typical flavor in cheese or decarboxylation of amino acids in fish marinades • Generation of desirable or objectionable products due to interactions of reactive groups induced by heating or chemical treatment, e.g., flavors or carcinogenic compounds in roasted meats or trans fatty acids in hydrogenated fats • Formation of different products of oxidation of food components, mainly of lipids, pigments, and vitamins • Loss of nutrients and deterioration of dried fish due to the attack by flies, mites, and beetles

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1.2 QUALITY OF FOODS The quality of a food product, i.e., the characteristic properties that determine the degree of excellence, is a sum of the attributes contributing to the satisfaction of the consumer with the product. The composition and the chemical nature of the food components affect all aspects of food quality. The total quality reflects at least the following attributes: • Compatibility with the local or international food laws, regulations, and standards, concerning mainly proportions of main components, presence of compounds regarded as identity indicators, contents of contaminants and additives, hygienic requirements, and packaging • Nutritional aspects, i.e., the contents of nutritionally desirable constituents, mainly proteins, essential amino acids, essential fatty acids, vitamins, fiber, and mineral components • Safety aspects affected by the compounds that may constitute health hazards for the consumers and affect the digestibility and nutritional use of the food, e.g., heavy metals, toxins of different origin, pathogenic microorganisms, parasites, and enzyme inhibitors • Sensory attributes — color, size, form, flavor, and taste — and rheological properties, obviously affected by the chemical composition of the product, as well as the effects resulting from processing and culinary preparation • Shelf life at specific storage conditions • Convenience aspects, which are reflected by the size and ease of opening and reclosing the container, suitability of the product for immediate use or for different types of thermal treatment, ease of portioning or spreading, and transport and storage requirements • Ecological aspects regarding suitability for recycling of the packaging material and pollution hazards For many foods one of the most important quality criterion is freshness. This is especially so in numerous species of vegetables, fruits, and seafood. Fish of valuable species at the state of prime freshness, suitable to be eaten raw, may have a market price that is ten times higher than that of the same fish after several days of storage in ice but still very fit for human consumption. The characteristic freshness attributes of different foods are usually evaluated by sensory examination and by determination of specific indices, e.g., nucleotide degradation products in fish.

1.3 FUNCTIONAL PROPERTIES OF FOOD COMPONENTS The term functional properties has evolved to have a broad range of meanings. That corresponding to the term technological properties implies that the given component present in optimum concentration, subjected to processing at optimum parameters, contributes to the desirable sensory characteristics of the product, usually by interacting with other food constituents. Hydrophobic interactions,

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Chemical and Functional Properties of Food Components

hydrogen bonds, ionic forces, and covalent bonding are involved. Thus the functional properties of food components are affected by the number of accessible reactive groups and by the exposure of hydrophobic areas in the given medium. Therefore the functional properties displayed in a system of given water activity and pH and in the given range of temperature can be to a large extent predicted from the structure of the respective saccharides, proteins, and lipids. They can also be improved by appropriate, intentional enzymatic or chemical modifications of the molecules, mainly those that affect the size, charge density, or hydrophilic and hydrophobic character of the compounds, or by changes in the environment, regarding both the solvent and other solutes. The functional properties of food components make it possible to manufacture products of desirable quality. Thus pectins contribute to the characteristic texture of ripe apples and make perfect jellies. Various other polysaccharides are good thickening and gelling agents at different ranges of acidity and concentration of various ions. Alginates in the presence of Ca2+ form protective, unfrozen gels on the surface of frozen products. Some starches are resistant to retrogradation, thereby retarding staling of bread. Fructose retards moisture loss from biscuits. Mono- and diacylglycerols, phospholipids, and proteins are used for emulsifying lipids and stabilizing food emulsions and foams. Antifreeze proteins decrease ice formation in various products, and gluten plays a major role in producing the characteristic texture of wheat bread. Technologically required functional effects can also be achieved by intentionally employing various food additives — food colors, sweeteners, and a host of other compounds — that are not regarded as foodstuffs per se, but are used to modify the rheological properties or acidity, increase the color stability or shelf life, or act as humectants or flavor enhancers (Rutkowski et al., 1997). During the recent two decades the term functional has also been given to a large group of products and components, also called designer foods, pharmafoods, nutraceuticals, or foods for specific health use, that are regarded as health enhancing. These foods, mainly drinks, meals, confectionery, ice cream, and salad dressings, contain various ingredients (e.g., oligosaccharides, sugar alcohols, or choline) that are claimed to have special physiological functions like neutralizing harmful compounds in the body and promoting recovery and general good health (Goldberg, 1994). Foods containing probiotics, mainly dairy products, have been treated in detail in Chapter 11.

1.4 ROLE OF CHEMISTRY AND PROCESSING FACTORS 1.4.1 INTRODUCTION The chemical nature of food components is of crucial importance for all aspects of food quality. It decides on the nutritional value of the product, its sensory attractiveness, the development of desirable or deteriorative changes due to interactions with other constituents and to processing, and the susceptibility and resistance to spoilage during storage. Food components that contain reactive groups, many of them essential for the quality of the products, are generally labile and

Food Components and Their Role in Food Quality

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easily undergo different enzymatic and chemical changes, especially when treated at elevated temperatures or in conditions promoting the generation of active species of oxygen.

1.4.2 EFFECT

ON

SAFETY

AND

NUTRITIONAL VALUE

Food is regarded as safe if it does not contain harmful organisms or compounds in concentrations above the accepted limits (see Chapter 13). The nutritional value of foods depends primarily on the contents of nutrients and nutritionally objectionable components in the products. Processing may increase the safety and biological value of food by inducing chemical changes, increasing the digestibility of the components, or by inactivating undesirable compounds, e.g., toxins or enzymes catalyzing the generation of toxic agents from harmless precursors. Freezing and short-term frozen storage of fish inactivate the parasite Anisakis, which could escape detection during visual inspection of herring fillets used as raw material for cold marinades produced at mild conditions. Thermal treatment brings about inactivation of myrosinase, the enzyme involved in hydrolysis of glucosinolanes. This arrests the reactions that lead to the formation of goitrogenic products in oilseeds of Cruciferae. Heat pasteurization and sterilization reduce the number of vegetative forms and spores, respectively, to the acceptable level of pathogenic microorganisms. Several other examples of such improvements of the biological quality of foods are given in the following chapters of this book. However, there are also nutritionally undesirable side effects of processing: destruction of essential food components as a result of heating, chemical treatment, and oxidation. Generally known is the partial thermal decomposition of vitamins, especially thiamine, loss of available lysine and sulfur-containing amino acids, or generation of harmful compounds (e.g., carcinogenic heterocyclic aromatic amines, lysinoalanine, and lanthionine or position isomers of fatty acids) not originally present in foods. Thanks to the unprecedented development of analytical chemistry, applying efficient procedures of enrichment and separation, combined with the use of highly selective and sensitive detectors, has made it possible to determine various products of chemical reactions in foods, even in very low concentrations. In recent years new evidence of side effects has been accumulated in respect to chemical processing of oils and fats. Commercial hydrogenation of oils not only brings about the intended saturation of selected double bonds in the fatty acids, and thereby the required change in the rheological properties of the oil, but also results in the generation of a large number of trans-trans and cis-trans isomers that are absent in unprocessed oils.

1.4.3 EFFECT

ON

SENSORY QUALITY

Many of the desirable sensory attributes of foods stem from the properties of the raw material: the color, flavor, taste, and texture of fresh fruits and vegetables or the taste of nuts and milk. These properties are in many cases carried through to the final products.

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Chemical and Functional Properties of Food Components

In other commodities the characteristic quality attributes are generated in processing. The texture of bread develops due to interactions of proteins, lipids, and saccharides with each other and with various gases (Eliasson, 1998; Wrigley et al., 1998; Preston, 1998). The bouquet of wine is due to fermentation of saccharides and a number of other biochemical and chemical reactions. The delicious color, flavor, texture, and taste of smoked salmon are generated as a result of enzymatic changes in the tissues and the effect of salt and smoke (Doe et al., 1998). Optimum foam performance of beer depends on the interactions of peptides, lipids, the surfaceactive components of hop, and gases (Hughes, 1999). The flavor, texture, and taste of cheese result from fermentation and ripening, while the appealing color and flavor of different fried products are due to reactions of saccharides and amino acids (Sikorski, 2001). The sensory attributes of foods are related to the contents of many chemically labile components. These components, however, just as most nutritionally essential compounds, are prone to deteriorative changes in severe heat treatment conditions, oxidizing conditions, or application of considerably high doses of chemical agents (e.g., acetic acid or salt), which are often required to ensure safety and sufficiently long shelf life of the products. Thus loss in sensory quality takes place in oversterilized meat products, due to degradation of sulfur containing amino acids and the development of an off-flavor; in toughening of the texture of overpasteurized ham or shellfish, due to excessive shrinkage of the tissues and drip; and in deterioration of the texture and arresting of ripening in herring, due to preservation at too high a concentration of salt. Optimum parameters of storage and processing ensure the retention of the desirable properties of the raw material and lead to development of intended attributes of the product. In the selection of these parameters the chemistry of food components and of the effect of processing must be studied. The eager food technology student can find all the necessary information in at least two excellent textbooks on food chemistry, published by Belitz et al. (2001) and Fennema (1996); in numerous books on food lipids, proteins, and saccharides; and in current international journals.

