[H.A_Modi]_Microbial_spoilage_of_foods(BookZZ.org).pdf

MICROBIAL SPOILAGE OF FOODS

Dr. H.A Modi is M.Sc., M.Phil. and Ph.D. in Microbiology. Presently, he is a Reader (Microbiology) at Department of Life Sciences, Gujarat University, Ahmedabad. He is visiting Professor at P.G. Departments of North Gujarat University, Saurastra University, Gujarat Vidyapeeth, South Gujarat University and other Universities of Gujarat. He is a recognized Ph.D. guiding teacher in Microbiology at Gujarat University and North Gujarat University, supervising Ph.D. students in the fields of Enzyme biotechnology, Bioconversion technology, Mushroom technology, Biocontrol and Food microbiology. He has presented several scientific presentations at National as well as International conferences, seminars and symposia. His research papers have been published in journals of national and international repute. He is a member of many prestigious academic and professional organizations. He has published many books. The important ones are Fermentation Technology in 2 vols., Introductory Food Microbiology, Dairy Microbiology, Food Microorganisms, Food-borne Illnesses and Elementary Microbiology in 2 vols. Dr. Modi has widely travelled in countries like Brazil, Germany, England etc. for academic purposes under Group Study Exchange Programmes. He has successfully carried out research projects funded by UGc, DBT, GUJCOST etc.

MICROBIAL SPOILAGE OF FOODS

Dr. H.A. Modi Microbiology Laboratory Department of Life Sciences University School of Sciences Gujarat University Ahrnedabad-380 009 (Guj.)

Aavishkar Publishers, Distributors Jaipur 302 003 (Raj) India

First Published in 2009 by Prern C. Bakliwal for

Aavishkar Publishers, Distributors 807, Vyas Building, Chaura Rasta Jaipur 302 003 (Raj) India Phone: 0141-2578159 e-mail: [email protected]

© Dr. H.A. Modi

ISBN 978-81-7910-285-5

All rights reserved. No part of this publication may be reproduced or copied for any purp('se by any means, manual, mechanical or electronic, without prior and written permission of the copyright owner and the Publishers.

Printed at

Sheetal Printers Jaipur 302 003 (Raj) India

PREFACE

The loss of food due to microbial spoilage has economic consequences for the producers, processors and consumers. With the increase in population in the world, loss of food due to microbial (as well as non-microbial) spoilage means less food is available for the hungry mouth. To fight against world hunger, efforts should be directed not only to increase food proouction, but also to minimize spoilage so that enough food is available for consumption. Except for sterile foods, all foods harbor microorganisms. Food spoilage stems from the growth of these microorganisms in food or is due to the action of microbial heat-stable enzymes. New marketing trends, the consumer's desire for foods that are not overly processed and preserved, extended shelf-life, and chances of temperature abuse between production and consumption of foods have greatly increased the chances of food spoilage and, in some instances, with new types of microorganisms. The major concerns are the economic loss and wastage of food. New concepts are being studied to reduce contamination as well as control the growth of spoilage microbes in foods. This book includes thirteen chapters. Introduction of Food Microbiology and its horizons are elaborated in Chapter 1. Basics of human nutrition are briefly summarized in Chapter 2. General principles of microbial spoilage of foods covering types of spoilage, microorganisms involved, favourable factors etc. are discussed in Chapter 3. Microbial spoilage of various food categories like meat and meat products (Chapter 4), poultry and eggs (Chapter 5), fish and other seafoods (Chapter 6), dairy products (Chapter 7), vegetables and fruits (Chapter 8) and cereal products (Chapter 9) are dealt in detail. Special

vi chapters on spoilage of canned foods (Chapter 10) and spoilage of frozen foods (Chapter 11) are included in this book due to popularity of such foods in present time. Indicators of microbial foods spoilage are mentioned in Chapter 12. Microbiological testing of food is very important, so it is elaborately explained in final Chapter 13. A list of selected bibliograpby is also given to assist students in expanding their knowledge on the subject. The materials in each chapter are arranged in logical, systematic and concise sequences. The attempt is done that the book should prove to be an useful source of information for the students of food microbiology, food science and technology, food biotechnology, agriculture, horticulhlre, food and nutrition, environmental studies, hotel and catering management and other food-related courses. This book will also be useful for the people who are working in food-processing industries, catering houses, food and agriculture department of Government. I am grateful to Aavishkar Publishers, Distributors, Jaipur for their concern, efforts and encouragement, especially for their excellent cooperation in the task of preparing and publishing this book.

H.A. Modi

CONTENTS Preface ............................................................................................ v 1. Food Microbiology and its Horizons .................................... 1-8 1.1 Prologue................................... .......................................... 1 1.2 Search of Microorganisms ............................................... 2 1.2.1 Spontaneous Generation Theory ...................... 3 1.2.2 Importance of Microorganisms ......................... 3 1.3 Early Developments in Food Microbiology .................. 4 1.4 Significance of Microorganisms in Foods ..................... 6 1.4.1 Foodbome Diseases ............................................ 6 1.4.2 Food Spoilage ...................................................... 6 1.4.3 Food Bioprocessing ............................................ 6 1.4.4 Food Biopreservation .......................................... 7 1.5 Roles of Food Microbiologists ........................................ 7 2. Basics of Human Nutrition ................................................. 9-42 2.1 Prologue ............................................................................. 9 2.2 Importance of Food .......................................................... 9 2.3 Constituents of Foods .................................................... 10 2.3.1 Carbohydrates ................................................... 10 2.3.1.1 Introduction and Classification ....... 10 2.3.1.2 Functions of Carbohydrates in the Diet ................................................ 13

viii Sources of Carbohydrate in the Diet Dietary Guidelines ............................. 2.3.2 ................................................................. Introduction and Chemical Structure .............................................. 2.3.2.2 Properties of Lipids ........................... 2.3.2.3 Functions of Lipids in the Diet ........ 2.3.2.4 Sources of Lipids in the Diet ............ 2.3.2.5 Dietary Guidelines ............................. 2.3.3 Proteins .............................................................. 2.3.3.1 Introduction and Chemical Structure ............... .......................... ..... 2.3.3.2 Functions of Proteins in the Body ... 2.3.3.3 Sources of Protein in the Diet ........... 2.3.3.4 Dietary Guidelines ............................. 2.3.4 Vitamins ............................................................. 2.3.4.1 Introduction and Types ..................... 2.3.4.2 Functions of Vitamins ....................... 2.3.5 Minerals ............................................................. 2.3.5.1 Introduction and Importance ........... 2.3.6 Water .................................................................. 2.3.6.1 Water Balance ..................................... Fate of Major Nutrients in the Body ............................ 2.4.1 Carbohydrates ................................................... 2.4.2 Fats ...................................................................... 2.4.3 Proteins .............................................................. Food and Energy ............................................................ 2.5.1 Energy Value of Nutrients ............................... 2.5.2 Energy Value of Foods ..................................... 2.5.3 Use of Energy by the Body .............................. Dietary Allowances for Indians (ICMR Recommendations) ............................................ Balanced Diets ................................................................ Classes of Natural Food Stuffs and their Nutritional Significance ................................................ 2.3.1.3 2.3.1.4 Lipids 2.3.2.1

2.4

2.5

2.6 2.7 2.8

14 14 14 14 16 17 18 18 18 18 19 19 20 20 20 21 23 23 26 26 27 27 27 27 28 28 28 28 30 32 37

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2.8.1 Cereals ................................................................ 37 2.8.2 Pulses ................................................................. 38 2.8.3 Nuts and Oilseeds ............................................ 38 2.8.4 Green Leafy Vegetables .................................... 39 2.8.5 Root Vegetables ................................................. 39 2.8.6 Other Vegetables ............................................... 40 2.8.7 ,Fruits ................................................................... 40 2.8.8 Milk and Milk Products ................................... 40 2.8.9 Sugar and Jaggery ............................................. 41 2.8.10 Fats and Oils ..................................................... 41 2.8.11 Flesh Foods ........................................................ 41 2.8.12 Eggs .................................................................... 41 2.8.13 Condiments and Spices ................................... 42 3. An Introduction to Microbial Spoilage of Foods ........... 43-66 3.1 Prologue ........................................................................... 43 3.2 Major Reasons for Food Spoilage ................................ 43 3.3 Food-spoilage Types ...................................................... 44 3.3.1 Mouldiness and 'whiskers' ............................. 44 3.3.2 Rots ..................................................................... 44 3.3.3 Sliminess ............................................................ 44 3.3.4 Colour Change .................................................. 44 3.3.5 Ropiness ............................................................. 45 3.3.6 Fermentative Spoilage ...................................... 45 3.3.7 Putrefaction ........................................................ 45 3.3.8 Aerobic Hydrolysis ..........: ................................ 46 3.4 The Organisms Involved in Food-spoilage ................. 46 3.4.1 Microbial Load .................................................. 46 3.4.2 Inter-relationships Between Organisms ......... 48 3.4.3 Moulds in Spoilage .......................................... 49 3.4.4 Yeasts in Spoilage ............................................. 51 3.4.5 Bacteria in Spoilage .......................................... 51 3.5 Food Qualities Responsible for Spoilage .................... 54 3.5.1 Water Content ................................................... 55 3.5.1.1 Dry Foods ............................................ 57 3.5.1.2 Deep-frozen Foods ............................. 59

x

3.5.2

3.5.3 3.5.4 3.5.5 3.5.6

3.5.1.3 Salted Foods ........................................ 3.5.1.4 Sweet Foods ........................................ pH ....................................................................... 3.5.2.1 Neutral Foods ..................................... 3.5.2.2 Acid Foods .......................................... 3.5.2.3 Canned Foods ..................................... Gaseous Conditions ......................................... Texture ................................................................ Nutrients ............................................................ Temperature .......................................................

59 60 61. 61 62 63 64 65 65 65

4. Spoilage of Meat and Meat Products .............................. 67-83 4.1 Prologue ........................................................................... 67 4.2 Spoilage of Fresh Meats ................................................ 67 4.2.1 Contamination of Tissues by Microorganisms ................................................ 67 4.2.2 Control of Microbial Growth ........................... 68 4.2.2.1 Initial Contamination ........................ 68 4.2.2.2 Glycogen Reserve ............................... 68 4.2.2.3 Oxidation-Reduction Potential ......... 69 4.2.2.4 The Rate of Cooling ........................... 69 4.2.3 Effect of Storage Temperature .......................... 69 4.2.3.1 Spoilage under Warm Conditions ... 69 4.2.3.2 Spoilage under Cool Conditions ...... 71 4.2.3.3 Spoilage under Refrigeration Conditions ........................................... 71 4.2.4 Chemical Changes Produced by Bacteria in Chilled Meats ................................ 73 4.3 Spoilage of Cured Meats ............................................... 74 4.3.1 Curing Agents ................................................... 74 4.3.2 The Curing Process .......................................... 74 4.3.3 The Microbiology and Spoilage of Bacon and Ham ................................................ 75 4.3.3.1 Unsmoked Bacon ............................... 75 4.3.3.2 Smoked Bacon .................................... 76 4.3.3.3 Ham ..................................................... 77

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4.4

Spoilage of Vacuum-packed Meats .............................. 4.4.1 Types of Packaging Materials ......................... 4.4.2 Influence of Packaging Materials on the Microbiological Flora ........................... 4.4.3 Spoilage of Packed Fresh Meats ..................... 4.4.4 Spoilage of Vacuum-packed Bacon ................

