Dairy science and technology

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Dairy science and technology

  1. 1. Dairy Science and Technology Second Edition© 2006 by Taylor & Francis Group, LLC
  2. 2. FOOD SCIENCE AND TECHNOLOGY A Series of Monographs, Textbooks, and Reference Books Editorial Advisory Board Gustavo V. Barbosa-Cánovas Washington State University–Pullman P. Michael Davidson University of Tennessee–Knoxville Mark Dreher McNeil Nutritionals, New Brunswick, NJ Richard W. Hartel University of Wisconsin–Madison Lekh R. Juneja Taiyo Kagaku Company, Japan Marcus Karel Massachusetts Institute of Technology Ronald G. Labbe University of Massachusetts–Amherst Daryl B. Lund University of Wisconsin–Madison David B. Min The Ohio State University Leo M. L. Nollet Hogeschool Gent, Belgium Seppo Salminen University of Turku, Finland James L. Steele University of Wisconsin–Madison John H. Thorngate III Allied Domecq Technical Services, Napa, CA Pieter Walstra Wageningen University, The Netherlands John R. Whitaker University of California–Davis Rickey Y. Yada University of Guelph, Canada 76. Food Chemistry: Third Edition, edited by Owen R. Fennema 77. Handbook of Food Analysis: Volumes 1 and 2, edited by Leo M. L. Nollet 78. Computerized Control Systems in the Food Industry, edited by Gauri S. Mittal 79. Techniques for Analyzing Food Aroma, edited by Ray Marsili 80. Food Proteins and Their Applications, edited by Srinivasan Damodaran and Alain Paraf 81. Food Emulsions: Third Edition, Revised and Expanded, edited by Stig E. Friberg and Kåre Larsson 82. Nonthermal Preservation of Foods, Gustavo V. Barbosa-Cánovas, Usha R. Pothakamury, Enrique Palou, and Barry G. Swanson 83. Milk and Dairy Product Technology, Edgar Spreer 84. Applied Dairy Microbiology, edited by Elmer H. Marth and James L. Steele 85. Lactic Acid Bacteria: Microbiology and Functional Aspects, Second Edition, Revised and Expanded, edited by Seppo Salminen and Atte von Wright© 2006 by Taylor & Francis Group, LLC
  3. 3. 86. Handbook of Vegetable Science and Technology: Production, Composition, Storage, and Processing, edited by D. K. Salunkhe and S. S. Kadam 87. Polysaccharide Association Structures in Food, edited by Reginald H. Walter 88. Food Lipids: Chemistry, Nutrition, and Biotechnology, edited by Casimir C. Akoh and David B. Min 89. Spice Science and Technology, Kenji Hirasa and Mitsuo Takemasa 90. Dairy Technology: Principles of Milk Properties and Processes, P. Walstra, T. J. Geurts, A. Noomen, A. Jellema, and M. A. J. S. van Boekel 91. Coloring of Food, Drugs, and Cosmetics, Gisbert Otterstätter 92. Listeria, Listeriosis, and Food Safety: Second Edition, Revised and Expanded, edited by Elliot T. Ryser and Elmer H. Marth 93. Complex Carbohydrates in Foods, edited by Susan Sungsoo Cho, Leon Prosky, and Mark Dreher 94. Handbook of Food Preservation, edited by M. Shafiur Rahman 95. International Food Safety Handbook: Science, International Regulation, and Control, edited by Kees van der Heijden, Maged Younes, Lawrence Fishbein, and Sanford Miller 96. Fatty Acids in Foods and Their Health Implications: Second Edition, Revised and Expanded, edited by Ching Kuang Chow 97. Seafood Enzymes: Utilization and Influence on Postharvest Seafood Quality, edited by Norman F. Haard and Benjamin K. Simpson 98. Safe Handling of Foods, edited by Jeffrey M. Farber and Ewen C. D. Todd 99. Handbook of Cereal Science and Technology: Second Edition, Revised and Expanded, edited by Karel Kulp and Joseph G. Ponte, Jr. 100. Food Analysis by HPLC: Second Edition, Revised and Expanded, edited by Leo M. L. Nollet 101. Surimi and Surimi Seafood, edited by Jae W. Park 102. Drug Residues in Foods: Pharmacology, Food Safety, and Analysis, Nickos A. Botsoglou and Dimitrios J. Fletouris 103. Seafood and Freshwater Toxins: Pharmacology, Physiology, and Detection, edited by Luis M. Botana 104. Handbook of Nutrition and Diet, Babasaheb B. Desai 105. Nondestructive Food Evaluation: Techniques to Analyze Properties and Quality, edited by Sundaram Gunasekaran 106. Green Tea: Health Benefits and Applications, Yukihiko Hara 107. Food Processing Operations Modeling: Design and Analysis, edited by Joseph Irudayaraj 108. Wine Microbiology: Science and Technology, Claudio Delfini and Joseph V. Formica 109. Handbook of Microwave Technology for Food Applications, edited by Ashim K. Datta and Ramaswamy C. Anantheswaran© 2006 by Taylor & Francis Group, LLC