REFERENCES Belitz, H.D., Grosch, W., and Schieberle, P., Lehrbuch der Lebensmittelchem, 4th ed., Springer-Verlag, Berlin, 2001. Decker, E.A., Livisay, S.A., Zhou S., Mechanism of endogenous skeletal muscle antioxidants: chemical and physical aspects, in Antioxidants in Muscle Foods: Nutritional Strategies to Improve Quality, Decker, E., Faustman, C., and Lopez-Bote, C.J., Eds., Wiley Interscience, New York, 2000, p. 25. Doe, P. et al., Basic principles, in Fish Drying and Smoking: Production and Quality, Doe, P.E., Ed., Technomic Publishing Co., Inc., Lancaster, PA, 1998, p. 13. Eliasson, A.C., Lipid-carbohydrate interactions, in Interactions: The Keys to Cereal Quality, Hamer, R.J. and Hoseney, R.C., Eds., American Association of Cereal Chemists, Inc., St. Paul, MN, 1998, p. 47. Fennema, O.R., Ed., Food Chemistry, 3rd ed., Marcel Dekker, New York, 1996.

Food Components and Their Role in Food Quality

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Fujihara, S., Kasuga, A., and Aoyagi, Y., Nitrogen-to-protein conversion factors for common vegetables in Japan, J. Food Sci., 66, 412, 2001. Goldberg, I., Functional Foods: Designer Foods, Pharmafoods, Nutraceuticals, Chapman & Hall, New York, 1994. Haard, N.F., Enzymic modification in food systems, in Chemical and Functional Properties of Food Proteins, Sikorski, Z.E., Ed., Technomic Publishing Co., Lancaster, PA, 2001, p. 155. Hughes, P., Keeping a head: optimizing beer foam performance, in Bubbles in Food, Campbell, G.M. et al., Eds., Eagan Press, St. Paul, MN, 1999, p. 129. Kritchevsky, D. and Bonfield, Ch., Eds., Dietary Fiber in Health & Disease, Eagan Press, St. Paul, MN, 1995. Matsumoto, M., Dietary delivery versus exogenous addition of antioxidants, in Antioxidants in Muscle Foods: Nutritional Strategies to Improve Quality, Decker, E., Faustman, C., and Lopez-Bote, C.J., Eds., Wiley Interscience, New York, 2000, p. 315. Preston, K.R., Protein-carbohydrate interactions, in Interactions: The Keys to Cereal Quality, Hamer, R.J. and Hoseney, R.C., Eds., American Association of Cereal Chemists, Inc., St. Paul, MN, 1998, p. 81. Rutkowski, A., Gwiazda, S., and Dabrowski, ˛ K., Food Additives and Functional Component, Agro & Food Technology, Katowice, 1997 (in Polish). Sikorski, Z.E., Chemical reactions in proteins in food systems, in Chemical and Functional Properties of Food Proteins, Sikorski, Z.E., Ed., Technomic Publishing Co., Inc., Lancaster, PA, 2001, p. 191. Sikorski, Z.E., Kolakowska , A., and Pan, B.S., The nutritive composition of the major groups of marine food organisms, in Seafood: Resources, Nutritional Composition, and Preservation, Sikorski, Z.E., Ed., CRC Press, Boca Raton, FL, 1990, p. 29. Sosulski, F.W. and Imafidon, G.I., Amino acid composition and nitrogen-to-protein conversion factors for animal and plant foods, J. Agric. Food Chem., 38, 1351, 1990. Wrigley, C.W. et al., Protein-protein interactions: essential to dough rheology, in Interactions: The Keys to Cereal Quality, Hamer, R.J. and Hoseney, R.C., Eds., American Association of Cereal Chemists, Inc., St. Paul, MN, 1998, p. 17.

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2

Chemical Composition and Structure of Foods Krystyna Palka

CONTENTS 2.1 2.2

Introduction ....................................................................................................11 Protein Food Products....................................................................................12 2.2.1 Meat....................................................................................................12 2.2.2 Milk and Milk Products.....................................................................14 2.2.3 Eggs....................................................................................................15 2.3 Saccharide Food Products..............................................................................16 2.3.1 Cereal and Cereal Products................................................................16 2.3.2 Potatoes ..............................................................................................18 2.3.3 Honey .................................................................................................20 2.3.4 Nuts ....................................................................................................20 2.3.5 Seeds of Pulses...................................................................................20 2.4 Edible Fats......................................................................................................21 2.5 Fruits and Vegetables .....................................................................................21 References................................................................................................................23

2.1 INTRODUCTION Foods are edible fragments of plant or animal tissues in a natural or processed state that, after being eaten and digested in the human organism, may be a source of different nutrients. Taking as a base the dominant nutritional component, food products may be divided into four groups: • • • •

Protein food products Saccharide food products Edible fats Fruits and vegetables

The particular groups of chemical constituents participate in building the structure of food products as components of specialized tissues. For this reason this chapter presents, in addition to chemical composition, the morphology of the selected products from each group. 1-5871-6149-4/02/$0.00+$1.50 © 2002 by CRC Press LLC

11

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Chemical and Functional Properties of Food Components

2.2 PROTEIN FOOD PRODUCTS 2.2.1 MEAT Meat is the edible part of animal, chicken, or fish carcasses. Its chemical composition is as follows: 60–85% water, 8–23% protein, 2–15% lipids, 0.5–1.5% saccharides, and about 1% inorganic substances (Table 2.1). These quantities change significantly depending on the kind, age, sex, level of fattening, and part of the animal carcass. The largest fluctuations are observed in the contents of water and lipids. Water is a solvent of organic and inorganic substances and an environment of biochemical reactions. It also participates in the maintenance of meat protein conformation. Meat proteins include sarcoplasmic, myofibrillar, and connective tissue proteins. Among the sarcoplasmic proteins are heme pigments and enzymes, which influence the color, smell, and structure of meat. Myofibrillar proteins and collagen are able to retain and hold water in meat structure and to emulsify fat. Therefore they influence the rheological properties of meat products. Mineral elements are in enzymatic complexes and other structures that play an important biochemical role. They can affect the technological properties of meat, e.g., water-holding capacity, as well as the sensory characteristics. Meat is also a good source of B group vitamins. The main structural unit of striated muscle tissue is a multinucleus cell called muscle fiber. Its length varies from several millimeters to hundredths of a millimeter, and the diameter is within the range 10–100 µm (Figure 2.1a). The thickness of muscle fibers affects the meat tenderness. The muscle fiber contains typical somatic cell compounds, sarcoplasmic reticulum, and myofibrils. The sarcoplasmic reticulum has the capacity of reversible binding of calcium ions. Myofibrils are the main structural element of muscle fiber, making up 80% of its volume. They have a diameter of 1–2 µm and are situated parallel to the long axis of the fiber (Figure 2.1b). The spaces between myofibrils are filled up with a semiliquid sarcoplasm, which forms the environment of enzymatic reactions and takes part in conducting nervous impulses into the muscle. Each myofibril consists of two different protein structures: myofilaments. These are myosin thick (15 nm × 1.5 µm) and thin (7 nm × 1 µm) filaments made from actin, tropomyosin, and troponin. Inside the muscle fiber there is also a cytoskeleton — the protein structures assuring the integrity of muscle cells. Cytoskeletal proteins such as titin and nebulin are located in myofibrils and anchored in the Z line. Desmin is made up of costamers, which connect the myofibrils; vinculin connects myofibrils and sarcolemma. Postmortem changes in cytoskeletal proteins probably play a role in the improvement of meat functional properties, especially its tenderness and water-holding capacity. The muscle fiber is covered by a thin membrane called sarcolemma and a layer of connective tissue called endomysium. Bundles of muscle fibers are surrounded by perimysium, and whole muscle is surrounded by epimysium. At the ends of the muscle epimysium forms tendons, which connect the muscle to the bone (Figure 2.2). Both the quantity and kind of connective tissue affect the technological and nutritional properties of meat.

Chemical Composition and Structure of Foods

13

TABLE 2.1 Chemical Composition of Foods Rich in Proteins

Product

Water (%)

Crude Protein Nx6.25 (%)

Lipids (%)

Saccharides (%)

Mineral Components (%)

Beef, lean Pork, lean Veal Lamb Chicken: Light meat Dark meat Herring Oyster Cow milk Sheep milk Sour cream (25%) Yogurt, low fat Quarg Ripened cheese Milk powder Whole egg, without shell White Yolk Whole egg powder

71.5 72.0 75.0 71.5

21.0 20.0 20.0 19.5

6.5 7.0 3.5 7.0

1.0 1.0 1.0 1.5

1.0 1.0 1.0 1.0

75.0 76.0 60.0 85.0 88.0 82.0 68.0 85.0 64.0–75.0 35.0–50.0 3.0

23.0 20.0 18.0 7.5 3.0 6.0 3.0 5.0 9.0–14.0 20.0–35.0 26.0

2.0 4.5 15.5 1.5 3.5 6.5 25.0 1.0 12.0–18.0 20.0–30.0 26.0

1.0 1.0 0.5–1.5 0.5–1.5 4.5 4.5 4.0 7.5 2.5 2.0 38.0.