77 77 78 79 81

5. Spoilage of Poultry and Eggs ............................................ 84-89 5.1 Prologue ........................................................................... 84 5.2 Effects of Poultry Processing on the Microbiological Flora ..................................................... 84 5.3 Spoilage of Chickens held at Chill Temperatures ..... 86 5.4 The Chicken's Egg and its Spoilage ............................ 87 5.5 Egg Products ................................................................... 88 6. Spoilage of Fish and other Seafoods ............................... 90-97 6.1 Prologue ........................................................................... 90 6.2 Bacteriology of the Newly Caught Fish ...................... 90 6.3 The Effect of Initial Processing and Storage in ice on Board Ship ...................................................... 91 6.4 The effect of Handling Ashore ..................................... 92 6.5 Chemical Changes induced by Bacteria in Fish ........ 93 6.6 Salted Fish ....................................................................... 94 6.7 Smoked Fish .................................................................... 95 6.8 Shell Fish ......................................................................... 95 6.8.1 Crustaceans ....................................................... 95 6.8.2 Molluscs ............................................................. 96 7. Spoilage of Dairy Products .............................................. 98-105 7.1 Prologue ........................................................................... 98 7.2 Microbiology of Raw Milk ............................................ 98 7.3 Pasteurization ................................................................. 99 7.4 UHT Milk ...................................................................... 100 7.5 Butter .............................................................................. 101 7.6 Cheese ............................................................................ 102 7.7 Yoghurt .......................................................................... 103

xii 8. Spoilage of Vegetables, Fruits and their Products .... 106-113 8.1 Prologue ......................................................................... 106 8.2 Vegetables ...................................................................... 106 8.3 Fruits .............................................................................. 107 8.4 Soft Drinks, Fruit Juices and Preserves, and Vegetable Juices .................................................... 107 8.5 Sauerkraut ..................................................................... 108 8.6 Wine ............................................................................... 109 8.7 Control of Microbial Spoilage ..................................... 109 9. Spoilage of Cereal Products .......................................... 114-118 9.1 Prologue ......................................................................... 114 9.2 Cereal Grains ................................................................ 114 9.3 Bread and Cakes .......................................................... 115 9.4 Refrigerated Dough ...................................................... 115 9.5 Pastas ............................................................................. 115 9.6 Liquid Sweeteners and Confectioneries .................... 116 9.7 Beer ................................................................................. 116 10. Spoilage of Canned Foods ............................................. 119-126 10.1 Prologue ......................................................................... 119 10.2 Leaker Spoilage ............................................................ 120 10.3 Spoilage due to Inadequate Heat Treatment ............ 123 10.3.1 Spoilage of Low Acid Foods ................. ~ ....... 124 10.3.2 Spoilage of High Acid Foods ........................ 125 11. Spoilage of Frozen Foods ............................................... 127-130 11.1 Prologue ......................................................................... 127 11.2 Factors affecting Viability of Microorganisms during Freezing ............................................................ 127 11.3 Effect of Cold Storage ................................................... 129 11.4 Freezing Injury to Cells ............................................... 129 11.5 Thawed Foods and their Spoilage ............................. 129 12. Indicators of Microbial Food Spoilage ........................ 131-137 12.1 Prologue ......................................................................... 131 12.2 Microbiological Criteria ............................................... 133

xiii

12.3 Chemical Criteria ......................................................... 12.4 Assay of Heat-stable Enzymes ................................... 12.4.1 Heat-stable Proteinases in Milk .................... 12.4.2 Heat-stable Lipases in Milk ..........................

135 136 136 136

13. Microbiological Testing of Food ................................... 138-161 13.1 Prologue ......................................................................... 138 13.2 Sampling ....................................................................... 139 13.2.1 Sampling Rate ................................................. 139 13.2.2 The Representative Sample ........................... 139 13.2.3 Sampling Techniques ..................................... 140 13.2.4 Treatment of Sample ....................................... 141 13.3 Microbiological Test Procedures in Common Usage 142 13.3.1 Total Viable Count .......................................... 142 13.3.1.1 The 'Pour Plate' Method ................. 143 13.3.1.2 The 'Spread Plate' Method ............. 143 13.3.1.3 The 'Drop Plate' Method ................. 144 13.3.1.4 The 'Agar Droplet' Method ............ 144 13.3.1.5 The 'Spiral Plate' Method ............... 144 13.3.2 Counting using Electrical Impedance Measurements ............................. 144 13.3.3 Counting by Measurement of Adenosine Triphosphate (ATP) .................... 145 13.3.4 Counting using the Direct Epifluorescent Filter Technique .............................................. 145 13.3.5 Direct Microscopic Count .............................. 146 13.3.6 Indicator Organisms ....................................... 146 13.3.6.1 Coliforms ........................................... 146 13.3.6.2 Enterococci ........................................ 147 13.3.6.3 Enterobacteriaceae ............................ 148 13.3.7 Food Poisoning Organisms ........................... 149 13.3.7.1 Salmonellas ....................................... 150 13.3.7.2 Clostridium perfringens and C. botulinum ............................... 151 13.3.7.3 Staphylococcus aureus ........................ 152 13.3.7.4 Bacillus cereus .................................... 153

xiv

13.3.7.5 Vibrio parahaemolyticus ..................... 153 13.3.8 Food Spoilage Organisms .............................. 154 13.3.8.1 Pseudomonas ....................................... 154 13.3.8.2 Micrococci .......................................... 155 13.3.8.3 Lactobacilli and Leuconostocs ....... 155 13.3.8.4 Streptococci ........................................ 156 13.3.8.5 Spore Formers ................................... 156 13.3.8.6 Yeasts and Moulds .......................... 157 13.3.9 Canned Foods ................................................. 157 13.3.9.1 Dextrose Tryptone Agar .................. 158 13.3.9.2 Reinforced Clostridial Medium ...... 158 13.3.9.3 Tomato Juice Agar ............................ 158 13.3.9.4 Malt Extract Agar ............................. 158 13.3.10 Frozen and Dehydrated Foods ..................... 158 13.3.11 Miscellaneous Tests ........................................ 159 13.3.11.1 Methylene Blue Reduction Test [MBRT] ...................................... 159 13.3.11.2 The Limulus Lysate Test ............... 159 13.3.11.3 Microcalorimetry ............................. 159 13.3.12 Compilation of Specifications ....................... 160 Selected Bibliography ...................................................... 162-178

FOOD MICROBIOLOGY AND ITS HORIZONS

1.1 PROLOGUE The choice of foods available today is extremely wide compared to a few years ago. An increasing amount of food is eaten outside the home. The public has a right to expect food to be of good quality. They do not expect it to make them ill, e.g. by food poisoning. In recent years there has also been more interest in the effect that food has on our health; however, food must still look and taste good. A good place to start is the question, "What is food?" Simply, food is any substance of plant or animal origin that when eaten and absorbed by the body produces energy, promotes the growth and repair of body tissues, or regulates the body processes. The components of food that perform these effects are called nutrients. Nutrition is the study of food (nutrients) and the effect it has on the body. It includes the factors that affect food intake. But food does not just contain nutrients. It contains water which is also essential to life, and various substances that give food colour and flavour. There are other substances that might be put into food such as additives or may just get into food accidentally such as pesticide residues. Many foods also contain a whole range of micro-organisms. The presence of these organisms may be beneficial or they may cause food spoilage or food poisoning.

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MICROBIAL SPOILAGE OF FOODS

Micro-organisms can be deliberately added to food during production, e.g. bread, wine and cheese, or the micro-organisms already present can spoil the food. As spoilage is ultimately inevitable, a whole range of preservation techniques have been developed to try to increase the shelf life (or keeping quality) of our food. Some micro-organisms contaminate our food and cause illness rather than spoilage. Every effort must be made to keep our food free from these hannful micro-organisms. To fully understand the mechanisms by which the processes of spoilage and contamination can be controlled, one needs to understand the basics of microbiology in general and food microbiology in particular.

1.2 SEARCH OF MICROORGANISMS The discovery of microorganisms ran parallel with the invention and improvement of the microscope. Around 1658, Athanasius Kircher reported that, using a microscope, he had seen minute living worms in putrid meat and milk. The magnification power of his microscope was so low that he could not have seen bacteria. In 1664, Robert Hooke described the structure of molds. However, probably the first person to see different types of microorganisms, especially bacteria, was the Dutch businessman turned naturalist Anton van Leeuwenhoek, using a microscope that probably had not above 300x magnification power. He observed bacteria in saliva, rainwater, vinegar, and other materials, sketched the three morphological groups (spheroids or cocci, cylindrical rods or bacilli, and spiral or spirilla), and also described some to be motile. He called them animalcules and in 1675 reported his observations to the newly formed leading scientific organization, The Royal Society of London, where his observations were read with fascination. As fairly good microscopes were not easily available at the time, during the course of the next 100 years, other interested individuals and scientists only confirmed Leeuwenhoek's observations. In the 19th century, as a result of the Industrial Revolution, improved microscopes became more easily available, and that stimulated many inquisitive minds to see and describe creatures they discovered under a microscope. By 1838, Ehrenberg (who introduced the term bacteria) had proposed at least 16 species in four genera and by 1875 Cohn had developed the preliminary classification system of bacteria. Cohn also was the first to discover that some bacteria produced spores. Although, like bacteria, the existence of submicroscopic viruses was recognized in the mid-19th century, they were observed only after the invention of the electron microscope in the 1940s.

FOOD MIRCROBIOLOGY AND ITS HORIZONS

3

1.2.1 Spontaneous Generation Theory Following Leeuwenhoek's discovery, althOl.:gh there were no bursts of activity, some scientific minds did have the curiosity to determine where the animalcules, found to be present in many different objects, were coming from. Society had just emerged from the Renaissance period and science, known as experimental philosophy, was in its infancy. The theory of spontaneous generation, i.e., the generation of some form of life from nonliving objects, had many strong followers among the educated and elite class. Since the time of the Greeks, the emergence of maggots from dead bodies and spoiled flesh was thought to be due to spontaneous generation. But around 1665, Redi disproved that theory by showing that the maggots in spoiled meat and fish could only appear if flies were allowed to contaminate them. The advocates of the spontaneous generation theory argued that the animalcules could not regenerate by themselves (biogenesis), but that they were present in different things only through abiogenesis (spontaneous generation). In 1749, Needham showed that boiled meat and meat broth, following storage in covered flasks, showed the presence of animalcules within a short time. This was used to prove the appearance of these animalcules by spontaneous generation. Spallanzani (1765) showed that boiling meat infusion in broth in a flask and sealing the flask immediately prevented the appearance of these microscopic organisms and thus disproved Needham's theory. This was the time when Antoine-Laurent Lavoisier and his coworkers showed the need of oxygen for life. The believers of abiogenesis rejected Spallanzani's observation, suggesting that there was not enough vital force (oxygen) present in the sealed flask for animalcules to appear through spontaneous generation. Later, Schulze (1830; by passing air through acid), Theodor Schwann (1838; by passing air through red hot tubes), and Schroeder (1854; by passing air through cotton) showed that bacteria failed to appear in boiled meat infusion even in the presence of air. Finally, in 1864, Louis Pasteur demonstrated that, in boiled infusion, bacteria could grow only if the infusions were contaminated with bacteria carried by dust particles in air. His careful and controlled studies proved that bacteria were able to reproduce (biogenesis) and life could not originate by spontaneous generation. John Tyndall, in 1870, showed that in a dust-free box, boiled infusion could be stored in dust-free air without microbial growth. 1.2.2 Importance of Microorganisms The involvement of invisible organisms in many diseases in humans was suspected as early as the 13th century by Roger Bacon. In

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MICROBIAL SPOILAGE OF FOODS

the 16th century, Francostro of Verona suggested that many human diseases were transmitted by small creahlres from person to person. This was also indicated by Kircher in 1658. In 1762, von Plenciz of Vienna suggested that different invisible organisms were responsible for different diseases. Schawnn (1837) and Hermann Helmholtz (1843) pointed out that putrefaction and fermentation were connected with the presence of the organisms derived from the air. Finally, Pasteur, in 1875, showed that wine fermentation from grapes and souring of wine were caused by microorganisms. He also proved that spoilage of meat and milk was associated with the growth of microorganisms. Later, he showed the association of microorganisms with several diseases in humans, cattle, and sheep, and later developed vaccines against several human and animal diseases, including the rabies virus. Robert Koch, in Germany (in the 1880s and 1890s), isolated bacteria in pure cultures responsible for anthrax, cholera, and tuberculosis. He also developed the famous Koch's postulates to associate a specific bacterium as a causative agent for a specific disease. Along with his associates, he also developed techniques of agar plating methods to isolate bacteria in pure cultures, the petri dish (by Petri in his laboratory), and staining methods for better microscopic observation of bacteria. With time, the importance of microorganisms in human and animal diseases, soil fertility, plant diseases, fermentation, food spoilage and foodborne diseases, and other areas was recognized, and microbiology was developed as a separate discipline. Later, it was divided into several disciplines, such as medical microbiology, soil microbiology, plant pathology, and food microbiology.