  4. 4. 110. Applied Dairy Microbiology: Second Edition, Revised and Expanded, edited by Elmer H. Marth and James L. Steele 111. Transport Properties of Foods, George D. Saravacos and Zacharias B. Maroulis 112. Alternative Sweeteners: Third Edition, Revised and Expanded, edited by Lyn O’Brien Nabors 113. Handbook of Dietary Fiber, edited by Susan Sungsoo Cho and Mark L. Dreher 114. Control of Foodborne Microorganisms, edited by Vijay K. Juneja and John N. Sofos 115. Flavor, Fragrance, and Odor Analysis, edited by Ray Marsili 116. Food Additives: Second Edition, Revised and Expanded, edited by A. Larry Branen, P. Michael Davidson, Seppo Salminen, and John H. Thorngate, III 117. Food Lipids: Chemistry, Nutrition, and Biotechnology: Second Edition, Revised and Expanded, edited by Casimir C. Akoh and David B. Min 118. Food Protein Analysis: Quantitative Effects on Processing, R. K. Owusu- Apenten 119. Handbook of Food Toxicology, S. S. Deshpande 120. Food Plant Sanitation, edited by Y. H. Hui, Bernard L. Bruinsma, J. Richard Gorham, Wai-Kit Nip, Phillip S. Tong, and Phil Ventresca 121. Physical Chemistry of Foods, Pieter Walstra 122. Handbook of Food Enzymology, edited by John R. Whitaker, Alphons G. J. Voragen, and Dominic W. S. Wong 123. Postharvest Physiology and Pathology of Vegetables: Second Edition, Revised and Expanded, edited by Jerry A. Bartz and Jeffrey K. Brecht 124. Characterization of Cereals and Flours: Properties, Analysis, and Applications, edited by Gönül Kaletunç and Kenneth J. Breslauer 125. International Handbook of Foodborne Pathogens, edited by Marianne D. Miliotis and Jeffrey W. Bier 126. Food Process Design, Zacharias B. Maroulis and George D. Saravacos 127. Handbook of Dough Fermentations, edited by Karel Kulp and Klaus Lorenz 128. Extraction Optimization in Food Engineering, edited by Constantina Tzia and George Liadakis 129. Physical Properties of Food Preservation: Second Edition, Revised and Expanded, Marcus Karel and Daryl B. Lund 130. Handbook of Vegetable Preservation and Processing, edited by Y. H. Hui, Sue Ghazala, Dee M. Graham, K. D. Murrell, and Wai-Kit Nip 131. Handbook of Flavor Characterization: Sensory Analysis, Chemistry, and Physiology, edited by Kathryn Deibler and Jeannine Delwiche© 2006 by Taylor & Francis Group, LLC
  5. 5. 132. Food Emulsions: Fourth Edition, Revised and Expanded, edited by Stig E. Friberg, Kare Larsson, and Johan Sjoblom 133. Handbook of Frozen Foods, edited by Y. H. Hui, Paul Cornillon, Isabel Guerrero Legarret, Miang H. Lim, K. D. Murrell, and Wai-Kit Nip 134. Handbook of Food and Beverage Fermentation Technology, edited by Y. H. Hui, Lisbeth Meunier-Goddik, Ase Solvejg Hansen, Jytte Josephsen, Wai-Kit Nip, Peggy S. Stanfield, and Fidel Toldrá 135. Genetic Variation in Taste Sensitivity, edited by John Prescott and Beverly J. Tepper 136. Industrialization of Indigenous Fermented Foods: Second Edition, Revised and Expanded, edited by Keith H. Steinkraus 137. Vitamin E: Food Chemistry, Composition, and Analysis, Ronald Eitenmiller and Junsoo Lee 138. Handbook of Food Analysis: Second Edition, Revised and Expanded, Volumes 1, 2, and 3, edited by Leo M. L. Nollet 139. Lactic Acid Bacteria: Microbiological and Functional Aspects: Third Edition, Revised and Expanded, edited by Seppo Salminen, Atte von Wright, and Arthur Ouwehand 140. Fat Crystal Networks, Alejandro G. Marangoni 141. Novel Food Processing Technologies, edited by Gustavo V. Barbosa-Cánovas, M. Soledad Tapia, and M. Pilar Cano 142. Surimi and Surimi Seafood: Second Edition, edited by Jae W. Park 143. Food Plant Design, edited by Antonio Lopez-Gomez; Gustavo V. Barbosa-Cánovas 144. Engineering Properties of Foods: Third Edition, edited by M. A. Rao, Syed S.H. Rizvi, and Ashim K. Datta 145. Antimicrobials in Food: Third Edition, edited by P. Michael Davidson, John N. Sofos, and A. L. Branen 146. Encapsulated and Powdered Foods, edited by Charles Onwulata 147. Dairy Science and Technology: Second Edition, Pieter Walstra, Jan T. M. Wouters and Tom J. Geurts© 2006 by Taylor & Francis Group, LLC
  6. 6. Dairy Science and Technology Second Edition Pieter Walstra Jan T. M. Wouters Tom J. Geurts Boca Raton London New York A CRC title, part of the Taylor & Francis imprint, a member of the Taylor & Francis Group, the academic division of T&F Informa plc.© 2006 by Taylor & Francis Group, LLC
  7. 7. Published in 2006 byCRC PressTaylor & Francis Group6000 Broken Sound Parkway NW, Suite 300Boca Raton, FL 33487-2742© 2006 by Taylor & Francis Group, LLCCRC Press is an imprint of Taylor & Francis GroupNo claim to original U.S. Government worksPrinted in the United States of America on acid-free paper10 9 8 7 6 5 4 3 2 1International Standard Book Number-10: 0-8247-2763-0 (Hardcover)International Standard Book Number-13: 978-0-8247-2763-5 (Hardcover)Library of Congress Card Number 2005041830This book contains information obtained from authentic and highly regarded sources. Reprinted material isquoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable effortshave been made to publish reliable data and information, but the author and the publisher cannot assumeresponsibility for the validity of all materials or for the consequences of their use.No part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic,mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, andrecording, or in any information storage or retrieval system, without written permission from the publishers.For permission to photocopy or use material electronically from this work, please access www.copyright.com(http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC) 222 Rosewood Drive,Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registrationfor a variety of users. For organizations that have been granted a photocopy license by the CCC, a separatesystem of payment has been arranged.Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used onlyfor identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Walstra, Pieter. Dairy science and technology / Pieter Walstra, Jan T.M. Wouters, T.J. Geurts.--2nd ed. p. cm. -- (Food science and technology ; 146) Rev. ed. of: Dairy technology / P. Walstra … [et al.]. c1999. Includes bibliographical references. ISBN 0-8247-2763-0 (alk. paper) 1. Dairy processing. 2. Milk. 3. Dairy products. I. Wouters, Jan T. M. II. Geurts, T. J. (Tom J.) III. Dairy technology. IV. Title. V. Food science and technology (Taylor & Francis) ; 146. SF250.5.D385 2005 637.1--dc22 2005041830 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com Taylor & Francis Group and the CRC Press Web site at is the Academic Division of T&F Informa plc. http://www.crcpress.com © 2006 by Taylor & Francis Group, LLC
  8. 8. PrefaceThe primary theme of this book is the efficient transformation of milk into high-quality products. This needs a thorough understanding of the composition andproperties of milk, and of the changes occurring in milk and its products duringprocessing and storage. Moreover, knowledge of the factors that determine prod-uct quality, including health aspects and shelf life, is needed. Our emphasis is onthe principles of physical, chemical, enzymatic, and microbial transformations.Detailed manufacturing prescriptions and product specifications are not given, asthey are widely variable. Aimed at university food science and technology majors, the book is writtenas a text, though it will also be useful as a work of reference. It is assumed thatthe reader is familiar with the rudiments of food chemistry, microbiology, andengineering. Nevertheless, several basic aspects are discussed for the benefit ofreaders who may be insufficiently acquainted with these aspects. The book con-tains no references to the literature, but suggestions for further reading are given. The book is made up of four main parts. Part I, “Milk,” discusses the chem-istry, physics, and microbiology of milk. Besides providing knowledge of theproperties of milk itself, it forms the basis for understanding what happens duringprocessing, handling and storage. Part II, “Processes,” treats the main unit oper-ations applied in the manufacture of milk products. These are discussed in somedetail, especially the influence of product and process variables on the (interme-diate) product resulting. A few highly specific processes, such as churning, arediscussed in product chapters. In Part III, “Products,” integration of knowledgeof the raw material and of processing is covered for the manufacture of severalproducts. The number of dairy products made is huge; hence, some product groupshave been selected because of their general importance or to illustrate relevantaspects. Procedures needed to ensure consumer safety, product quality, and pro-cessing efficiency are also treated. Part IV, “Cheese,” describes the processes andtransformations (physical, biochemical, and microbial) in the manufacture andripening of cheese. Here, the processes are so specific and the interactions sointricate that a separate and integrated treatment is needed. It starts with genericaspects and then discusses specific groups of cheeses. Several important changes have been introduced in this second edition. Thereasons were, first, to improve the didactic quality of the book and, second, tomake it more useful as a reference source. More basic and general aspects arenow treated, especially physicochemical and microbiological ones. Part I hasbeen substantially enlarged, one reason why the title of the book has beenbroadened. The nutritional aspects of milk components are now included, andthose of some products are enlarged. A section on milk formation has been added.© 2006 by Taylor & Francis Group, LLC
  9. 9. Naturally, the text has been updated. Moreover, several parts have been reorga-nized or rewritten. Factual information has been increased and partly moved toan Appendix. Pieter Walstra Jan Wouters Tom Geurts Wageningen, The Netherlands© 2006 by Taylor & Francis Group, LLC