1.0 1.0 0.5 0.7 1.5 5.0 6.0

73.5 88.0 48.5 3.5

13.0 11.0 16.0 47.5

12.0 traces 32.0 43.0

1.0 0.5 1.0 to 0.5

1.0 0.5 1.0 4.0

Source: Adapted from Hedrick, H.B. et al., Principles in Meat Science, Kendall-Hunt Publishing Co., Dubuque, 1994; Kirk, R.S. and Sawyer, R., Pearson’s Composition and Analysis of Foods, Longman Science, London, 1991; Renner, E., Cheese: Chemistry, Physics and Microbiology, Vol. 1, Fox, P.F., Ed., Chapman & Hall, London, 1993, 557; Sikorski, Z.E., Seafood Raw Materials, WNT, Warsaw, 1992; Tamime, A.Y. and Robinson, R.K., Yoghurt: Science and Technology, CRC Press, Boca Raton, FL, 1999.

FIGURE 2.1 Scanning electron microscope (SEM) micrographs of bovine semitendinosus muscle: transverse section (a) and longitudinal section (b). F, muscle fiber; MF, myofibril. (From Palka, K., unpublished.)

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Chemical and Functional Properties of Food Components

FIGURE 2.2 Schematic structure of skeletal muscle: tendon (1), epimysium (2), perimysium (3), endomysium (4), sarcolemma (5), myofibril (6), muscle fiber (7), and bundle of muscle fibers (8).

2.2.2 MILK

AND

MILK PRODUCTS

Milk is a liquid secretion of the mammary glands of female mammals, consisting of 80–90% water and 10–20% dry mass. It is an oil-in-water (O/W) emulsion composed of fat and fat-soluble vitamins; the aqueous phase contains proteins, mineral salts, lactose, and water-soluble vitamins. The chemical composition of milk (Table 2.1) depends on species and breed, lactation period, and nutritional and health conditions of the animal. The proteins of milk are made up of caseins and whey proteins. Milk proteins, caseins, and several enzymes, mainly hydrolases and oxidoreductases, are very important in the manufacturing of cheeses and yogurts (Figure 2.3). After drying they are used in the food industry as milk powder, caseinates, and casein hydrolyzates. Nonprotein nitrogenous compounds constitute about 0.2% of milk.. The milk fat is made up of about 98% triacylglycerols and 1% phospholipids. It also contains smaller amounts of di- and monoacylglycerols, sterols, higher fatty acids,

FIGURE 2.3 SEM micrograph of protein matrix in yogurt. (From Domagala, J., unpublished. With permission.)

Chemical Composition and Structure of Foods

15

carotenoids, and vitamins. In cow milk fat over 500 fatty acid residues have been identified. The polyenoic fraction constitutes about 3% of the total fatty acids and is composed mainly of linoleic and oc-linolenic acids. The milk fat is easily digestible because of a relatively low melting temperature and great dispersion (droplets of 5–10 µm in diameter). Because of the latter, it is susceptible to hydrolysis and oxidation. The main saccharide of milk is lactose. During heat treatment of milk, lactose is involved in Maillard reactions. Lactose is used for the production of baby formulas, low-caloric foods, bread, drugs, and microbiological media. The milk minerals are composed mainly of calcium and phosphorus in the form of calcium phosphate. Phosphorus is also present in milk in the form of phosphoproteins. These components have important nutritional and technological significance. The total content of Ca and P in milk is about 0.12 and 0.10%, respectively. About 6–9% of milk volume is made up of gases, mainly CO2, N2, and O2. Oxygen present in milk may cause oxidation of unsaturated fatty acids. For this reason air is removed from milk during processing. Milk contains vitamins essential for the growth and development of young organisms, especially vitamins from the B group and vitamin A. The quantity of vitamin A depends on the season.

2.2.3 EGGS The hen egg consists of a shell, egg white, and yolk (Figure 2.4). The 0.2- to 0.4mm-thick shell constitutes 10–12% of the egg mass and consists of about 3.5% organic and 95% mineral components. The shell has a many-layer structure. Two of the layers are made of keratin and collagen fibers. The next two layers are calcinated and on the surface are covered by a thin membrane (cuticula) that contains two thirds of the shell pigments. The shell protects the egg against microbiological contamination and makes the exchange of gases possible.

FIGURE 2.4 Schematic structure of hen egg: shell (1), membranes (2), air chamber (3), rare white (4), dense white (5), yolk (6), and chalazae (7).

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Chemical and Functional Properties of Food Components

The egg white — about 60% of the egg mass — composed mainly of water and a mixture of proteins, has a many-layer structure too. Starting from the shell there are four fractions of white: external thin, external thick, internal thin, and internal thick, which make up 23, 57, 17, and 2.5%, respectively, of the egg white mass. Mucin structures called chalazae keep the yolk in central position in the egg. During long storage the chalazae lose their elasticity, and the egg white loses part of its water, due to evaporation. The egg yolk — about 30% of the egg mass — has a spherical shape, a diameter of about 3–3.5 cm, and a color ranging from dark to light orange, depending on the quantity of lipids and carotenoid pigments in the fodder. It is surrounded by a thin and elastic vittelin membrane build of keratin and mucin fibers. The egg yolk, being an O/W emulsion, stabilized by lecithin, has a very high viscosity. The viscosity of the yolk decreases during storage, as a result of water permeation from the white through the vittelin membrane. Egg yolks are utilized as a stabilizer in manufacturing mayonnaise. The chemical composition of the egg (Table 2.1) is rather stable. As the only source of food for the embryo, it contains all substances essential for life. There is about 6.6 g of very well-balanced proteins in one egg. About two thirds of yolk mass are lipids, mainly unsaturated. Cholesterol makes up about 2.5% of the dry mass of the yolk. The egg is also a source of vitamins A, B, D, E, and K and the best dietary source of choline. The minerals S, K, Na, P, Ca, Mg, and Fe are in free form or bound to proteins and lipids. The eggs and egg products, thanks to their texture-improving properties, emulsifying effect, and foaming ability, are multifunctional components used in food technology in liquid or dried form.

2.3 SACCHARIDE FOOD PRODUCTS 2.3.1 CEREAL

AND

CEREAL PRODUCTS

Cereals are fruits of cultivated grasses that may be used as raw materials for production of food and feed. The major cereals are: wheat, rye, barley, oats, millet, rice, sorghum, and maize. The share of cereal products in the human diet is estimated at 50–60%. The shape of grains varies from elongated (rye) to spherical (millet), but the anatomical structure of cereal grains is rather similar. The essential anatomical elements of cereal grains are: seed coat (bran), endosperm, and germ. Commercially the most important cereal is wheat. A wheat grain is about 1 cm long and has a diameter of 0.5 cm. It is egg-shaped with a deep crease running along one side and a number of small hairs, called the beard, at one end (Figure 2.5). The grain is surrounded by a five-layer coat called bran that makes up 15% of the mass of the whole grain. It is rich in B vitamins and contains about 50% of the total mass of minerals of the grain. The bran consists of cellulose and is indigestible for humans. It is separated during flour production and used as animal fodder. The germ, about 3% of the mass of the grain, is situated at the base of the grain. It contains the embryo, which is rich in lipids, proteins, B vitamins, vitamin E, and

Chemical Composition and Structure of Foods

17

FIGURE 2.5 Schematic structure of wheat grain: longitudinal section (a) and transverse section (b); beard (1), bran (2), endosperm (3), crease (4), scutellum (5), and germ (6).

minerals, mainly iron. A membranous tissue called scutellum separates the germ from the endosperm. It is a rich source of thiamine — about 60% of all its content in the grain. The starchy endosperm makes up 80–90% of the wheat grain and is a reserve of food for the germ. The starch granules are embedded in a protein matrix, while the periphery of the endosperm is composed of a single aleurone layer. The aleurone layer is rich in proteins and contains high amounts of minerals, vitamins, and enzymes. However, it is usually removed during milling. Considering the size, most of the starch granules in the endosperm cells of wheat may be located in two ranges — large, 15–40 µm in diameter; and small, 1–10 µm in diameter — whereas those in the subaleurone endosperm cells are 6–15 µm in diameter. The chemical composition of cereal (Table 2.2) is dependent on species, cultivate, and time and conditions of growth, harvest, and storage. The starch constitutes about 80% of the grain dry mass. In bread making the most important properties of starch are its water-holding capacity, gelatinization, and susceptibility to hydrolysis. The protein content of cereal grains is in the range of 7–18%. From the technological point of view, proteins, mainly gluten proteins, as well as enzymes — amylases, proteases, and lipases — are important during dough making. Cereal grains also contain 2–4% of lipids, mainly triacylglycerols of unsaturated fatty acids and phospholipids.