1.3 EARLY DEVELOPMENTS IN FOOD MICROBIOLOGY (PRIOR TO 1900 A.D.) It is not known exactly when our ancestors recognized the importance of the invisible creatures, now designated as microorganisms, in food. But it had to be around 8000 B.C. in the Near East after they developed agriculture and animal husbandry. They produced more foods than they could consume within the short growing season, and a portion of the produce was lost due to spoilage. They solved the problems and secured uniform food supplies throughout the year by developing different preservation techniques. Between 8000 and 2000 B.C., they used drying, cooking, smoking, salting, low temperature, baking, modified atmosphere, fermentation, spices, and honey to extend the storage life of different types of raw and processed foods. Although we are not sure if they had perceptions

FOOD MlRCROBIOLOGY AND ITS HORIZONS

5

about the cause of foodbome diseases, they definitely associated food spoilage with some invisible factors and developed successful preventative measures. From the time of the Greeks until the discovery of biogenesis, spoilage of foods, especially of meat and fish, was thought to be due to spontaneous generation, such as the development of maggots. When the presence of different types of bacteria in many foods was diScovered, their appearance through spontaneous generation was explained to be the cause of food spoilage. Schawnn (1837) and Helmholtz (1843) associated the presence of microorganisms (bacteria) in food with both putrefactive and fermentative changes of foods. However, they did not believe in spontaneous generation, but they could not explain how microorganisms could bring about those changes. Finally, Pasteur resolved the mystery by explaining that contamination of foods with microorganisms from the environment and their subsequent metabolic activities and growth were the causes of fermentation of grapes, souring of milk, and putrefaction of meat. Diseases caused by the consumption of certain foods (foodbome disease) was recognized at least during the Middle Ages. Ergot poisoning in Europe was related to the consumption of grains (infested with molds) in the 12th century. In 1857, consumption of raw milk was suspected to be the cause of typhoid fever. In 1870, Selmi related certain food poisoning with ptomaine (histamine). Gaertner was the first to isolate Salmonella from a meat implicated in a foodbome disease in 1888. Denys, in 1894, was able to estClblish Staphylococcus aureus with food poisoning and, in 1896, Ermengem isolated Clostridium botulinum from food. The association of many other pathogenic bacteria and viruses to foodbome diseases was established after 1900 A.D. Pasteur, in the 1860s, recognized the role of yeasts in alcohol fermentation. He also showed that souring of wine was due to growth of acetic acid-producing bacteria (Acetobacter acet!), and developed the pasteurization process (heating at 145°F for 30 min.) to selectively eliminate these undesirable bacteria from wine. Like fermentation, cheese ripening was suggested by Martin in 1867 to be of microbial origin. John Lister, in 1873, was able to isolate milk-souring bacteria (LactOCOCCIlS lactis) by the serial dilution (dilution to extinction) procedure. Cienkowski, in 1878, isolated the bacteria (Leuconostoc mesenteroides) associated with slime formation in sugar. In 1895, microbial enumeration of milk was developed by Von Geuns. After 1900 A. D., the involvement of different microorganisms in food spoilage and food fermentation was demonstrated.

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MICROBIAL SPOILAGE OF FOODS

1.4 SIGNIFICANCE OF MICROORGANISMS IN FOODS Since 1900 AD., our understanding of the importance of microorganisms in food has increased greatly. Their role in food can be either desirable (food bioprocessing) or undesirable (foodbome diseases and food spoilage), which is briefly discussed here.

1.4.1 Foodbome Diseases Many pathogenic microorganisms (bacteria, molds, and viruses) can contaminate food during various stages of their handling between production and consumption. Consumption of these foods can cause foodbome diseases. Foodbome diseases not only can be fatal, but they can cause large economic losses. Foods of animal origin are associated more with foodbome diseases than foods of plant origin. Mass production of foods, introduction of new technologies in the processing and storage of foods, changes in food consumption patterns, and the increase in imports of food from other countries have increased the chances of large outbreaks as well as the introduction of new pathogens. Effective intervention technologies are being developed to ensure the safety of consumers against foodbome diseases. New methods are also being developed to effectively and rapidly identify the pathogens in contaminated foods.

1.4.2 Food Spoilage Except for sterile foods, all foods harbor microorganisms. Food spoilage stems from the growth of these microorganisms in food or is due to the action of microbial heat-stable enzymes. New marketing trends, the consumers' desire for foods that are not overly processed and preserved, extended shelf life, and chances of temperature abuse between production and consumption of foods have greatly increased the chances of food spoilage and, in some instances, with new types of microorganisms. The major concerns are the economic loss and wastage of food. New concepts are being studied to reduce contamination as well as control the growth of spoilage microbes in foods.

1.4.3 Food Bioprocessing Many food-grade microorganisms are ~d to produce different types of fermented foods using raw materials from animal and plant sources. Consumption of these foods has increased greatly over the last 20 to 30 years and is expected to increase still more in the future. There have been great changes in the production and availability of these microorganisms (starter cultures) to meet the large demand. Also, novel

FOOD MIRCROBIOLOGY AND ITS HORIZONS

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and better strains are being developed using genetic engineering techniques. Microbial enzymes are also being used to produce food and food additives. Genetic recombination techniques are being used to obtain better enzymes and from diverse sources. Many types of additives from microbial sources are being developed and used in food. 1.4.4 Food Biopreservation Antimicrobial metabolites of desirable microorganisms are being used in foods in place of nonfood preservatives to control pathogenic and spoilage microorganisms in food. Economic production of these antimicrobial compounds and their effectiveness in food systems have generated wide interest.

1.5 ROLES OF FOOD MICROBIOLOGISTS From the above discussion, it becomes apparent what, as a discipline, food microbiology has to offer. Prior to the 1970s, food microbiology was regarded as an applied science mainly involved in the microbiological quality control of food. Since then, the technology used in food production, processing, distribution and retailing, and food consumption patterns have changed dramatically. These changes have introduced new problems that no longer can be solved by just using applied knowledge. Thus, modem-day food microbiology needs to include a great deal of basic science to effectively solve the microbiological problems in food. The discipline not only includes the microbiological aspects of food spoilage and foodbome diseases and their effective control and bioprocessing of foods, it also includes basic information of microbial physiology, metabolism, and genetics. This information is helping to develop methods for rapid and effective detection of spoilage and pathogenic bacteria, to develop desirable microbial strains by recombinant DNA technology, to produce fermented foods of better quality, to develop thermostable enzymes in enzyme processing of food and food additives, to develop methods to remove bacteria from food and equipment surfaces, and to combine several control methods for effective control of spoilage and pathogenic microorganisms in food. An individual who has completed courses in food microbiology (both lecture and laboratory) should gain knowledge in the following areas: 1. Determination of microbiological quality of foods and food ingredients using appropriate techinques.

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MICROBIAL SPOILAGE OF FOODS

2.

Determination of microbial type(s) involved in spoilage, health hazards, and the identification of the sources 3. Design corrective procedures to control the spoilage and pathogenic microorganisms in food 4. Identify how new technologies adapted in food processing can have specific microbiological problem and design methods to overcome the problem 5. Design effective sanitation procedures to control spoilage and pathogen problems in food processing facilities 6. Effective use of desirable microorganisms to produce fermented foods 7. Design methods to produce better starter cuitures for use in fermented foods and probiotics 8. Food regulations (state, federaC international) To be effective, in addition to the knowledge gained, one has to be able to communicate with different groups of people about the subject (food microbiology and its relation to food science).

DDD

BASICS OF HUMAN NUTRITION

2.1 PROLOGUE This chapter describes all the nutrients that we must obtain from food, the types of foods that we get them from and the effect of cooking or processing on nutrients. Although water is not a nutrient, it is essential for life and so water is also included in this section. How our body obtains the nutrients from the food we eat is essential for an understanding of nutrition. The last section introduces you to the concept of food energy or Calories as it is more widely known. Dietary allowances for Indians (ICMR recommendations) as well as balanced diets are also discussed. At the end, various types of natural foodstuffs available in India and their nutrient contents are summarized.

2.2 IMPORTANCE OF FOOD By way of introduction, hunger is one of our strongest drives and we do not need a bell to remind us to eat. We have a physiological signal. Our blood usually contains about one-tenth per cent sugar and draws on the body to maintain this sugar level. A large portion of the sugar obtained from a meal is stored in the liver and slowly released on demand. When the blood sugar falls, we begin to feel sluggish, and the muscular contractions of an empty stomach tells us we must eat. We can eat sugar to boost our blood sugar level temporarily but a stomach must be at least partially filled and coated, even thinly, with fat to overcome hunger.

10

MICROBIAL SPOILAGE OF FOODS

In addition to the physiological aspects, there is a mental response to hunger which stimulates the flow of saliva. Our interest in food is basic to our existence. Food is an absolute need, our energy source. Food should contain the building blocks we need to grow and to repair tissues. Food should contain the nutrients we need to regulate our body systems. Our life process can be seen as a constant use of energy and a constant exchange of materials. While occasionally our own tissues can yield some nutrients to meet a temporary shortage, we cannot do so long. We need food, and there must be a balance between the food we eat and our nutrient needs if we are to achieve a feeling of well being. 2.3 CONSTITUENTS OF FOODS Most people now know that plants and animals as a group contain sugars, proteins, fats, vitamins, minerals and water. Eaten as food, starch and sugars are either burned, stored as fat, or stored as glycogen (animal starch). Protein is broken down into amino acids which are used to build and repair body tissues, burned for energy, changed into sugar, or contribute to the production of fat. Fat is broken down in digestion, passed into the body, recombined into fat, changed into important body chemicals and burned for energy or stored as fat in tissues. All these substances (sugar, protein, and fat) may be used from a variety of food sources as fuel by our bodies. 2.3.1 Carbohydrates

2.3.1.1 Introduction and Classification Carbohydrates are an important group of nutrients in the diet; their main function is providing energy. Thei.r structures are all based on a common lmit called a saccharide unit. This unit is nearly always glucose, and the grouping or classification of carbohydrates depends primarily on the number of saccharide units each carbohydrate contains. This can vary from one to many thousands. Sugars MONOSACCHARIDES one unit (mono meaning one) DISACCHARIDES two lmits (di meaning two) POLYSACCHARIDES NOll-sugars many units (poly meaning many) (A) Monosaccharides (simple sugars) Three types of sugar are important:

BASICS OF HUMAN NUTRITION

11

Glucose (sometimes called dextrose) is t~e most important monosaccharide. It occurs naturally in fruit and plant juices, and in the blood of living animals. Most carbohydrates in food are ultimately converted to glucose during digestion. 2. Fructose occurs naturally in some fruit and vegetables and especially in honey. It is the sweetest sugar known and is often called fruit sugar. 3. Galactose does not exist as such in foods but is produced when lactose (a disaccharide) is broken down during digestion (see below). (B) Disaccharides Disaccharides consist of two monosaccharides linked together: 1. Sucrose occurs naturally in sugar cane and sugar beet and in some roots (e.g. carrots) and fruits. Table sugar, as sucrose is often called, is a chemical combination of glucose and fructose. 2. Maltose is formed during the breakdown of starch by digestion and during the germination or sprouting of barley (important in beer production). It is a combination of two glucose units. 3. Lactose occurs only in milk, including human milk. It is less sweet than sucrose or glucose and is a combination of glucose and galactose. We are being encouraged to eat more fruit and vegetables, hence our intake of intrinsic sugars should rise. However, our intake of the extrinsic sugar sucrose is already extremely high and so most authorities recommend a reduction in this type of sugar. Properties of sugars (Monosaccharides & Disaccharides) 1. All sugars are white crystalline compounds which are soluble in water (i.e. they dissolve in water). 2. All sugars a!e sweet but they do not have the same degree of sweetness. 3. When sugars are heated they caramelise. 4. Sugars can act as preservatives if large amounts are present in a food, e.g. jam. Sugars are also grouped on the basis of where they occur in foods (i.e. inside or outside the cell walls of foods). (Table 2.1) 1.