  10. 10. AcknowledgmentsFirst, we want to stress that much of the present book derives from the substantialcontributions that our then-coauthors, Ad Noomen, Arend Jellema, and Tiny vanBoekel, made to the first edition. We are grateful that we could benefit from theirextensive expertise. Several people have provided information and advice. Professors NormanOlson (University of Wisconsin, Madison), Marie Paulsson (Lund University, Swe-den), and Zdenko Puhan (Technical University, Zürich, Switzerland) scrutinized(parts of) the first edition and gave useful advice. We consulted several colleaguesfrom our department, from NIZO Food Research (Ede, the Netherlands), andfrom the Milk Control Station (Zutphen, the Netherlands). We also receivedinformation from the following Dutch companies: Campina (Zaltbommel andWageningen), Carlisle Process Systems (formerly Stork, Gorredijk), FrieslandFoods (Deventer), and Numico (Wageningen). We thank all of the people involvedfor their cooperation and for the important information given.© 2006 by Taylor & Francis Group, LLC
  11. 11. ContentsPart IMilkChapter 1 Milk: Main Characteristics .....................................................31.1 Composition and Structure..........................................................................3 1.1.1 Principal Components .....................................................................3 1.1.2 Structural Elements .........................................................................41.2 Milk Formation............................................................................................71.3 Some Properties of Milk ...........................................................................111.4 Variability ..................................................................................................121.5 Changes......................................................................................................13Suggested Literature ...........................................................................................16Chapter 2 Milk Components..................................................................172.1 Lactose.......................................................................................................17 2.1.1 Chemical Properties.......................................................................17 2.1.2 Nutritional Aspects ........................................................................19 2.1.3 Physicochemical Aspects...............................................................202.2 Salts............................................................................................................26 2.2.1 Composition and Distribution among the Phases.........................26 2.2.2 Properties of the Salt Solution ......................................................30 2.2.3 Colloidal Calcium Phosphate ........................................................32 2.2.4 Nutritional Aspects ........................................................................33 2.2.5 Changes in Salts ............................................................................332.3 Lipids .........................................................................................................37 2.3.1 Constituent Fatty Acids .................................................................38 2.3.2 Lipid Classes .................................................................................42 2.3.3 Nutritional Aspects ........................................................................47 2.3.4 Autoxidation ..................................................................................48 2.3.5 Triglyceride Crystallization...........................................................522.4 Proteins ......................................................................................................63 2.4.1 Chemistry of Proteins....................................................................63 2.4.2 Survey of Milk Proteins ................................................................72 2.4.3 Serum Proteins...............................................................................76 2.4.4 Casein ............................................................................................79 2.4.5 Nutritional Aspects ........................................................................83© 2006 by Taylor & Francis Group, LLC
  12. 12. 2.5 Enzymes.....................................................................................................84 2.5.1 Enzyme Activity ............................................................................85 2.5.2 Some Milk Enzymes .....................................................................88 2.5.3 Inactivation ....................................................................................922.6 Other Components.....................................................................................93 2.6.1 Natural Components......................................................................93 2.6.2 Contaminants .................................................................................95 2.6.3 Radionuclides ................................................................................972.7 Variability ..................................................................................................98 2.7.1 Sources of Variability ....................................................................98 