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Chemical and Functional Properties of Food Components

TABLE 2.2 Chemical Composition of Cereals and Cereal Products

Product

Water (%)

Crude Protein Nx6.25 (%)

Saccharides (%)

Lipids (%)

Mineral Components (%)

Grains Wheat Rye Maize Rice paddy Millet

15.0 15.0 15.0 15.0 15.0

11.0 9.0 10.0 7.5 10.5

68.5 70.5 67.0 75.5 65.0

2.0 1.5 4.5 0.5 4.0

1.5 1.5 1.5 1.0 3.0

Wheat flour (97%) Wheat flour (50%) Rye flour (97%) Rye flour (60%)

13.5 13.5 13.5 13.5

10.0 8.5 7.5 5.5

70.5 75.0 73.0 78.5

3.0 1.5 2.0 1.5

1.5 0.5 1.5 0.5

Wheat bread Rye bread Rusks

37.5 46.0 7.0

8.0 6.5 8.5

57.5 45.0 75.0

1.5 1.0 5.5

2.0 2.0 1.5

Flour

Bread

Source: Adapted from Fox, B.A. and Cameron, A.G., Food Science: A Chemical Approach, Hodder and Stoughton, London, 1986; Kent, N.L., Technology of Cereals with Special Reference to Wheat, Pergamon Press, Oxford, 1975.

The mineral elements, mainly P and K, and to a smaller extent, also Mg and Ca, make about 2% of the grain mass. Vitamins of the B group and vitamin E are also present in the grains. The milling technique can be modified to increase or decrease the yield of flour from a given amount of grain. The percentage of flour produced is termed the extraction rate of flour. Whole flour, containing the bran, germ, scutellum, and endosperm of the grain, has an extraction rate of 100%. An extraction rate of 70% means that the flour is almost entirely composed of crushed endosperm. As the percentage of flour increases, the amount of dietary fiber in flour increases too. It is an important nutritional aspect connected with cereal products.

2.3.2 POTATOES The potato is a swollen underground stem or tuber that contains a store of food for the plants. In the tuber a bud end and a stem end can be distinguished. The hollows, called eyes, are spirally arranged around the tuber surface. The tuber section is divided into the pith, parenchyma, vascular system, cortex, and periderm (Figure 2.6). Each potato tuber is a single living organism, and its water is indispensable in all the vital processes. Water transports any substances moving in the interior of the

Chemical Composition and Structure of Foods

19

FIGURE 2.6 Schematic structure of potato, longitudinal section: eye (1), periderm (skin) (2), parenchyma (3), vascular ring (4), and pith (5).

tuber. It also protects the tubers against overheating (by transpiration). The water constitutes about 75% of the potato (Table 2.3). The major constituent of the potato is starch (about 20%). With regard to starch content there are potato cultivates of a low (to 14%), medium (15–19%), and high (above 20%) starch content. Potato is also a valuable source of ascorbic acid — up to 55 mg/100 g. Its ash consists of about 60% K and 15% P2O5. The chemical composition of potato tubers changes during storage, due to evaporation and catabolic processes. In many parts of the world potatoes are the main saccharide source in human food and animal fodder and are also widely used as raw material for starch manufacture and in the fermentation industry.

TABLE 2.3 Chemical Composition of Potato and Honey Potato Water Dry matter: Starch Saccharides Proteins Cellulose Lipids Mineral components

% 76.0 24.0 17.5 1.5 2.0 1.0 0.5 1.0

Honey Water Saccharides: Fructose Glucose Maltose Trisaccharides Saccharose Proteins, vitamins, and mineral components

% 17.0 82.5 38.5 31.0 7.0 4.0 1.5 0.5

Source: Adapted from Lisi´n ska, G. and Leszczy´nski, W., Potato Science and Technology, Elsevier Applied Science, London, 1989; Ramsay, I., Honey as a food ingredient, Food Ingredient Process. Int., 10, 16, 1992.

20

Chemical and Functional Properties of Food Components

FIGURE 2.7 SEM micrographs of starch granules in different starchy raw materials: potato (a), wheat (b), and maize (c). (From Juszczak, L., unpublished. With permission.)

The shape and size of starch granules are specific for different starchy raw materials (Figure 2.7).

2.3.3 HONEY Honey is produced by honeybees from the flower nectar of plants. Fresh honey is a clear, very aromatic, dark-amber-colored liquid. It is very sticky and hygroscopic, with a density of about 1.40 g/cm3. Honey is an oversaturated solution of glucose and fructose, easy crystallizing. After crystallization its color is brighter. It is a very stable product. At a temperature of 8–10°C and a humidity of 65–75% it may be stored for many years. Honey is a high-caloric food easy assimilated by the human organism. Honey is used in the manufacturing of alcoholic beverages, i.e., in wine production. In medicine it is prescribed for heart, liver, stomach, skin, and eye illnesses. In the food industry honey is used as a very effective sweetener (25% sweeter than sucrose); a very well binding, concentrating, and covering additive; and a taste intensifier. The chemical composition of honey (Table 2.3) is dominated by glucose and fructose. Honey also contains many other valuable components, like enzymes, organic acids, mineral elements, nonprotein nitrogenous compounds, vitamins, aroma substances, and pigments.

2.3.4 NUTS Nuts are composed of a wooden-like shell and a seed, covered by a yellow or brown skin. Each part makes up about 50% of the nut mass. Inside of the seed is a germ. The seeds of nuts consist of about 60% lipids rich in unsaturated fatty acids, 16–20% easily digested proteins, 7% saccharides, vitamins B1 (10 mg/100 g) and C (30–50 mg/100 g), and P, Mg, K, and Na. With regard to their high quantity of easily assimilated nutrients, nuts may be used in diets of convalescents and children.

2.3.5 SEEDS

OF

PULSES

To this group belong peas, beans, lentils, soybeans, and peanuts. All of them have fruits in the form of pods. Their shape and size depend on the cultivar. Inside the pod are seeds used as raw material in the food industry.

Chemical Composition and Structure of Foods

21

The dry mass of pulse seeds consists of saccharides (14–63%), proteins (28–44%), and lipids (1–50%). The other constituents are mineral elements (mainly K and P) and vitamins from the B group. Soybean is the most valuable pod plant, due to its high quantity and good quality of protein. Soy products in the form of meat extenders and analogs are used all over the world. Soybean is also a raw material in the oil industry.

2.4 EDIBLE FATS Food products like butter, lard, margarine, and plant oils are regarded as “visible” fats. They make up about 45% of the total fat consumed by man, while the “invisible” fats, which are natural components of foods like meat, fish, eggs, and bakery products, make up about 55%. Visible fats are composed mainly of triacylglycerols. They also contain fatsoluble vitamins A, D, and E and additives added during processing, like antioxidants, colorants, or preservatives. The consistency of fats depends on the content of unsaturated fatty acid residues. The oils of plant and fish origin are rich in long-chain polyenic fatty acids. Butter consists of 16–18% water, 80–82.5% lipids, 0.5% proteins, and 0.5% saccharides.

2.5 FRUITS AND VEGETABLES Fruits and vegetables are rich sources of vitamins and minerals, as well as terpenes, flavonoids, tannins, chinons, and phytoncides. They make food more attractive because of smell and color. The fruits and vegetables are living organisms, and their chemical composition is very changeable. The predominant constituent of fruits and vegetables is water, which may represent up to about 96% of the total weight of the crop. The water in fruits and vegetables may be in free or bound form. A relatively high amount of free water improves the taste of fruits and vegetables consumed in their raw state, as well as the accessibility of soluble components. Most of the solid matter of fruits and vegetables is made of saccharides and smaller amounts of protein and fat. The total saccharide content in the fresh weight of fruits and vegetables ranges from about 2% in some pumpkin fruits to above 30% in starchy vegetables. Generally vegetables contain less than 9% saccharides. The polysaccharides — cellulose and hemicellulose — are largely confined to the cell walls. The di- and monosaccharides — sucrose, glucose, and fructose — are accumulated mainly in the cell sap. The proportions of the different saccharide constituents can fluctuate due to metabolic activity of the plant, especially during fruit ripening. The majority of proteins occurring in fruits and vegetables play enzymatic roles that are very important in the physiology and postmortem behavior of the crop. The protein content in vegetables is lower than 3%, except in sweet maize (above 4%). In fruits it ranges from below 1% to above 1.5%. They are found mainly in the cytoplasmic layers.