12

MICROBIAL SPOILAGE OF FOODS

Table 2.1 : Classification of Sugars

Intrinsic (sugars contained inside the cell walls of foods)

1

Sugars Extrinsic (sugars not contained within the cell walls of foods) /Ex as in external "non-milk: milk and milk products

1

.

lactose mostly sucrose (table sugar and walls of fruits and baked foods, also vegetables honey) (C) Polysaccharides These are formed from a varying number of monosaccharide units. They are usually insoluble in cold water (i.e. do not dissolve in cold waterfand are tasteless. Polysaccharides in food fall into three groups: 1. Starch is the most important polysaccharide. It is the major food reserve of plants and is a mixture of two polysaccharides called amylose and amylopectin. Starch is a white powder and does not have a sweet taste. If you heat a mixture of starch in water it eventually thickens; this is the reason why sauces thicken when you heat them. On heating, the starch granules swell and eventually gelatinise. 2. Glycogen is a carbohydrate found only in animals, where small amounts are stored in the liver and muscles, and act as an energy reserve. Glycogen is composed of branched chains of glucose units, but unlike amylopectin it is soluble in water. We do not eat very much glycogen because it breaks down again to glucose after the animal is slaughtered. 3. Non-Starch Polysaccharides (NSP) (or dietary fibre) provide the rigid and fibrous structure of vegetables, fruits and cereal grains. They form the main part of food that is not digested. The term' dietary fibre' was widely used to describe this part of food. However, the amount of fibre

fructose, glucose and sucrose within cell

BASICS OF HUMAN NUI'RITION

13

found in foods appeared to vary depending on which method of chemical analysis had been used. Therefore, to obtain some standardisation, a specific method of chemical analysis has been agreed upon. This measures the amount of non-starch polysaccharide (NSP) in a food (Le. the amount of polysaccharide other than starch). The term NSP may replace dietary fibre in future. NSP is made up of the following: a. Cellulose consists of many thousands of glucose units. It cannot be digested by man because we do not have the necessary enzymes to break it down. Cellulose is important for providing roughage or bulk in the diet and therefore assisting in the passage of digestible materials and waste products through the intestines. b Pectin and other similar polysaccharides are found in many fruits and some root vegetables, e.g. turnips. Apples and the peel of citrus fruits are particularly rich in pectin. Its main importance is as a gelling agent, e.g. in jam making. c. Hemicelluloses and other polysaccharides are found in small amounts. The old term 'dietary fibre' included all the above plus other non-digestible plant material such as the woody material lignin.

2.3.1.2 Functions of carbohydrates in the diet After eating foods containing polysaccharides and disaccharides, they are hydrolysed (broken down) by digestive enzymes. All carbohydrates are absorbed as monosaccharides. As part of the digestive process the monosaccharides fructose and galactose are converted into glucose. Thus almost all digested carbohydrates are eventually converted to glucose. Glucose has two main functions in the body: 1. Energy: Glucose is oxidised in the cells with the release of energy. This energy can be used for physical activity but more usually it is needed by body cells for normal functioning: 19 of carbohydrate provides 3.75 kcal (16 kJ). 2. Converted into body fat: Any carbohydrate you eat that you do not immediately need for energy may be converted into body fat. This conversion takes place in the liver but the fat is stored all over the body, mainly in the adipose tissue under the skin.

14

MICROBIAL SPOILAGE OF FOODS

2.3.1.3 Sources of carbohydrate in the diet 1.

Cereals and Cereal Foods : All cereals contain a high percentage of starch. The main cereals consumed in the world are wheat, rice, maize (corn), oats, rye and barley. Cereal foods account for 40% of the total carbohydra te content of the average British diet. 2. Refined Sugar (sucrose) : Sugar is eaten in large quantities, as table sugar and in manufachlred goods such as biscuits, sweets., ice-cream, jams and soft drinks. Sugar and preserves account for 11 % of the total carbohydrate content of the average British diet. 3. Vegetables: Vegetables contain starch and sugars in varying amounts. Potatoes are the richest source of carbohydrate although pulse vegetables (e.g. beans) also contain significant amounts. They account for 14% of the total carbohydrate content of the average British diet. 4. Fruits: As fruit ripens starch is turned into sugar. Most fruits contain between 5% and 10% sugar. Bananas are the only fruit which contain starch as well as sugar when ripe. Fruits contribute on average 7% of the total carbohydrate content. 5. Milk: Milk and milk products such as yoghurt contain the sugar lactose. They contribute 8% of the total carbohydrate content of the average British diet. Foods such as cheese and butter made from milk do not contain lactose because the whey part of milk which contains lactose is discarded during cheese and butter production.

2.3.1.4 Dietary guidelines We are recommended to eat more starchy foods like cereals and also fruit and vegetables. Starches should provide 39% of our daily energy (Calorie) intake and sugars (mainly sucrose) 11 %. In addition we should aim to eat 18g/day of NSP with a range of 12-24g/ day. Because of their smaller body weight children should eat less.

2.3.2 Lipids [Fats and Oils]

2.3.2.1 Introduction and chemical structure Fats and oils (or lipids) include not only 'visible fats' such as butter and margarine, cooking fats and oils and the fat on meat, but also the 'invisible fats' which occur in milk, nuts, lean meat and other

15

BASICS OF HUMAN NUTRITION

foods. They are a more concentrated source of energy than carbohydrates; much of the energy reserve of animals and some seeds is stored in this form. Although 'lipids' is the correct term, the term 'fats' is often used to mean both fats and oils. Fats and oils found in food consist mainly of mixtures of triglycerides. Each triglyceride is a combination of three fatty acids with a unit of glycerol (glycerine). The differences between one fat or oil and another are the result of different proportions of the various fatty acids in each.

Glycerol 'backbone'

-f

Fatty acid Fatty acid Fatty acid

Fig. 2.1 : Triglyceride structure.

(a) Fatty acids There are many different fatty acids found in nature. They consist of chains of carbon atoms with hydrogen atoms attached (this structure is called a hydrocarbon chain). They differ in the number of carbon atoms and double bonds which they contain. Each carbon atom can make links with a maximum of four other atoms. (b) Saturated fatty acids These have no double bonds and therefore are more stable. This structure is 'saturated' with hydrogen; no more hydrogen atoms can be fitted in. H H H H H H H I

I

I

I

I

I

I

I

I

I

I

I

I

I

- C - C - C - C - C - C - C - COOH (Acid Group) H H H H H H H Fig. 2.2 : Part of the hydrocarbon chain of a saturated fatty acid. (c) Unsaturated fatty acids

Here the carbon chain is not saturated with hydrogen and therefore has one or more double bonds. These react gradually with air making the fat rancid. HHHHHHH I

I

I

I

I

I

I

-C-C=C-C-C=C-C-COOH I

H

I

H

I

H

Fig. 2.3 : Part of the structure of an unsaturated fatty acid.

16

MICROBIAL SPOILAGE OF FOODS

Because each carbon atom can make links with four other atoms, it is possible to fit more hydrogen atoms into this structure by breaking the double bond; hence this structure is 'unsaturated'. Unsaturated fatty acids may be either: Monounsaturated, containing one double bond Polyunsaturated, containing more than one double bond. (d) Cis and trans fatty acids The arrangement of atoms at the double bond may vary and both mono and polyunsaturated fatty acids can be either: 1. CIS fatty acids with the two hydrogen atoms on the same side of the double bond. H

H

I

I

-C=CNaturally occurring unsaturated fatty acids are usually in the CIS configuration. OR

2.

TRANS, fatty acids with the hydrogen atoms on geometrically opposite sides of the double bond. H I

-C=CI

H

Small amounts are found naturally in some foods but larger amounts can occur as a result of certain types of lipid processing. There is some concern about high intakes of TRANS fatty acids and hence current recommendations are that we do not increase our intake of this type of lipid.

2.3.2.2 Properties of lipids 1.

Fats are lipids which are solid at low temperatures and become liquid when they are heated. Oils are lipids which are liquid at room temperature, usually as a result of their higher content of unsaturated fatty acids, and will solidify on refrigeration, e.g. olive oil. 2. Oils and fats do not dissolve in water but may be emulsified with water by vigorous mixing as when butter and margarine are made.

BASICS OF HUMAN NUTRITION

17

3.

Lipids make an important contribution to the texture and palatability of foods. 4. Furthermore, because they are digested comparatively slowly, foods rich in lipids have a high satiety value. If you eat a fatty meal you won't feel hungry as quickly as if you had eaten a high carbohydrate meal. 5. Food lipids usually contain small amounts of other fat soluble substances, e.g. flavour compounds and the fat soluble vitamins.

2.3.2.3 Functions of lipids in the diet 1. Energy: Fat is broken down in the body by oxidation and energy is released. 19 of fat providEs 9kcal (37k1). Fat has more than twice the calorific value of carbohydrates and is therefore a more concentrated source of energy. 2. Formation of adipose tissue : Excess fat, which is not immediately required for energy, is stored in the adipose tissue under the skin where it has three functions. a. an energy reserve; b. it forms an insulating layer and helps to prevent excessive heat loss from the body. It therefore assists in the maintenance of a constant body temperature; and c. when stored around delicate organs such as the kidneys, it protects these organs from physical damage. 3 Essential fatty acids: Some fatty acids are essential in small amounts for the functioning of the body. Linoleic acid and one form of linolenic acid (alpha - linolenic acid) are probably the only truly essential fatty acids. Linoleic acid is needed for the formation of cell membranes. Derivatives of the essential fatty acids are used to form prostaglandins, a group of hormone-like substances which help to regulate many body functions. If the diet provides 1-2% of its energy content as essential fatty acids then deficiency is unlikely. Most diets contain more than 2%. 4. Fat soluble vitamins: The inclusion of certain fats included in the diet help to ensure an adequate intake of the fat soluble vitamins A, 0, E and K.

18

MICROBIAL SPOILAGE OF FOODS

2.3.2.4 Sources of lipids in the diet Fats and oils are obtained from both animals and plants. 1. Meat and fish: All meat contains fat, though the percentage of fat varies from animal to animal, and from one part of an animal to another. Meat provides 25% of the total fat content of the average British diet. Oily or fatty fish such as herring and mackerel contain up to 20% oil, but they contribute little fat to the British diet as they are eaten infrequently. 2. Butter and margarine: Butter contributes 6% and margarine a further 6% of the total fat content of the average diet. 3. Milk, cream and cheese: Full cream milk contains between 3% and 4% fat; some products made from milk, such as cream and cheese contain much larger amounts. Milk and milk products account for 11% of the total fat content of the average British diet, and cheese contributes a further 6%. 4. Other foods : Other important sources of fat are baked goods (cakes, pastries, biscuits) 8%, and vegetable oils, 8%. Many other foods contain a considerable amount of fat. These include ice-cream, chocolates, some sweets, nuts and salad dressings. Most vegetables and fruits do not contain significant amounts of fat except the soya bean (24% fats) and the avocado pear (8% fat). 2.3.2.5 Dietary guidelines It is recommended that, on average, 35% of our energy (Calorie) intake comes from fats and oils with only 11% from saturated fatty acids. At present we eat considerably more than this and hence we are advised to eat less fat, particularly saturated fat.