2.7.2 Nature of the Variation ................................................................103 2.7.3 Some Important Variables ...........................................................106Suggested Literature .........................................................................................108Chapter 3 Colloidal Particles of Milk..................................................1093.1 Basic Aspects...........................................................................................109 3.1.1 Surface Phenomena .....................................................................111 3.1.2 Colloidal Interactions ..................................................................118 3.1.3 Aggregation .................................................................................122 3.1.4 Size Distributions ........................................................................1253.2 Fat Globules.............................................................................................127 3.2.1 Properties .....................................................................................127 3.2.2 Emulsion Stability .......................................................................130 3.2.3 Interactions with Air Bubbles .....................................................134 3.2.4 Creaming......................................................................................136 3.2.5 Lipolysis ......................................................................................1393.3 Casein Micelles .......................................................................................140 3.3.1 Description...................................................................................141 3.3.2 Changes........................................................................................145 3.3.3 Colloidal Stability........................................................................150 3.3.4 Gel Formation and Properties .....................................................155Suggested Literature .........................................................................................157Chapter 4 Milk Properties....................................................................1594.1 Solution Properties ..................................................................................1594.2 Acidity .....................................................................................................1604.3 Redox Potential .......................................................................................1624.4 Flavor .......................................................................................................1644.5 Density.....................................................................................................1664.6 Optical Properties ....................................................................................1674.7 Viscosity ..................................................................................................169 4.7.1 Some Fluid Rheology..................................................................169 4.7.2 Liquid Milk Products ..................................................................173Suggested Literature .........................................................................................174© 2006 by Taylor & Francis Group, LLC
  13. 13. Chapter 5 Microbiology of Milk .........................................................1755.1 General Aspects .......................................................................................175 5.1.1 Microorganisms ...........................................................................175 5.1.2 Bacteria ........................................................................................176 5.1.3 Yeasts and Molds.........................................................................179 5.1.4 Enumeration of Microorganisms.................................................181 5.1.5 Growth .........................................................................................182 5.1.6 Milk as a Substrate for Microorganisms ....................................1875.2 Undesirable Microorganisms...................................................................190 5.2.1 Pathogenic Microorganisms ........................................................190 5.2.2 Spoilage Microorganisms ............................................................1945.3 Sources of Contamination .......................................................................197 5.3.1 Microbial Ecology .......................................................................197 5.3.2 Microorganisms Present in the Udder ........................................198 5.3.3 Contamination during and after Milking ....................................1995.4 Hygienic Measures ..................................................................................201 5.4.1 