22

Chemical and Functional Properties of Food Components

The lipids of fruits and vegetables are, like the proteins, largely confined to the cytoplasmic layers, in which they are especially associated with the surface membranes. Their content in fruits and vegetables is always lower than 1%. Lipid and lipid-like fractions are particularly prominent in the protective tissues at the surfaces of plant parts — epidermal and corky layers. Plant tissues also contain organic acids formed during metabolic processes. For this reason fruits and vegetables are normally acidic in reaction. The quantity of organic acids is different, from very low, about 2 miliequivalents of acid/100 g in sweet maize and pod seeds, to very high, up to 40 miliequivalents/100 g in spinach. For the majority of fruits and vegetables the dominant acids are citric acid and malic acid, each of which can, in particular examples, constitute over 2% of the fresh weight of the material. Lemons contain over 3% of citric acid. Tartaric acid accumulates in grape and oxalic acid in spinach. The fruits in general show a decrease in overall acidity during the ripening process. The total amount of mineral components in fruits and vegetables is in the range of 0.1% (in sweet potatoes) up to about 4.4% (in kohlrabi). The most abundant mineral constituent in fruits and vegetables is potassium (Table 2.4). Generally vegetables are a better source of minerals than fruits. The mineral elements influence not only the growth and crop of fruits and vegetables, but also their texture (Ca), color (Fe), and metabolic processes (microelements). The diversity of form shown by fruit and vegetable structures is extremely wide. Among the vegetables there are representatives of all the recognizable morphological divisions of the plant body — whole shoots, roots, stems, leaves, and fruits. Fruits may also be classified into a number of structural types. The individual seed-bearing structures of the flower called carpels constitute the gynoecium. The seed-containing cavity of a carpel is called the ovary, and its wall develops into the pericarp of the fruit. The edible fleshy part of a fruit most commonly develops from the ovary wall, but it may be also derived from the enlarged tip of stem from which floral organs arise, and sometimes leaf-like structures protecting the flowers may also become fleshy, e.g., in pineapple.

TABLE 2.4 Mineral Components of Fruits and Vegetables (mg/100 g of Raw Mass) Component

mg

Rich Source

K Na Ca Mg P Cl S Fe

350 65 150 50 120 90 80 2

Parsley (above 1000 mg) Celery Spinach (up to 600 mg) Sweet maize Seeds and young growing parts Celery Plants with higher quantity of proteins Parsley (up to 8 mg)

Source: Adapted from Duckworth, R.B., Fruit and Vegetables, Pergamon Press, London, 1966.

Chemical Composition and Structure of Foods

23

The most metabolic activity of plants is carried out in the tissue called parenchyma, which generally makes up the bulk of the volume of all soft edible plant structures. The epidermis, which sometimes is replaced by a layer of corky tissue, is structurally modified to protect the surface of the organ. The highly specialized tissues collenchyma and sclerenchyma provide mechanical support for the plant. Water, minerals, and products of metabolism are transported from one part to another of the plant through the vascular tissues, xylem and phloem, which are the most characteristic anatomical features of plants on the cross section. The structure of fruits is dominated by soft parenchymatous tissue, while conducting and supporting structures are rather poorly developed. An exception is the pineapple, in which conducting tissues are very prominently represented. The subtle structure and proportions of individual tissues influence the texture, properties, and suitability for processing of fruits and vegetables.

REFERENCES Duckworth, R.B., Fruit and Vegetables, Pergamon Press, London, 1966. Fox, B.A. and Cameron, A.G., Food Science: A Chemical Approach, Hodder and Stoughton, London, 1986. Hedrick, H.B. et al., Principles of Meat Science, Kendall-Hunt Publishing Co., Dubuque, IA, 1994. Kent, N.L., Technology of Cereals with Special Reference to Wheat, Pergamon Press, Oxford, 1975. Kirk, R.S. and Sawyer, R., Pearson’s Composition and Analysis of Foods, Longman Science, London, 1991. Lisi´nska, G. and Leszczy´nski, W., Potato Science and Technology, Elsevier Applied Science, London, 1989. Ramsay, I., Honey as a food ingredient, Food Ingredient Process. Int., 10, 16, 1992. Renner, E., Nutritional aspects of cheese, in Cheese: Chemistry, Physics and Microbiology, Vol. 1, Fox, P.F., Ed., Chapman & Hall, London, 1993, p. 557. Sikorski, Z.E., Seafood Raw Materials, WNT, Warsaw, 1992 (in Polish). Tamime, A.Y. and Robinson, R.K., Yoghurt: Science and Technology, CRC Press, Boca Raton, FL, 1999.

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3

Water and Food Quality Barbara Cybulska and Peter Edward Doe

CONTENTS 3.1 3.2

Introduction ....................................................................................................25 Structure and Properties of Water..................................................................26 3.2.1 Water Molecule ..................................................................................26 3.2.2 Hydrogen Bonds ................................................................................27 3.2.3 Properties of Bulk Water ...................................................................28 3.2.4 Thermal Properties of Water..............................................................32 3.2.5 Water as a Solvent .............................................................................33 3.2.6 Water in Biological Materials............................................................36 3.2.6.1 Properties ............................................................................36 3.2.6.2 Water Transport...................................................................39 3.3 Water in Food.................................................................................................40 3.3.1 Introduction ........................................................................................40 3.3.2 Sorption Isotherms and Water Activity .............................................41 3.3.2.1 Principle ..............................................................................41 3.3.2.2 Measurement of Water Activity..........................................43 3.3.3 Water Activity and Shelf Life of Foods ............................................44 3.4 Water Supply, Quality, and Disposal.............................................................45 3.4.1 Water Supply......................................................................................45 3.4.2 Water Quality .....................................................................................45 3.4.2.1 Standards and Treatment ....................................................45 3.4.2.2 Water Pollution ...................................................................47 3.4.3 Wastewater Treatment and Disposal..................................................48 References................................................................................................................49

3.1 INTRODUCTION Water is the most popular and most important chemical compound on our planet. It is a major chemical constituent of Earth’s surface, and it is the only substance that is abundant in solid, liquid, and gaseous form. Because it is ubiquitous, it seems to be a mild and inert substance. In fact, it is a very reactive compound characterized by unique physical and chemical properties that make it very different from other popular liquids. The peculiar water properties determine the nature of the physical and biological world.

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25

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Chemical and Functional Properties of Food Components

Water is the major component of all living organisms. It constitutes 60% or more of the weight of most living things, and it pervades all portions of every cell. It existed on our planet long before the appearance of any form of life. The evolution of life was doubtlessly shaped by physical and chemical properties of the aqueous environment. All aspects of living cells’ structure and function seem to be adapted to water-unique properties. Water is the universal solvent and dispersing agent, as well as a very reactive chemical compound. Biologically active structures of biomacromolecules are spontaneously formed only in aqueous media. Intracellular water is not only a medium in which structural arrangement and all metabolic processes occur, but an active partner of molecular interactions, participating directly in many biochemical reactions as a substrate or a product. Its high heat capacity allows water to act as a heat buffer in all organisms. Regulation of water contents is important in the maintenance of homeostasis in all living systems. Only 0.003% of all sweet water reserve participates in its continuous circulation between the atmosphere and the hydrosphere. The remaining part is confined in the Antarctic ice. The geography of water availability has determined, to a large degree, the vegetation, food supply, and habitation in the various areas of the world. For example, Bangladesh has one of the world’s highest population densities — made possible through the regular flooding of the Ganges River and the rich slits it deposits in its wake. In Bangladesh, the staple food — rice — grows abundantly and is readily distributed. In other societies the food must be transported long distances or kept over winter. Stability, wholesomeness, and shelf life are significant features of such foods. These features are, to a large degree, influenced by the water content of the food. Dried foods were originally developed to overcome the constraints of time and distance before consumption. Canned and frozen foods were developed next. The physical properties, quantity, and quality of water within food have a strong impact on food effectiveness, quality attributes, shelf life, textural properties, and processing.

3.2 STRUCTURE AND PROPERTIES OF WATER 3.2.1 WATER MOLECULE Water is a familiar material, but it has been described as the most anomalous of chemical compounds. Although its chemical composition, HOH or H2O, is universally known, the simplicity of its formula belies the complexity of its behavior. Its physical and chemical properties are very different from compounds of similar complexity, such as HF and H2S. To understand the reasons for water’s unusual properties, it is necessary to examine its molecular structure in some detail. Although a water molecule is electrically neutral as a whole, it has a dipolar character. The high polarity of water is caused by the direction of the H–O–H bond angle, which is 104.5°, and by an asymmetrical distribution of electrons within the molecule. In a single water molecule, each hydrogen atom shares an electron pair with the oxygen atom in a stable covalent bond. However, the sharing of electrons between H and O is unequal, because the more electronegative oxygen atom tends

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FIGURE 3.1 Water molecule as an electric dipole.

to draw electrons away from the hydrogen nuclei. The electrons are more often in the vicinity of the oxygen atom than of the hydrogen atom. The result of this unequal electron sharing is the existence of two electric dipoles in the molecule, one along each of the H–O bonds. The oxygen atom bears a partial negative charge δ–, and each hydrogen a partial positive charge δ+. Since the molecule is not linear, H–O–H has a dipole moment (Figure 3.1). Because of this, water molecules can interact through electrostatic attraction between the oxygen atom of one water molecule and the hydrogen of another.