2.3.3 Proteins

2.3.3.1 Introduction and chemical structure Proteins are found in all living cells of animals and plants. Protein must be provided in the diet for the growth and repair of the body, but any excess is used to provide energy. Proteins consist of chains of hundreds or even thousands of amino acid units. Only about

19

BASICS OF HUMAN NUTRITION

20 different amino acids are involved, but the number of ways in which they can be arranged is almost infinite. It is the unique sequence of these units which gives each protein its characteristic properties. Of the 20 amino acids commonly found in proteins, 8 are essential in the diet (9 for children) and must be supplied by the foods we eat since they cannot be made in the body. The non-essential amino acids can be synthesised in the body by converting one amino a~id into another within the body's cells. (Table 2.2) Table 2.2: Essential and Non-essential Amino Acids Essential Isoleucine Leucine Lysine Methionine Phenylalanine Threonine Tryptophan Valine Histidine (essential for infants)

Non-essential Alanine Arginine Asparagine Aspartic acid Cysteine Glutamic acid Glutamine Glycine Proline Serine Tyrosine

2.3.3.2 Functions of proteins in the body I. Growth and Maintenance : Proteins are the main constituents of the cells of the body. The number of cells in the body increases during periods of growth, therefore during childhood and adolescence, protein requirements are high. In addition, protein in the tissues is constantly being broken down and must be replaced from the amino acids supplied in the diet. Protein is also necessary for the formation of enzymes, antibodies and some hormones. II. Energy: The diet may supply more protein than is required for growth and maintenance. Any excess protein may be used for energy. 19 of protein provides 4kcal (17kJ). 2.3.3.3 Sources of protein in the diet Protein can be obtained from both animal and plant sources.

20

MICROBIAL SPOILAGE OF FOODS

1.

2.

3. 4. S.

Meat and fish: Meat makes an important contribution to the protein content of the average British diet. Fish is eaten less frequently and hence makes a smaller contribution to protein intake. Bread and cereals : Bread contains a significant amount of protein and is one of the most important and cheapest sources in the British diet. Other cereal foods such as rice, pasta, breakfast cereals, cakes and biscuits are also significant sources of protein. Milk and cheese : Milk and cheese are valuable sources of good quality protein in the British diet. Eggs: These are an excellent source of high quality protein although their contribution to the diet is small. Nuts: Nuts are a major source of protein in the diets of many vegans and vegetarians.

2.3.3.4 Dietary guidelines In the world most people eat much more protein than they need. It is recommended that we obtain 15% of our energy from protein which, for an adult, is between 45 and SSg protein per day. 2.3.4 Vitamins

2.3.4.1 Introduction and types Vitamins are substances which the body requires in small amounts, yet cannot make for itself, at least in sufficient quantities. They must, therefore, be eaten as part of the diet. As the vitamins were discovered, each was first labelled with a letter, but once a vitamin has been isolated and its structure identified, it was given a specific name. For example, the chemical found in milk which promoted growth was given the name vitamin A. As the vitamins were identified it became possible to divide them into two groups: 1. Fat soluble vitamins Vitamin A - retinol Vitamin D - cholecalciferol Vitamin E - tocopherol Vitamin K - phylloquinone 2. Water soluble vitamins Vitamin B1 - thiamin Vitamin B2 - riboflavin

21

BASICS OF HUMAN NUTRITION

Nicotinic acid Vitamin B6 - pyridoxine Vitamin B12 - cyanocobalamin Folic acid Pantothenic acid Biotin Vitamin C - ascorbic acid Fat soluble vitamins can be stored in the human body, water soluble vitamins cannot. This means that an adequate daily intake of water soluble vitamins such as vitamin C is particularly important. One drawback to the body's ability to store fat soluble vitamins is that toxic levels can accumulate in the body, although this condition is rare. In general, any excess of the water soluble vitamins is immediately excreted in the urine.

2.3.4.2 Functions of vitamins • promote health and help prevent disease • regulate the building and repair of body cells • help regulate the chemical reactions which release energy in body cells. A well-balanced diet will contain all the vitamins in the recommended quantities but as some foods are very poor sources of certain vitamins it is essential to choose appropriate foods. It is rare for people in the developed countries to suffer a severe deficiency of vitamins. Some people may suffer minor deficiencies with symptoms such as tiredness, broken nails, poor condition of skin, h~r and teeth. These symptoms may be due to a shortage of one or of several vitamins. The amount of each vitamin needed to promote health is more than the amount needed to prevent disease. Active form, biochemical functions and deficiency of various fat-soluble and water-soluble vitamins are summarized in Table 2.3. Table 2.3 : Biochemical Functions of Vitamins Vitamin Fat-soluble: Vitamin A [Retinol]

Active Form

Function

ll-cis-retinal retinoic acid

vision, growth factor, xerophthalmia, maintenance of kera tomalacia or blindness epithelial integrity contd ....

Deficiency

22

MICROBIAL SPOILAGE OF FOODS

...contd. / Vitamin Vitamin D [Calciferol] Vitamin E [Tocopherol]

Active Form I, 25-dihydroxycholecalciferol

Function calcium and phosphate metabolism a-tocopherol biological antioxidant, intra-cellular respiration, cell and vascular integrity, central nervous system and muscle integrity 2 methyl-3-phytyl- formation of certain I, 4-naphthoblood clotting factors quinone

Vitamin K [Phylloquinone] Water-soluble: Tniamin (B-1) thiamin pyrophosphate Riboflavin (B-2)

flavin mononucleotide, flavin adenine dinucleotide

Niacin

nicotinamide adenine dinucleotide; nicotinamide adenine dinucleotide phosphate tetrahydrofolic acid and enzymes carboxybiotin and biotin enzymes

Folacin Biotin

Pantothenic acid

co-enzyme A

DeficicncI rickets, osteomalacia microcytic anemia and edema in prematures; creatinuria, red cell hemolysis, ceroid pigment hemorrhage, decreased clotting

aldehyde transfer, metabolism of carbohydrates hydrogen and electron transfer, role in metabolism of macronutrients

beriberi

hydrogen transfer, degradation and synthesis of fatty acids, carbohydrates and amino acids formyl transfer, C 1 metabolism CO transfer, 2 carboxyl group transfer, fatty acid biosynthesis acyl transfer, role in macro-nutrient metabolism

pellagra

glossitis, dermatitis cheilosis, ocular symptoms

macrocytic anemia seborrheic dermatitis

gastrointestinal and nervous disorders contct ....

23

BASICS OF HUMAN NUTRITION

... contd. Vitamin

Active Fonn

Function

Deficien~

Pyridoxine

pyridoxal phosphate, and decarboxy lases, deaminases, transaminases B-12 co-enzymes and enzymes ascorbic acid, dehydroascorbic acid

NH transfer, other

convulsions, dermatitis

(8-6)

Cobalamin (B-12) Vitamin C

Choline

choline, acetylcholine

2

functions in amino acid metabolism

dehydrogena tion, methylation integrity of intercellular substances, oxidation-reduction systems fat metabolism, compound of phospholipids, methyl donor for transmethylation

pernicious anemia scurvy

fatty liver

2.3.5 Minerals

2.3.5.1 Introduction and Importance Apart from hydrogen, carbon and oxygen (the main elements of which protein, fat and carbohydrate are composed) the body also requires around 20 other elements for a variety of reasons. Fifteen of them are known to be essential and a further three or more are necessary for normal life in other animal species and may prove to be necessary for man: no dietary deficiency has yet been shown. Minerals have four main functions: • body building e.g. constituents of bones and teeth • control of body processes, e.g. transmission of nerve impulses • essential part of body fluids and cells • form part of many enzymes and other proteins which are necessary for the release and utilisation of energy. Some mineral elements are required in relatively large amounts and are known as major minerals. These include:

24

MICROBIAL SPOILAGE OF FOODS

1. Major Minerals:

Calcium Sodium Phosphorus Chloride Magnesium Potassium Zinc Iron Sulphur Others are required in minute amounts and are known as trace elements. 2. Trace Elements: Iodine Fluoride Molybdenum Copper Cobalt Sulphur Chromium Selenium Manganese Information about the various major minerals are summarized in Table 2.4. Similarly, main functions and rich sources of trace elements are mentioned in Table 2.5. Table 2.4: Major Minerals Calcium

Iron

Sodium

Phosphorus

Main Functions Growth and development of bones and teeth; blood clotting and hormone secretion Red blood cell formation; oxygen transport and transfer. Deficiency can lead to anaemia Maintenance of constant body water content, muscle and nerve activity. High intakes have been related to high blood pressure Component of all cells; combines with other

Rich Sources Milk, cheese, yoghurt, flou~bread,green

vegetables, canned fish Red meat (particularly offal), bread, flour and cereal products, green leaf vegetables Table and cooking salt, bread, cereal products, meat products

Milk, milk products, bread, cereal products, contd ....

25

BASICS OF HUMAN NUTRITION ... contd.

Magnesium

Chloride

Potassium Zinc

Sulphur

Main Functions minerals (eg. calcium) to give strength to bones and teeth Muscle tone; activation of enzymes-especially in protein synthesis Assists sodium and potassium

Rich Sources meat, meat products

Milk, bread, cereal products, potatoes, other vegetables Table and cooking salt, bread, cereal products, meat products, milk Vegetables, meat, milk, Maintenance of constant fruit and fruit juices body water content Meat and meat Bone metabolism; activation of enzymes; release of products, eggs, fish Vitamin A; growth; immune system; taste; insulin release Component of certain Protein containing foods essential amino acids; metabolism of drugs; bone metabolism Table 2.5 : Trace Elements

Iodine

Copper

Main Functions Constituent of thyroid hormones, which regulate many body processes Growth; component of many enzymes - including those needed for formation of blood and bone - and in the body's defence system; neurotransrnittor function; cell respiration

Rich Sources Milk and milk products, meat, eggs, fish Wholegrain cereals, meat, vegetables

contd ....

26

MICROBIAL SPOILAGE OF FOODS

... contd. Cobalt Chromium

Manganese SeleniuI"

Molybdenum

Fluoride

Main Functions Component of Vitamin BI2 Enhances the action of insulin, which controls the utilisation of glucose Helps to maintain structure of cells; enzymes Part of an enzyme involved in protection of membranes and lipids against oxidative damage Component of several enzymes-including one concerned in the formation of uric acid; possibly in the utilisation of iron Increases the resistance of teeth to decay

Rich Sources Animal products Widely distributed, particularly wholegrain cereals and vegetables Wholegrain cereals, nuts and tea Cereals, fish, offal, meat, cheese, eggs, milk

Widely distributed, particularly in vegetables and pulses

Tea, fish, water

2.3.6 Water By the end of this section you should be able to understand why although water is not a nutrient, it is essential to life.

2.3.6.1 Water balance All living organisms contain water; the human body consists of about 65% water. It is the medium in which nutrients, enzymes and other chemical substances can be dispersed and in which the chemical reactions necessary for maintaining life take place. It is also necessary as a means of transport within the body. Nutrients are carried to cells and waste products are transported from the cells by blood plasma which is 90% water. It is possible to exist for several weeks without food, but the body can only survive a few days without water. Water comes from 'solid' foods as well as from drinks and is lost by evaporation in the breath and sweat as well as in the urine. The balance of water retained in the body is normally very carefully regulated by the kidneys. Excessive losses can occur usually as a result of vomiting or diarrhoea, in illness or from heavy sweating

BASICS OF HUMAN NUTRITION

27

due to strenuous activity or a hot climate. If water intake is not increased, dehydration may result. The amount of water taken into the body is determined mainly by habit and social custom. It is also regulated by thirst which arises as a result of the concentration of sodium in the blood. The body cannot store water and any excess passes into the urine.

2.4 FATE OF MAJOR NUTRIENTS IN THE BODY? 2.4.1 Carbohydrates The simple sugars entering the intestinal wall are carried by the blood stream directly to the liver. They may then be: • passed as glucose to all the cells of the body to be used directly for energy • converted into glycogen and stored in the liver and skeletal muscles as a readily available source of energy • converted into fatty acids and stored in the body fat (adipose tissue) as a source of energy.

2.4.2 Fats Almost all the fatty acids which enter the intestinal wall are immediately rebuilt into triglycerides which are carried to the blood stream by lymph. Fat may be further transformed by the liver and some of it is finally deposited in the adipose tissue. The reservoir of fat is constantly available as a source of energy.

2.4.3 Proteins When the pep tides enter the intestinal wall, they are split into amino acids which are carried in the blood directly to the liver. Then: • they may be passed into the general circulation where they enter the body's 'pool' of essential and non-essential amino acids. These are then built into the structural proteins and specific enzymes which each cell needs • the excess of some amino acids may be converted into those that are lacking • any excess of amino acids will be used as a source of energy or converted to body fat.

28

MICROBIAL SPOILAGE OF FOODS

2.5 FOOD AND ENERGY 2.5.1 Energy value of nutrients All living organisms require a source of energy. The chemical energy in food is released in the cells of animals by oxidation. Some of the energy is used to maintain metabolic processes in the cells, some is converted into heat to maintain body temperature and some is converted into mechanical energy which is used for physical activity. The unit of energy is the joule. Since the joule is too small for practical nutrition the kilojoule (kJ) is used. However, traditionally the Calorie or more correctly, the kilocalorie (kcal) unit was used and so normally energy values are given in both types of unit. 1 kilocalorie (lkcal) = 4.18kJ An even larger unit, the megajoule (MJ) is also used: 1MJ = 1,000,000 joules = 1000kJ The three groups of nutrients which provide the body with energy are carbohydrates, fats and proteins. • 19 of carbohydrate provides 3.75kcal (16kJ) • 19 of fat provides 9kcal (37kJ) • 19 of protein provides 4.0kcal (17kJ) 1£ alcohol is consumed this also contributes to the body's energy intake. • 19 of alcohol provides 7kcal (29kJ) 2.5.2 Energy value of foods The energy value of food, often called the Calorie content, depends on the quantities of carbohydrate, fat and protein (and sometimes alcohol) in the food. The total energy value of a food in Calories will be the total of the amount of Carbohydrate (g) X 3.75, Protein (g) X 4, Fat (g) X 9 plus Alcohol(g) X 7.1£ the, energy value is required in kJ thel'. the kJ conversion factors given above will need to be used. 2.5.3 Use of energy by the body 1.

Basal Metabolism: This is the term used to describe the basic metabolic processes which keep the body alive. Energy is needed to keep the heart beating and the lungs

29

BASICS OF HUMAN NUTRITION

functioning, to maintain body temperature and muscle tone and for the numerous chemical reactions taking place in body cells. The rate at which energy is used up in maintaining basal metabolism is called basal metabolic rate (BMR). 2. Physical Activity: In addition to basal metabolism, energy is used by the body for muscular activity. The energy requirements for-various activities have been determined by measuring oxygen uptake during different activities. (Table 2.6). Table 2.6 : Energy consumption for various activities Activity Sitting Writing Standing Washing-up Domestic work Walking Running Tennis Football Swimming

kcallmin 1.4 1.7 1.7 2.4 2.9 3.3-5.0 6.0-15.6 4.S-S.4 4.S-S.4 4.S-12

Although these figures are not accurate for any individual, they provide useful comparisons. 3. Growth, Pregnancy and Lactation: Additional energy is needed during growth to provide for the extra body tissue. During pregnancy and lactation all the infant's needs for energy must be supplied by the mother. Up to SO,OOO kcal of extra food energy may be needed during pregnancy mostly during the final months. 4. Heat: Some of the energy in food will be used as a source of heat, some of which will be used to keep the body warm. The dietary energy required by an individual who is neither gaining nor losing weight exactly equals the energy expended on maintenance and physical activity. In practice this balance is achieved over periods of a few days with remarkable accuracy.

30

MICROBIAL SPOILAGE OF FOODS

2.6 DIETARY ALLOWANCES FOR INDIANS The preceding pages have provided an account of the importance of the different nutrients present in food. In order to prevent the ill-effects due to deficiency of particular nutrients and to sustain a vigorous and healthy life, it is necessary to know in quantitative terms the amounts of the different nutrients needed. Obviously, this need will vary with factors such as age, sex and type of work. A schedule of dietary allowances will have to meet at least the minimum nutritional needs of the majority of persons for whom it is applied and at the same time provide reasonable margin to allow for physiological nonavailability of some nutrients from particular foods. Such a schedule will help a group of persons to select the proper foods to make up a diet that will provide the nutrients in the amounts indicated. Also, on a national level such allowances will be useful in enabling governments to plan their food production policies, to judge the adequacy or otherwise of the national supply of foods, and to point out the areas in which improvements are called for. The recommendations of ICMR (Indian Council of Medical Research) for dietary allowances for Indians are mentioned in Table 2.7. Explanatory Notes on Table 2.7 (1) The dietary allowances suggested for adults are for a reference man weighing 55 kg. and for a reference woman weighing 45 kg. The allowances for calories and proteins and for B-complex vitamins should be increased or decreased depending on the body weight. (2) The allowance for protein recommended by Nutrition Expert Group of Indian Council of Medical Research for adult is about 1 gm. per Kg. body weight per day and it is assumed that the dietary protein is derived from a mixture of vegetable foods. Proteins of animal origin are superior in biological value as compared to vegetable proteins. However, it is possible to improve the biological value of vegetable proteins through a proper admixture of foodstuffs, and it is for this reason that it is not insisted that a certain proportion of the total protein should be derived from animal foods. For infants and children and for pregnant and nursing women, however, it is desirable to supply some part (about 25%) of the total protein from animal foods such as milk, egg, flesh foods, etc. (3) The requirements for fats have not been indicated in the Table and the subject has been discussed in the text. It would appear to be unnecessary to have a fat intake which supplies more than 15% of the calories in the diet. About 15 gms. of vegetable oils, however, should be present in the diet to meet the essential fatty acid requirement.

Table 2.7 : Daily Allowances of Nutrients for Indians

Group

Man

Woman

Infants ChiIdrt'n

Adolescents

Particulars

Sedentary work Moderate work Heavy work Sdedentary work Mod~rate work Heavy work Pragnancy (.ccond half of pregnancy) Lactation (upto 1 year) 0-6 months 7-12 months

2400 2800

0.4 to 0.5

] 55

30

750

40

750

1.2 3000 1.4 2.0 1.0 3000 1.1 1.5 3000 +0.2

30

1150

4600

20

750

3900

1900 2200 3000 +300

J45

0.4 to 05

"0]

i700

+20

120/kg. 100/kg.

2.3-1.8/kg 1.8-1.5/kg

1.3 1.5 2.2 1.0 1.2 1.7 +0.2

+0.4

16] 19

50

100

13] 15 20

50

100

+2

50

150-300]

+5

80

150

26

1.0

1.5

0.5-0.6

1 mg/kg 400 kg. 300

1200

15-20] 250

lOOO

+0.4

25

0.2

1 year]

2 years 3 years 4-6 years 7-9 years 10-12 years

1200 1500 1800 2100

13-15 yrs. Boys Girls 16-18 yrs. Boys Girls

2500 2200 3000 2200

18

17] 20

~

0.4 to 05

0.6

0.7

8

600

1200 0.1l 1600 0.9 2400 1.0

0.8 1.0 1.2

10 12 14

750

3000

1.4 1.2 17 1.2

17 14 21 14

300 400

41 0.6-0.7

0.5-0.6

~J ~J

1.3 1.1

750

3000 1.5 1.1

30-50 50-100

0.5-1.0

32

MICROBIAL SPOILAGE OF FOODS

(4) Figures for carbohydrates are also not given in the Table; but about 70% of the calories in a diet can be from carbohydrates. (5) Most of the ingredients of a diet are rich in phosphorus, and it is for this reason the allowances for this element are not listed. (6) Minerals such as magnesium, copper, iodine, etc., are also essential in nutrition, but they are needed only in small amounts. Normally, if a diet is well-balanced and is adequate with reference to other nutrients, the requirements for these trace elements can be assumed to have been met. (7) Dietary allowances for vitamin A are given both in terms of retinol (preformed vitamin A) and /3-carotene, and the required amounts of vitamin A can be obtained from either or both. Although by definition 1 mg. of /3-carotene is equivalent to more than O. 5 mg. (1666 LU.) of retinol, some studies indicate that for all practical purposes it may be taken as equivalent to 0.25 mg. of retinol because of the inefficiency of utilization of carotene as a source of vitamin A. The total vitamin A value of a diet in terms of retinol can be calculated as follows:Total vitamin A value as Retinol (Ilg) = Retinol (Ilg) + /3 carot:ne(llg) (8) A part of the vitamin D requirement is undoubtedly met by the action of sunlight on the skin. However, it may not be advisable to rely entirely on sunshine for obtaining the vitamin D requirements, especially in the case of children. (9) The requirements for thiamine, riboflavin and nicotinic acid are related to calorie intake and the recommended allowances per 1,000 calories are: thiamine, 0.5 mg., riboflavin, O. 55 mg. and nicotinic acid, 6.6 mg. Nicotinic acid allowances include contribution from dietary tryptophan, 60 mg. of tryptophan being equivalent to 1 mg. of nicotinic acid.

2.7 BALANCED DIETS In the preceding pages, the importance of the various nutrients

in human nutrition has been considered. We shall now consider the planning of diets which would provide these essential nutrients in the needed amounts and proportions. A 'balanced diet' is one which contains different types of foods in such quantities and proportions that the need for calories, minerals, vitamins and other nutrients is adequately met and a small provision is made for extra nutrients to withstand short durations of leanness. Taking into account the foods which commonly form part of the Indian diets, balanced diets have been suggested for various groups of population, and the composition of such diets is given in tables 2.8, 2.9, 2.10 and 2.11.

0:1

Table 2.8 : Balanced Diets for Adult Man Sedentary Work Vegetarian Nonvegetarian

Moderate Work Vegetarian Nonvegetarian

> 5!l n til

Heavy Work 0.., Vegetarian Non::r:: vegetarian ~ > z (gm.) (gm.) Z 7 6 650 650 0 z 80 65 125 125 100 100

(gm.)

(gm.)

(gm.)

(gm.)

1 Cereals Pulses

2 400 70

Green leafy vegetables Other vegetables

100 75

3 400 55 100 75

4 475 80 125 75

5 475 65 125

Roots and tubers Fruits Milk

75 30 200

100

100

100

100

30 200

30 100

30 200

30 100

Fats and oils Meat and fish

35

75 30 100 40

40

40

50

50

Eggs Sugar and jaggery Groundnuts

30 30 30

30

75

~

30 30 40

40

* An additional 30 gm. of fats and oils can be included in the diet in place of groundnuts.

30 30 55 50

55 50* w w

YJ

Table 2.9 : Balanced Diets for Adult Woman Sedentary Work Vegetarian

Cereals Pulses Green leafy vegetables Other vegetables Roots and tubers Fruits Milk Fats and oils Sugar and jaggery Meat and fish Eggs Groundnuts

Moderate Work

~

Heavy Work

Vegetarian NonVegetarian NonNon:· vegetarian vegetarian vegetarian

Additional Allowances During Pregnancy Lactation

(~m.)

(~m.)

(~m.)

(~m.)

(~m.)

(~m.)

(~.)

(~.)