3.2.2 HYDROGEN BONDS Such interactions, which arise because the electrons on one molecule can be partially shared with the hydrogen on another, are known as hydrogen bonds. The H2O molecule, which contains two hydrogen atoms and one oxygen atom in a nonlinear arrangement, is ideally suited to engage in hydrogen bonding. It can act both as a donor and as an acceptor of hydrogens. The nearly tetrahedral arrangement of the orbital about the oxygen atom allows each water molecule to form hydrogen bonds with four of its neighbors (Figure 3.2). An individual, isolated hydrogen bond is very labile. It is longer and weaker than a covalent O–H bond (Figure 3.3). The hydrogen bond’s energy, i.e., the energy required to break the bond, is about 20 kJ/mol. These bonds are intermediate between those of weak van der Waals interactions (about 1.2 kJ/mol) and those of covalent bonds (460 kJ/mol). Hydrogen bonds are highly directional; they are stronger when the hydrogen and the two atoms that share it are in a straight line (Figure 3.4). Hydrogen bonds are not unique to water. They are formed between water and different chemical structures, as well as between other molecules or even within a molecule. They are formed wherever an electronegative atom (oxygen or nitrogen) comes in close proximity to a hydrogen covalently bonded to another electronegative atom. Some representative hydrogen bonds of biological importance are shown in Figure 3.5.

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Chemical and Functional Properties of Food Components

FIGURE 3.2 Tetrahedral hydrogen bonding of five water molecules.

Hydrogen bond 0.177 nm

Covalent bond 0.0965

FIGURE 3.3 Two water molecules connected by hydrogen bonds.

Intra- and intermolecular hydrogen bonding occurs extensively in biological macromolecules. A large number of the hydrogen bonds and its directionality confers very precise three-dimensional structures upon proteins and nucleic acids.

3.2.3 PROPERTIES

OF

BULK WATER

The key to understanding water structure in solid and liquid form lies in the concept and nature of the hydrogen bonds. In the crystal of ordinary hexagonal ice (Figure 3.6), each molecule forms four hydrogen bonds with its nearest neighbors.

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FIGURE 3.4 Directionality of the hydrogen bonds.

FIGURE 3.5 Some hydrogen bonds of biological importance.

Each HOH acts as a hydrogen donor to two of the four water molecules and as a hydrogen acceptor for the remaining two. These four hydrogen bonds are spatially arranged according to the tetrahedral symmetry. The crystal lattice of ice occupies more space than the same number of H2O molecules in liquid water. The density of solid water is thus less than that of liquid water, whereas simple logic would have the more tightly bound solid structure more

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Chemical and Functional Properties of Food Components

FIGURE 3.6 Structure of ice.

dense than its liquid. One explanation for ice being lighter than water at 0°C proposes a reforming of intermolecular bonds as ice melts so that, on average, a water molecule is bound to more than four of its neighbors, thus increasing its density. But as the temperature of liquid water increases, the intermolecular distances also increase, giving a lower density. These two opposite effects explain the fact that liquid water has a maximum density at a temperature of 4°C. At any given instant in liquid water at room temperature, each water molecule forms hydrogen bonds with an average of 3.4 other water molecules (Lehninger et al., 1993). The average translational and rotational kinetic energies of a water molecule are approximately 7 kJ/mol, the same order as that required to break hydrogen bonds; therefore, hydrogen bonds are in a continuous state of flux, breaking and reforming with high frequency on a picosecond time scale. A similar dynamic process occur in aqueous media with substances that are capable of forming hydrogen bonds. At 100°C liquid water still contains a significant number of hydrogen bonds, and even in water vapor there is strong attraction between water molecules. The very large number of hydrogen bonds between molecules confers great internal cohesion on liquid water. This feature provides a logical explanation for many of its unusual properties. For example, its large values for heat capacity, melting point, boiling point, surface tension, and heat of various phase transitions are all related to the extra energy needed to brake intermolecular hydrogen bonds. That liquid water has structure is an old and well-accepted idea; however, there is no consensus among physical chemists as to the molecular architecture of the hydrogen bond’s network in the liquid state. The available measurements on liquid water do not lead to a clear picture of liquid water structure. It seems that the majority

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of hydrogen bonds survive the melting process, but obviously rearrangement of molecules occurs. The replacement of crystal rigidity by fluidity gives molecules more freedom to diffuse about and to change their orientation. Any molecular theory for liquid water must take into account changes in the topology and geometry of the hydrogen bond network induced by the melting process. Many models have been proposed, but none has adequately explained all properties of liquid water. “Iceberg” models postulated that liquid water contains disconnected fragments of ice suspended in a sea of unbounded water molecules. The most popular, the so-called “flickering clusters” model, suggests that liquid water is highly organized on a local basis: the hydrogen bonds break and reform spontaneously, creating and destroying transient structural domains (Figure 3.7). However, because the half-life of any hydrogen bond is less than a nanosecond, the existence of these clusters has statistical validity only; even this has been questioned by some authors who consider water to be a continuous polymer. Experimental evidence obtained by x-ray and neutron diffractions strongly support the persistence of a tetrahedral hydrogen bond order in the liquid water, but with substantial disorder present. Stillinger (1980) created a qualitatively water-like structure by computer simulation. The view that emerges from these results is the following: liquid water consists of a macroscopically connected, random network of hydrogen bonds. This network has a local preference for tetrahedral geometry, but it contains a large proportion of strained and broken bonds that are continually undergoing topological reformation. The properties of water arise from the competition between relatively bulky ways

FIGURE 3.7 “Flickering clusters” of H2O molecules in bulk water.

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Chemical and Functional Properties of Food Components

of connecting molecules into local patterns characterized by strong bonds and nearly tetrahedral angles and more compact arrangements characterized by more strain and bond breakage. According to the model proposed by Wiggins (1990), two types of water structure can be distinguished: high-density water and low-density water. In dense water the bent, relatively weak hydrogen bonds predominate over straight, stronger ones. Low-density water has many ice-like straight hydrogen bonds. Although hydrogen bonding is still continuous through the liquid, the weakness of the bonds allows the structure to be disrupted by thermal energy extremely rapidly. High-density water is extremely reactive and more liquid, whereas low-density water is inert and more viscous. A continuous spectrum of water structures between these two extremes could be imagined. The strength of water–water hydrogen bonding, which is the source of water density and reactivity, has great functional significance; this explains solvent water’s properties and its role in many biological events. A common feature of all theories is that a definite structure of liquid water is due to the hydrogen bonding between molecules and that the structure is in the dynamic state as the hydrogen bonds break and reform with high frequency.

3.2.4 THERMAL PROPERTIES

OF

WATER

The unusually high melting point of ice, as well as the heat of water vaporization and specific heat, is related to the ability of water molecules to form hydrogen bonds and to the strength of these bonds. A large amount of energy, in the form of heat, is required to disrupt the hydrogenbonded lattice of ice. In the common form of ice, each water molecule participates in four hydrogen bonds. When ice melts, most of the hydrogen bonds are retained by liquid water, but the pattern of hydrogen bonding is irregular, due to the frequent fluctuation. The average energy required to break each hydrogen bond in ice has been estimated to be 23 kJ/mol, while that to break each hydrogen bond in water is less than 20 kJ/mol (Ruan and Chen, 1998). The heat of water vaporization is much higher than that of many other liquids. As is the case with melting ice, a large amount of thermal energy is required for breaking hydrogen bonds in liquid water, to permit water molecules to dissociate from one another and to enter the gas phase. Perspiration is an effective mechanism of decreasing body temperature because the evaporation of water absorbs so much heat. A relatively large amount of heat is required to raise the temperature of 1 g of water by 1°C because multiple hydrogen bonds must be broken in order to increase the kinetic energy of the water molecules. Due to the high quantity of water in the cells of all organisms, temperature fluctuation within cells is minimized. This feature is of critical biological importance, since most biochemical reactions and macromolecular structures are sensitive to temperature. The unusual thermal properties of water make it a suitable environment for living organisms, as well as an excellent medium for the chemical processes of life.

Water and Food Quality

3.2.5 WATER

AS A

33

SOLVENT

Many molecular parameters, such as ionization, molecular and electronic structure, size, and stereochemistry, will influence the basic interaction between a solute and a solvent. The addition of any substance to water results in altered properties for this substance and for water itself. Solutes cause a change in water properties because the hydrate envelopes that are formed around dissolved molecules are more organized and therefore more stable than the flickering clusters of free water. The properties of solutions that depend on solute and its concentration are different from those of pure water. The differences can be seen in such phenomena as the freezing point depression, boiling point elevation, and increased osmotic pressure of solutions. The polar nature of the water molecule and the ability to form hydrogen bonds determine its properties as a solvent. Water is a good solvent for charged or polar compounds and a relatively poor solvent for hydrocarbons. Hydrophilic compounds interact strongly with water by an ion–dipole or dipole–dipole mechanism, causing changes in water structure and mobility and in the structure and reactivity of the solutes. The interaction of water with various solutes is referred to as hydration. The extent and tenacity of hydration depends on a number of factors, including the nature of the solute, salt composition of the medium, pH, and temperature. Water dissolves dissociable solutes readily, because the polar water molecules orient themselves around ions and partially neutralize ionic charges. As a result, the positive and negative ions can exist as separate entities in a dilute aqueous solution without forming ion pairs. Sodium chloride is an example where the electrostatic attraction of Na+ and Cl– is overcome by the attraction of Na+ with the negative charge on the oxygens and Cl– with the positive charge on the hydrogen ions (Figure 3.8). The number of weak charge–charge interactions between water and the Na+ and Cl– ions is sufficient to separate the two charged ions from the crystal lattice. To acquire their stabilizing hydration shell, ions must compete with water molecules, which need to make as many hydrogen bonds with one another as possible. The normal structure of pure water is disrupted in solution of dissociable solutes. The ability of a given ion to alter the net structure of water is dependent on the strength of its electric field. Among ions of a given charge type (e.g., Na+ and K+ or Mg+2 and Ca+2), the smaller ions are more strongly hydrated than the larger ions, in which the charge is dispersed over a greater surface area. Most

FIGURE 3.8 Hydration shell around Na+ and Cl–.