300 60 125

300 45 125

350 70 125

350 55 125

475 70 125

475 55 125

50

100 10 25

75 50 30 200 30 30

75 50 30 100 35 30 30 30

75 75 30 200 35 30

75 75 30 100 40 30 30 30

100 100 30 200 40 40

100 100 30 100 45 40 30 30 40""

40""

25

125 10

125 15 20

~

Q ~

;;

I""

Vl

~>

C) tT1

0

'T1

""An additional 25 gm of fats and oils can be included in place of groundnuts.

8 0



I"" (J)

g I""

>

C') rT1

~ "Tl

§ til

BASICS OF HUMAN NUTRITION

37

2.8 CLASSES OF NATURAL FOODSTUFFS AND THEIR NUTRITIONAL SIGNIFICANCE Diets with composition shown in the tables supply all the essential nutrients in adequate amounts and keep the majority of individuals consuming them in a state of good health. It may be pertinent then at this stage to consider how each class of foodstuffs suggested in the above diets supplies our daily requirement of the various nutrients. 2.8.1 Cereals Rice, wheat and millets Gowar, bajra, ragi etc.} are the main cereal grains consumed in India. They are the cheapest sources of calories and they contribute as much as 70 to 80% of the calories in the diets of a majority of population in our country. In view of the large amounts in which cereals are included in the diet, they form important sources of nutrients in an average Indian diet. Most cereal grains contain 0 to 12 per cent protein, and in general cereal proteins are somewhat deficient in the essential amino acid lysine which limits the protein quality. Rice protein, however~ is richer in lysine compared to the other cereal proteins and for this reason rice protein is of better quality. Most cereal grains are poor in mineral content and rice is an especially poor source of two important minerals, calcium and iron. However, ragi is very rich in these minerals, especially calcium, and inclusion of this millet in adequate amounts in the diet will go a long way in making up the deficiencies of some of the minerals in the diet. Bajra is also a good source of iron. Whole cereal grains are important sources of B-vitamins, especially thiamine and nicotinic acid. Since these vitamins are present in the cereal grain in the outer bran layers, the vitamin content of the finished product depends on the degree of removal of the outer layers. Particularly in the case of raw rice, the vitamin content decreases with the increase in the degree of milling and polishing given to the grain. Highly poliShed raw rice, therefore, has a very poor content of vitamins. Parboiled rice, on the other hand, contains significant amounts of thiamine because during the course of parboiling in which paddy is subjected to steaming or boiling in water, the vitamin seeps into the inner portions of the grain so that even if the grain is milled and

38

MICROBIAL SPOILAGE OF FOODS

polished, significant amounts of the vitamin are still retained in the grain. Except yellow maize, which contains some amounts of carotene, cereal grains in general do not contain much vitamin A activity and vitamin C.

2.8.2 Pulses Pulses (or legumes as they are also called) are rich in proteins. In diets in which flesh foods are present only in small amounts, pulses are therefore important as a source of protein. Pulse proteins, however, are of relatively low biological value because of the deficiency of the essential amino-acid methionine. Red gram is deficient in tryptophan also. However, pulse proteins are rich in lysine and they are therefore, of good supplementary value to cereal diets. The lysine deficit in cereals is made good by the lysine present in pulses and thus the overall biological value of cereal-pulse diets is better. In the amounts consumed, pulses cannot be considered rich sources of minerals, but they are rich in B-vitamins, especially thiamine and folic acid. Dried pulses do not contain vitamin C in any significant amounts, but when they are germinated, significant amounts of vitamin C are elaborated so that sprouted pulses, especially sprouted green gram and Bengal gram become rich source of this vitamin.

2.8.3 Nuts and Oilseeds Like pulses, nuts and oilseeds are also rich in proteins and in addition they contain fat so that they are rich in calories also. Most of the oilseeds produced in the country are used for extraction of edible oils and the cake left behind is even richer in protein than the original seed. Oilseed cakes were not being used as human food to any significant extent till recently because the methods used so far for extraction of oil were not good enough to produce a wholesome cake. Also, with country 'ghanis' the removal of oil is not complete, and the oil that is retained in the cake turns rancid in course of time and gives rise to off-flavours in addition to posing storage problems. However, improved extraction procedures followed in large mills in recent years have enabled the production of clean products practically free fJ::om offflavours. The meal can be used as such in various ways as food for humans, and procedures are-Cilso available for production of 'protein isolates' from the oil meals.

BASICS OF HUMAN NUTRITION

39

In common with other proteins of plant origin, oilseed proteins

are also low in biological value because of a relative deficiency of the amino-acid methionine, and groundnut protein is particularly poor in methionine. Gingelly (sesame) protein, however, is relatively richer in this amino-acid, as also is sunflower seed protein. Besides protein, oilseeds are rich sources of B-complex vitamins also. Groundnut especially is very rich in thiamine and in nicotinic acid. Some work carried out in recent years in various parts of the world showed that many foodstuffs can become contaminated with fungi (moulds) if they are stored under humid and unhygienic conditions. Some of these fungi produce toxins which are pOSitively deleterious to health. Groundnut is shown to be particularly prone to infestation with a fungus known as Aspergillus flavus which produces aflatoxin, and this toxin has been shown to cause damage to the liver in many experimental animals including monkeys. Only clean and wholesome groundnuts should, therefore, be used as food, and when dealing with the deoiled cake it should be seen to it that it does not contain aflatoxin in amounts above the accepted safe and permissible limits. 2.8.4 Green Leafy Vegetables

Many types of green leaves such as palak, amaranth, fenugreek leaves, drumstick leaves, mint etc., are consumed all over the country as vegetables, and most of them are rich sources of calcium, iron, carotene, vitamin C, riboflavin and folic acid. These vegetables are, therefore, inexpensive sources of many nutrients which are essential for growth and maintenance of normal health. Deficiency of these nutrients is commonly seen in our country and steps should, therefore, be taken to encourage cultivation of green leafy vegetables in kitchen gardens and school gardens. Consumption of such vegetables in adequate amounts especially by pregnant and nursing women and by children should also be encouraged. 2.8.5 Root Vegetables

Some of the important foodstuffs belonging to the group of root vegetables are tapioca, potato, sweet potato, carrots, yam and colocasia. They are all rich in carbohydrates and hence they yield mainly energy. Foods like carrots and yellow varieties of yam are also rich in carotene,

40

MICROBIAL SPOILAGE OF FOODS

and foods like potato contain significant amounts of vitamin C. Some root vegetables like tapioca, which is consumed commonly in Kerala, are such high yielders per acre of land that they have served as emergency or famine foods in times of cereal shortage. 2.8.6 Other Vegetables Other vegetables are those which do not fall under the category of leafy and root vegetables. Many such vegetables like brinjals (eggplant), ladies fingers (okra), French beans, various gourds, etc., are consumed mainly to add variety to the diet. Some of them are also fair sources of vitamins and minerals. 2.8.7 Fruits Fruits are in general good sources of vitamin C, and amla is an especially rich source of this vitamin. Yellow fruits like mango and papaya contain carotene in addition and dried fruits like dates and raisins are sources of iron. The commonly used banana is a fruit rich in carbohydrate, and it therefore yields energy also. If green leafy vegetables are included in the diet in adequate amounts, the need for fruit as an essential item in the diet is much reduced. 2.8.8 Milk and Milk Products Milk is an ideal food for infants and children, and it is a good supplementary food for adults. It contains proteins of good quality and also other nutrients in proper proportion and it is thus a complete food. It is, however, deficient in vitamin C and in iron. With only minor exceptions, the overall nutritive value of milk of different species can be said to be similar. Human milk contains more lactose (milk sugar) and buffalo milk contains more fat as compared to cow's milk. Cow's milk contains more protein than does human milk. Unless the whey is thrown away, the products derived from milk retain most of the nutrients contained originally in milk. For example, curd, which is the form in which milk is consumed to a significant extent in India, is for all practical purposes the same in nutritive value as milk is. The nutrient composition of dried milk (milk powder) is more or less the same, as that of milk on a moisture-free basis. The requirement for milk by persons of different age groups are given in the suggested balanced diets. It may be noted that these

BASICS OF HUMAN N1ITRITION

41

amounts are low, but it should be pointed out that these low figures are suggested as practical levels in the context of the prevailing low per capita availability of milk in the country. It should be our aim in food planning to achieve a much higher figure than this. In the more advanced countries and also in some regions in our country, the daily intake of milk is nearly 600 ml. per person. Renewed and vigorous efforts should be made to increase the average level of milk consumption, and in the meantime the available milk should be channelized to meet the priority needs of infants, growing children and pregnant and nursing women. 2.8.9 Sugar and Jaggery Sugar and jaggery are used as sweetening agents in beverages and other foods to increase their palatability. They are mainly sources of energy although jaggery contains in addition, iron. 2.8.10 Fats and Oils The visible fats that enter the diet are fats such as butter and ghee and the various vegetable oils and sometimes also vanaspati made by hydrogenation of oils. Irrespective of the type, all fats and oils yield the same amount a energy. However, vegetable oils have necessarily to be included in the die to the extent of 15 gms. per day to obtain the necessary amounts of essential fatty acids required by the body. Vegetable oils, especially safflower oil, are rich in polyunsaturated rated fatty acids.

2.8.11 Flesh Foods Flesh foods such as fish and meat are rich in proteins of high biological value and in B-vitamins. Especially vitamin B12 is contained only in foods of animal origin and not in plant foods. Flesh foods are generally not good sources of vitamin A, but liver, which is very rich in vitamin A, is an exception. Fish is a good source of calcium, especially the small varieties which are consumed whole.

2.8.12 Eggs Egg is a rich source of all nutrients except vitamin C. The protein contained in egg is considered to be a perfect protein, and because of its high biological value and digestibility, egg protein is used in nutrition work as a reference protein for comparison with other proteins. Egg of different species of birds can be said to be similar in

42

MICROBIAL SPOILAGE OF FOODS

nutritive value. Raw egg-white, however, contains a protein known as avidin which renders the vitamin biotin unavailable to the body. Duck egg-white contains in addition a substance known as trypsin-inhibitor which inhibits the action of trypsin on protein. Heating egg, as, for instance, in the preparation of boiled egg destroys both avidin and the trypsin-inhibitor. 2.8.13 Condiments and Spices These are accessory foodstuffs mainly used for flavouring food preparations. Some of the condiments like chillies and coriander are good sources of carotene. Green chillies supply vitamin C, and turmeric and tamarind are fair sources of iron. However, because of the small amounts in which many of the condiments and spices are used, they do not add substantially to the l\utritive value of the diet. Some spices like garlic and asafoetida are believed to contain active principles which inhibit the growth of putrefactive bacteria in the intestinal tract.

DOD

AN INTRODUCTION TO MICROBIAL SPOILAGE OF FOODS

3.1 PROLOGUE Spoilage of food involves any change which renders food unacceptable for human consumption and may result from a variety of causes. It is often difficult to decide when a food is actually spoiled since views differ on what is and is not acceptable and fit or unfit to eat. These differences of opinion are particularly evident when viewed on a worldwide basis as can be illustrated by the following well-known example. The British prefer game meat to be 'hung' for several days to allow organoleptic changes to take place which encourage the development of a 'strong' flavour. Whilst the British consider such flavoured meat to be a delicacy other nationalities, including Americans, regard it as spoiled and unacceptable.