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Chemical and Functional Properties of Food Components

cations, except the largest ones, have a primary hydration sphere containing four to six molecules of water. Other water molecules, more distant from the ion, are held in a looser secondary sphere. The electrochemical transfer experiments indicate a total of 16 molecules of water around Na+ and about 10 around K+. The bound water is less mobile and more dense than HOH molecules in the bulk water. At some distance, the bonding arrangements melt into a dynamic configuration of pure water. Water is especially effective in screening the electrostatic interaction between dissolved ions, because, according to Coulomb’s law, the force (F) between two charges q+ and q– separated by a distance r is given as: F = q+ · q–/εr2

(3.1)

where ε is the dielectric constant of the medium. For a vacuum, ε = 1 Debye unit, whereas for bulk water, ε = 80; this implies that the energies associated with electrostatic interactions in aqueous media are approximately 100 times smaller than the energies of covalent association, but increase considerably in the interior of a protein molecule. In thermodynamic terms, the free energy change, ∆G, must have a negative value for a process to occur spontaneously. ∆G = ∆H – T∆S

(3.2)

where ∆G represents the driving force, ∆H (the enthalpy change) is the energy from making and breaking bonds, and ∆S (the entropy change) is the increase in randomness. Solubilization of a salt occurs with a favorable change in free energy. As salt such as NaCl dissolves, the Na+ and Cl– ions leaving the crystal lattice acquire greater freedom of motion. The entropy (∆S) of the system increases; where ∆H has a small positive value and T∆S is large and positive, ∆G is negative. Water in the multilayer environment of ions is believed to exist in a structurally disrupted state because of conflicting structural influences of the innermost vicinal water and the outermost bulk-phase water. In concentrated salt solutions, the bulkphase water would be eliminated, and the water structure common in the vicinity of ions would predominate. Small or multivalent ions, such as Li+, Na+, H3O+, Ca+2, Mg+2, F–, SO4–2, and PO4–3, which have strong electric fields, are classified as water structure formers because solutions containing these ions are less fluid than pure water. Ions that are large and monovalent, most of the negatively charged ions and large positive ions, such as K+, Rb+, Cs+, NH4+, Cl–, Br–, I–, NO–3, ClO4, and CNS–, disrupt the normal structure of water; they are structure breakers. Solutions containing these ions are more fluid than pure water (Fennema, 1985). Through their varying abilities to hydrate and to alter water structure and its dielectric constant, ions influence all kinds of water solute interactions. The conformation of macromolecules and the stability of colloids are greatly affected by the kinds and concentrations of ions present in the medium.

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Water is a good solvent for most biomolecules, which are generally charged or polar compounds. Solubilization of compounds with functional groups such as ionized carboxylic acids (COO –), protonated amines (NH3+), phosphate esters, or anhydrides is also a result of hydration and charge screening. Uncharged but polar compounds possessing hydrogen bonding capabilities are also readily dissolved in water, due to the formation of hydrogen bonds with water molecules. Every group that is capable of forming a hydrogen bond to another organic group is also able to form hydrogen bonds of similar strength with water. Hydrogen bonding of water occurs with neutral compounds containing hydroxyl, amino, carbonyl, amide, or imine groups. Saccharides dissolve readily in water, due to the formation of many hydrogen bonds between the hydroxyl groups or carbonyl oxygen of the saccharide and water molecules. Water–solute hydrogen bonds are weaker than ion–water interactions. Hydrogen bonding between water and polar solutes also causes some ordering of water molecules, but the effect is less significant than with ionic or nonpolar solutes. The introduction into water of hydrophobic substances such as hydrocarbons, rare gases, and the apolar groups of fatty acids, amino acids, or proteins is thermodynamically unfavorable because of the decrease in entropy. The decrease in entropy arises from the increase in water–water hydrogen bonding adjacent to apolar entities. Water molecules in the immediate vicinity of a nonpolar solute are constrained in their possible orientations, resulting in a shell of highly ordered water molecules around each nonpolar solute molecule (Figure 3.9a). The number of water molecules in the highly ordered shell is proportional to the surface area of hydrophobic solute. In the case of dissolved hydrocarbons, the enthalpy of formation of the new hydrogen bonds often almost exactly balances the enthalpy of creation in water, a cavity of Hydrophylic "head group"

(a)

(b)

FIGURE 3.9 Cage-like water structure around the hydrophobic alkyl chain (a) and hydrophobic interactions (b).

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Chemical and Functional Properties of Food Components

the right size to accommodate the hydrophobic molecule. However, the restriction of water mobility results in a very large decrease in entropy. According to ∆G = ∆H – T∆S

(3.3)

if ∆H is almost zero and ∆S is negative, ∆G is positive. To minimize contact with water, hydrophobic groups tend to aggregate; this process is known as hydrophobic interaction (Figure 3.9b). The existence of hydrophobic substances barely soluble in water but readily soluble in many nonpolar solvents, and their tendency to segregate in aqueous media, has been known for a long time. However, the origin of this hydrophobic effect is still somewhat controversial. The plausible explanation is that hydrophobic molecules disturb the hydrogen bonded state of water, without having any compensatory ordering effects. Apolar molecules are water structure formers: water molecules cannot use all four possible hydrogen bonds when in contact with hydrophobic, water-hating molecules. This restriction results in a loss of entropy, a gain in density, and increased organization of bulk water. Amphipathic molecules, i.e., compounds containing both polar or charged groups and apolar regions, disperse in water if the attraction of the polar group for water can overcome possible hydrophobic interactions of the apolar portions of the molecules. Many biomolecules are amphipathics: proteins, phospholipids, sterols, certain vitamins, and pigments have polar and nonpolar regions. When amphipathic compounds are in contact with water, the two regions of the solute molecule experience conflicting tendencies: the polar or charged hydrophilic regions interact favorably with water and tend to dissolve, but the nonpolar hydrophobic regions tend to avoid contact with water. The nonpolar regions of the molecules cluster together to present the smallest hydrophobic area to the aqueous medium, and the polar regions are arranged to maximize their interactions with the aqueous solvent. In aqueous media, many amphipathic compounds are able to form stable structures, containing hundreds to thousands of molecules, called micelles. The forces that held the nonpolar regions of the molecules together are due to hydrophobic interactions. The hydrophobic effect is a driving force in the formation of clathrate hydrates and the self-assembly of lipid bilayers. Hydrophobic interactions between lipids and proteins are the most important determinants of biological membrane structure. The three-dimensional folding pattern of proteins is also determined by hydrophobic interactions between nonpolar side chains of amino acid residues.

3.2.6 WATER

IN

BIOLOGICAL MATERIALS

3.2.6.1 Properties Water behaves differently in different environments. Properties of water in heterogenous systems such as living cells or food remain a field of debate. Water molecules may interact with macromolecular components and supramolecular structures of biological systems through hydrogen bonds and electrostatic interactions. Solvation of biomolecules such as lipids, proteins, nucleic acids, or saccharides resulting from these interactions determines their molecular structure and function.

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Various physical techniques, i.e., nuclear magnetic resonance (NMR), x-ray diffraction, and chemical probes (exchange of H by D), indicate that there is a layer of water bound to protein molecules, phospholipid bilayers, and nucleic acids, as well as at the surface of the cell membranes and other organelles. Water associated at the interfaces and with macromolecular components may have quite different properties from those in the bulk phase. Water can be expected to form locally ordered structures at the surface of water-soluble, as well as waterinsoluble, macromolecules and at the boundaries of the cellular organelles. Biomacromolecules generally have many ionized and polar groups on their surfaces and tend to align near polar water molecules. This ordering effect exerted by the macromolecular surface extends quite far into the surrounding medium. According to the association–induction theory proposed by Ling (1962), fixed charges on macromolecules and their associated counterions constrain water molecules to form a matrix of polarized multilayers having restricted motion, compared with pure water. The monolayer of water molecules absorbed on the polar sorption site of the molecule is almost immobilized and thus behaves, in many respects, like part of the solid or like water in ice. It has different properties than additional water layers defined as multilayers have. The association–induction theory has been shared by many researchers for many years. Unfortunately, elucidation of the nature of individual layers of water molecules has been less successful, due to the complexity of the system and lack of appropriate techniques. Measurements of the diffusion coefficients of globular protein molecules in solution yield values for molecular size that are greater than the corresponding radii determined by x-ray crystallography. The apparent hydrodynamic radius can be calculated from the Stokes–Einstein relation: D = kBT/6πηaH

(3.4)

where D is the diffusion coefficient, kB is the Boltzman constant, T is the temperature, η is the solution viscosity, and aH is the molecule radius (Nossal and Lecar, 1991). Similarly, studies utilizing NMR techniques show that there is a species of associated water that has a different character than water in the bulk phase. By these and other methods it was found that, for a wide range of protein molecules, approximately 0.25–0.45 g of H2O is associated with each gram of protein. The hydration forces can stabilize macromolecular association or prevent macromolecular interactions with a strength that depends on the surface characteristic of the molecules and the ionic composition of the medium. The interaction between a solute and a solid phase is also influenced by water. Hydration shells or icebergs associated with one or the other phase are destroyed or created in this interaction and often contribute to conformational changes in macromolecular structures — and ultimately to changes in biological and functional properties important in food processing. Biophysical processes involving membrane transport are also influenced by hydration. The size of the hydration shell surrounding small ions and the presence of water in the cavities of ionic channels or in the defects between membrane lipids strongly affect the rates at which the ions cross a cell membrane.