3.2 MAJOR REASONS FOR FOOD SPOILAGE The main causes of food spoilage are: 1. Physical damage in transporting, storage, etc., resulting in changes in texture such as bruising. 2. Insect, rodent or other animal activity. 3. Chemical breakdown or chemical contamination resulting in deterioration in quality. 4. Autolytic enzymes which catalyze reactions within the food resulting in texture breakdown and the food becoming

44

MICROBIAL SPOILAGE OF Fboos

soft and pulpy. It may also be more vulnerable to attack by insects, rodents and microorganisms. 5. Micro-organisms, whose entry into the food is aided by I, 2, 3 and 4 above, which grow and change the texture, colour, taste, smell and quality of the food. Spoilage caused by micro-organisms is recognized by changes in foods which are often given common names - such as 'slime', 'rots' and 'putrefaction'. The main features of microbial spoilage are that the texture of the food degenerates and it gradually becomes soft and sticky and eventually fluid. These changes are often accompanied by odours which become more marked as time passes-some odours are very distinct, of which the 'sulphur stinker' spoilage of canned foods is an extreme example. Spoiled food can also change in colour, although this is dependant on the type of organism present. The characteristics of these microorganisms which cause them to spoil foods are those which also make them beneficial in the normal decay of organic material in the soil and in water. They break down the organic components of foods for their own use and in so doing convert them to simpler compounds. 3.3 FOOD-SPOILAGE TYPES The main types of spoilage are: 3.3.1 Mouldiness and 'whiskers' Moulds, being aerobic, grow mainly on the outside surfaces of the affected foods, initially as small separate colonies-'spots' which may later merge. Foods become sticky, 'whiskery' and locally coloured. 3.3.2 Rots A general word used to refer to spoilage of fruit, vegetables, eggs and other foods, for example, black rot of eggs, watery soft rots of fruit and vegetables. 3.3.3 Sliminess Growth of bacteria on moist surfaces of vegetables, meat, fish, etc., may cause taints and odours and can result in such deterioration of the food that it degenerates into slime. Pigmentation may occur at the same time. 3.3.4 Colour Change Many microbes produce brightly coloured colonies or pigments which give colour to the spoiling food, for example Serratia marcescens-

AN INTRODUCTION TO MICROBIAL SPOILAGE OF FOODS

45

red, Sarcina lutea-yellow, Pseudomonas fluorescens-green with fluorescence, Aspergillus niger-black, Penicillium species-green. 3.3.5 Ropiness Rope is the formation of a viscous sticky material closely allied to slime and caused by a wide variety of organisms such as LeUconostoc

mesenteroides, Leuconostoc dextranicum, Bacillus subtilis, Lactobacillus plantarum and others. In some foods, especially high sugar foods, the rope organisms produce copious capsules, and as the number of cells increases, the rope appears. Rope is also caused by microbial hydrolysis of starch and protein to produce glutinous non-capsular materials. Rope can affect soft drinks, wine, pickling brine, vinegar, milk and bread. 3.3.6 Fermentative Spoilage Many types of organisms, especially yeasts, aerobic and anaerobic sporing bacteria, and lactobacilli are able to ferment carbohydrates. Yeasts usually convert sugars into alcohols and carbon dioxide, 'homofermentative' lactic acid bacteria convert sugars into lactic acid, while the heterofermentative bacteria produce several acids, such as butyric and propionic acid, in addition to lactic acid, and the gases carbon dioxide and hydrogen. Bone taint refers to fermentative spoilage which arises close to the bone in meat. Flat sours occur in canned foods in non-gas producing fermentative spoilage. Blown cans occur as a result of gas producing fermentation in which such copious quantities of gas are evolved that the pressure within the can distorts the sides and ends of the can and it may eventually blow. Fermentative spoilage may occur in foods which are produced by fermentation'wild' organisms flourishing to the detriment of the product. This can be a problem in beer manufacture, for example. 3.3.7 Putrefaction The anaerobic decomposition of proteins into pep tides or amino acids causes the production of foul odours in the food due to hydrogen sulphide, ammonia, methyl and ethyl sulphides, amines and other strong smelling products. Foods which are likely to deteriorate in this way are those which have been poorly processed and packed to provide anaerobic conditions-for example improperly processed canned meat and vegetables.

46

MICROBIAL SPOILAGE OF FOODS

- 3.3.8 Aerobic Hydrolysis Aerobic hydrolysis of proteins leads to the development of bitter flavours in foods-which are not necessarily unpleasant and sometimes enhance the flavour. 3.4 THE ORGANISMS INVOLVED IN FOOD-SPOILAGE The way that spoilage develops in a food. depends on the types of organisms present and whether the food, under its existing conditions of storage, can support the growth of any or all of them. 3.4.1 Microbial load The microbial load-the f,umbers and species of organisms which a food carries is initially determined by the food type and its origins. Later it will be altered by the handling and processing to which the food is subjected. Raw foods carry their own characteristic flora. Soil crops carry on their surfaces organisms which are saprophytic or parasitic, as well as soil organisms; meat carries organisms derived from the animal bowel, skin and fur; fish, organisms derived from the fish skin, intestine and from the water in which it lived; milk, organisms from the udder; etc. Subsequent treatment of the food will either reduce or increase the total load. Procedures such as the removal of soil from root vegetables, peeling fruit or vegetables, washing foods and heat treatments, such as pasteurization, cooking or canning, tend to reduce the microbial load. Storage of food under warm conditions-such as grain in inadequately aerated silos, or fish or other food in a warm shop window-tends to increase the microbial load. Table 3.1 lists some treatments of food and their probable effect on the microbial load of the food. Table 3.1 : Some Treatments of Foods and their Probable Effect on the Microbial Load Food type

Treatment

Grain

Harvesting Milling-removes outer layers

Probable effect on microbial load Adds organisms in dust to the natural flora Reduces flora contct ....

AN INTRODUCfION TO MICROBIAL SPOILAGE OF FOODS

47

... contd. Food type Flour

Milk

Bulk egg

Vegetables

Fruit

Probable effect on microbial load Storage under dry conditions Dormant organisms gradually reduce in numbers Storage under damp conditions Moulds and yeasts very likely to proliferate Numbers of bacteria Kept warm increase Chilled immediately after Numbers of bacteria increase slowly. Psychrotrophic milking organisms increase Pasteurized Numbers of organisms reduced-heat sensitive organisms killed Sterilized Cells destroyed-no growth Pasteurized, chilled Very slow growth of remaining organisms Pasteurized, frozen No growth while frozen Harvesting Adds organisms from dust and soil Washing Removes some surface organisms Washing, followed by storage Organisms multiply leading to rapid wet spoilage or damp Bruising Aids penetration of organisms-leads to rapid spoilage Treatment

contd. ...

48

MICROBIAL SPOILAGE OF FOODS

... contd. Food type

Treatment Storage wet

Meat

Warm storage Prolonged chilling

Frozen storage

Fish

Warm storage Wet cool storage

Probable effect on microbial load Rapid increase in numbers of organisms Rapid increase in numbers of organisms Increase in numbers of organisms especially of psychrotrophs ~o increase in numbers of organisms while frozen Very rapid increase in numbers of organisms Increase in numbers of organisms

3.4.2 Inter-relationships Between Organisms If a food contains a single species of organism, or only very closely related species, then their growth will not be in competition with any others and the growth rate will be determined by the environmental conditions. If spoilage occurs it will probably be very characteristic, such as the flat souring of canned foods by Bacillus species. In a mixed population of organisms the different species affect each other's growth in several ways. The rate at which an organism is able to multiply in a food determines whether it will achieve dominance, the fastest growing organisms having the greatest opportunity. When bacteria, yeasts and moulds are present in a food which is capable of supporting the growth of all three it is most likely that the bacteria will become dominant first. Mould or yeast spoilage may occur at a later stage if the conditions in the food at that time permit. The waste products that the dominant organisms produce may either stimulate or inhibit the growth of other organisms present. For example, some moulds of the Penicillium species may produce antibiotics in their growth which are inhibitory to other organisms; some bacteria may produce acids which favour the growth of acidophiles.

AN INTRODUCfION TO MICROBIAL SPOILAGE OF FOODS

49

Sequential spoilage occurs when the initial wave of growth due to one or several species of organism dies down due to factors such as overcrowding, depletion of food supply and build up of waste products to toxic levels. The conditions now existing may favour the rapid growth of a second group of organisms whose growth up to this point has been repressed. In a similar way to the first wave, the second wave may later die down to be replaced by a third and also perhaps a fourth wave of growth. The spoilage of the food which results from these population changes may be distinctive-as when mould follows bacterial growth. The conditions of storage and the treatment of a food affect which categories of organisms can become dominant, for example: 1. Pasteurization destroys heat sensitive bacteria, yeasts and moulds, and leaves heat resistant spoilage organisms. 2. Storaee of food under chilled conditions may discourage mesophiles but allow psychrophiles to grow unchecked. 3. Vacuum-packed food can spoil anaerobically whereas packed aerobically it would spoil in a different way. 3.4.3 Moulds in Spoilage Mould growth is initiated when a ripe spore is able to germinate and start mycelium growth. The affected food becomes coloured, musty, softer and sticky or slimy. Because moulds are aerobic, spoilage generally begins at the surface, although the mycelium later penetrates deep into the food. As well as spoiling the more perishable foods, moulds are often associated with the spoilage of 'dry' foods especially those stored under damp conditions and those foods containing high concentrations of sugar or salt. Moulds important in food spoilage

1.

Non-septate moulds-reproduce by asexual and sexual

spores. (a)

(b)

(c)

Genus Rhizopus: Widespread. Fluffy, luxuriant

mycelium. 'Pin head' sporangia which become dark as they ripen. Spoilage: 'bread mould', soft rots in fruit and vegetables, spoil chilled meat. Genus Mucor: Widespread. Approximately 150 species. Hyphae pale; sporangia become greyish as they ripen. Spoilage: wide range of foods affected. Genus Thaminidium: Not common. Greyish brown sporangia. Psychrophilic. Spoilage: chilled meat.

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MICROBIAL SPOILAGE OF FOODS

2. Septate moulds-usually reproduce by asexual spores only. (a) Genus Aspergillus: Widespread. Compact colonies - white, buff, green, black. Bear conidia in 'globose' heads. Two important groups: Aspergillus glaucus group grey, green. Grow well in a low ~. Spoilage: dried foods and those preserved in sugar and in salt. Optimal temperature range for growth 1520°C. Aspergillus niger group Black conidia. Spoilage: bread, black rots of fruit and vegetables. Optimal temperature for growth 30°C. (b) Genus Penicillium: Widespread. Compact velvety grey-green, or white colonies. Bear conidia in 'brushes'. Spoilage: soft rots in citrus fruits, 'blue rot'; greenish patches on stored meat, yellow or green spots in eggs, greenish spoilage of cheddar and other cheese, bread, and so on. Optimal temperature for growth 20-25°C. (c) Genus Trichothecum. Trichothecum roseum causes spoilage of stored moist fruits. (d) Genus Geotrichum: Commonly found in dairy produce; compact felt-like colonies-white, yellow, red or orange. Hyphae break to form arthrospores. Spoilage: dairy produce-yoghurt, cheese, bread, stored citrus fruits and chilled meat. (e) Genus Monilia: Associated with the spoilage of bread-pink loose textured growth requiring moist conditions. (f) Genus Sporotrichum: Compact white colonies; requires high aw • Spoilage: stored chilled meats. (g) Genus Cladosporium: Common. Dark colonies. Spoilage: green rot of fruit, vegetables; black spot of meat, eggs, cheese. Wide temperature range for growth, favouring low temperatures. (h) Genus Alternaria: Dirty green mycelium, brown multicellular conidia. Spoilage: fruit and vegetables. (i) Genus Fusarium: Produce sickle-shaped multicellular conidia. Spoilage: rot fruit and vegetables; cause discolouration in butter.

AN INTRODUCfION TO MICROBIAL SPOILAGE OF FOODS

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3.4.4 Yeasts in Spoilage Yeasts tend to grow in acid conditions and where the sugar concentration is high. They grow both in aerobic and anaerobic conditions. Fermentative yeasts break down sugars to produce carbon dioxide, alcohols and acids. Oxidative or film yeasts oxidize sugars, organic acids and alcohol, and in their growth raise the pH; they tend to grow on the surface of liquors forming a skin or film. Osmophilic yeasts tolerate conditions of low