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Chemical and Functional Properties of Food Components

The idea that intracellular water exhibits properties different from those of bulk water has been around for a long time. The uniqueness of the cytoplamic water was deduced from: • The observation that cells may be cooled far below the freezing point of a salt solution iso-osmotic with that of the cytoplasm. • Properties of the cytoplasm, which in the same conditions should bind water like a gel. • Osmotic experiments in which it has often been observed that part of cell water is not available as a solvent. This water has been described as osmotically inactive water, bound water, or compartmentalized water. According to a recent view, three different kinds of intracellular water can be distinguished: a percentage of the total cell water appears in the form of usual liquid water. A relevant part is made up of water molecules that are bound to different sites of macromolecules in the form of hydration water, while a sizeable amount, although not fixed to any definite molecular site, is strongly affected by macromolecular fields. This kind of water has been termed vicinal water. Most of the vicinal water surrounds the elements of the cell cytoskeleton. Vicinal water has been extensively investigated, and it has been found that some of its properties are different from those of normal water. It does not have a unique freezing temperature, but an interval ranging from –70 to –50°C; it is a very bad solvent for electrolytes, but nonelectrolytes have the same solubility properties in it as in usual water; its viscosity is enhanced, and its NMR response is anomalous (Giudice et al., 1986). The distribution of various types of water inside the living cells is a question that cannot be answered yet, especially because in many cells marked changes have been noted in the state of intracellular water as a result of biological activity. The possibility that water in living cells may differ structurally from bulk water has incited a search for parameters of cell water that deviate numerically from those of bulk water. The diffusion coefficient for water in the cytoplasm of various cells has been determined with a satisfactory precision. It has been found that the movement of water molecules inside living cells is not much different and is reduced by a factor of between 2 and 6, compared with the self-diffusion coefficient for pure water. According to Mild and Løvtrup (1985), the most likely explanation of the observed values is that part of the cytoplasmic water, the vicinal water close to the various surface structures in the cytoplasm, is structurally changed to the extent that its rate of motion is significantly reduced, compared with the bulk phase. In heterogenous biological materials and foods, water exists in different states. It is thought that water molecules in different states function differently. Water associated with proteins and other macromolecules has traditionally been referred to as bound water. However, to designate such water as bound can be misleading because, for the most part, the water molecules are probably only transiently associated, and at least a portion of the associated water has to be constantly rearranged, due to the thermal perturbations of weak hydrogen bonds.

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Water molecules are constantly in motion, even in ice. In fact, the translational and rotational mobility of water directly determines its availability. Water mobility can be measured by a number of physical methods, including NMR, dielectric relaxation, ESR, and thermal analysis (Chinachoti, 1993). The mobility of water molecules in biological systems may play an important role in a biochemical reaction’s equilibrium and kinetics, formation and preservation of chemical gradients and osmotic pressure, and macromolecular conformation. In food systems, the mobility of water may influence the engineering processes — such as freezing, drying, and concentrating chemical and microbial activities, and textural attributes (Ruan and Chen, 1998). 3.2.6.2 Water Transport Water transport is associated with various physiological processes in whole living organisms and single cells. When cells are exposed to hyper- or hypo-osmotic solutions, they immediately lose or gain water, respectively. Even in an isotonic medium a continuous exchange of water occurs between living cells and their surroundings. Most cells are so small and their membrane so leaky that the exchange of water molecules measured with isotopic water reaches equilibrium in a few milliseconds. The degree of water permeability differs considerably between tissues and cell types. Mammalian red blood cells and renal proximal tubules are extremely permeable to water molecules. Transmembrane water movements are involved in diverse physiological secretion processes. How water passes through cells has begun to come clear only in the last five years. Water permeates living membranes through both the lipid bilayer and specific water transport proteins. In both cases water flow is passive and directed by osmosis. Water transport in living cells is therefore under the control of ATP and ion pumps. The most general water transport mechanism is diffusion through lipid bilayers, with a permeability coefficient of 2–5 × 10-4 cm/sec. The diffusion through lipid bilayers depends on lipid structure and the presence of sterol (Subczy´nski et al., 1994). It is suggested that the lateral diffusion of the lipid molecules and the water diffusion through the membrane is a single process (Haines, 1994). A small amount of water is transported through certain membrane transport proteins, such as a glucose transporter or the anion channel of erythrocytes. The major volume of water passes through water transport proteins. The first isolated water transporting protein was the channel-forming integral protein from red blood cells. The identification of this protein has led to the recognition of a family of related water-selective channels, the aquaporins, that are found in animals, plants, and microbial organisms. Water flow through the protein channel is controlled by the number of protein copies in the membrane. In red blood cells, there are 200,000 copies/cell; in apical brush border cells of renal tubules, it constitutes 4% of the total protein (Engel et al., 1994).

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Chemical and Functional Properties of Food Components

3.3 WATER IN FOOD 3.3.1 INTRODUCTION Water, with a density of 1000 kg m-3, is denser than the oil components of foods; oils and fats typically have densities in the range 850–950 kg m-3. Glycerols and sugar solutions are denser than water. Unlike the solid phase of most other liquids, ice is less dense than liquid water; ice has lower thermal conductivity than water. These properties have an effect on the freezing of foods that are predominantly water based; the formation of an ice layer on the surface of liquids and the outside of solids has the effect of slowing down the freezing rate. Because a molecule of water vapor is lighter (molecular weight = 18) than that of dry air (molecular weight = about 29), moist air is lighter than dry air at the same temperature. This is somewhat unexpected, because the popular conception is that humid air (which contains more water) is heavier than dry air. At room temperature, water has the highest specific heat of any inorganic or organic compound, with the sole exception of ammonia. It is interesting to speculate why the most commonly occurring substance on this planet should have one of the highest specific heats. One of the consequences of this peculiarity in the food industry is that heating and cooling operations for essentially water-based foods are more energy demanding. To heat a kilogram of water from 20 to 50°C requires about 125 kJ of energy, whereas heating the same mass of vegetable oil requires only 44 kJ. A sponge holds most of its water as liquid held in the intestacies of the sponge structure. Most of the water can be wrung out of the sponge, leaving a matrix of air and damp fibers. Within the sponge fibers the residual water is more tenuously held — absorbed within the fiber of the sponge. If the sponge is left to dry in the sun, this adsorbed water will evaporate, leaving only a small proportion of water bound chemically to the salts and to the cellulose of the sponge fibers. Like water in sponge, water is held in food by various physical and chemical mechanisms (Table 3.1). It is a convenient oversimplification to distinguish between “free” and “bound” water. The definition of bound water in such a classification poses problems. Fennema (1985) reports seven different definitions of bound water. Some of these definitions are based on the freezability of the “bound” component, and others rely on its availability as a solvent. He prefers a definition in which bound water is “that which exists in the vicinity of solutes and other nonaqueous constituents, exhibits reduced molecular activity and other significantly altered properties as compared with ‘bulk water’ in the same system, and does not freeze at –40°C.” The moisture content can be measured simply by weighing a sample and then oven drying it, usually at 105°C overnight; the difference in mass is the moisture content in the original sample. However, much confusion is caused by reporting the moisture content simply as a percentage without specifying the basis of the calculation. It should be made clear whether the moisture content is calculated on a wet basis (moisture content divided by original mass) or on a dry basis (moisture content

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TABLE 3.1 Classification of Water States in Foods Class of Water Constitutional Vicinal

Multilayer

Free

Entrapped

Description An integral part of nonaqueous constituent Bound water that strongly acts with specific hydrophilic sites of nonaqueous constituents to form a monolayer coverage; water–ion and water–dipole bonds Bound water that forms several additional layers around hydrophilic groups; water–water and water–solute hydrogen bonds Flow is unimpeded; properties close to those of dilute salt solutions; water–water bonds predominate Free water held within matrix or gel that impedes flow

Porportion of Typical 90% (Wet Basis) Moisture Content Food