Protection of the Consumer against Pathogenic Microorganisms ........................................................202 5.4.2 Measures against Spoilage Organisms........................................202Suggested Literature .........................................................................................203Part IIProcessesChapter 6 General Aspects of Processing............................................2076.1 Introduction..............................................................................................2076.2 Preservation Methods ..............................................................................2096.3 Quality Assurance....................................................................................212 6.3.1 Concepts ......................................................................................212 6.3.2 Hazard Analysis/Critical Control Points (HACCP)....................214 6.3.3 Quality Assurance of Raw Milk .................................................2156.4 Milk Storage and Transport ....................................................................217 6.4.1 Milk Collection and Reception ...................................................217 6.4.2 Milk Storage ................................................................................218 6.4.3 Transport of Milk in the Dairy ...................................................2216.5 Standardizing ...........................................................................................222Suggested Literature .........................................................................................223Chapter 7 Heat Treatment ....................................................................2257.1 Objectives ................................................................................................2257.2 Changes Caused by Heating ...................................................................226© 2006 by Taylor & Francis Group, LLC
  14. 14. 7.2.1 Overview of Changes ..................................................................226 7.2.2 Reactions of Proteins...................................................................229 7.2.3 Reactions of Lactose ...................................................................233 7.2.4 Heat Coagulation .........................................................................2367.3 Heating Intensity .....................................................................................242 7.3.1 Processes of Different Intensity ..................................................242 7.3.2 Kinetic Aspects............................................................................245 7.3.3 Inactivation of Enzymes ..............................................................252 7.3.4 Thermobacteriology.....................................................................2557.4 Methods of Heating.................................................................................263 7.4.1 Considerations .............................................................................263 7.4.2 Equipment....................................................................................265 7.4.3 Heat Regeneration .......................................................................270 7.4.4 Control .........................................................................................271Suggested Literature .........................................................................................272Chapter 8 Centrifugation......................................................................2738.1 Cream Separation ....................................................................................2738.2 Removal of Particles ...............................................................................276Suggested Literature .........................................................................................277Chapter 9 Homogenization ..................................................................2799.1 Objectives ................................................................................................2799.2 Operation of the Homogenizer................................................................2809.3 Effects of Turbulence ..............................................................................2829.4 Factors Affecting Fat Globule Size.........................................................2859.5 Surface Layers .........................................................................................2879.6 Colloidal Stability....................................................................................2899.7 Homogenization Clusters ........................................................................2909.8 Creaming..................................................................................................2929.9 Other Effects of Homogenization ...........................................................2939.10 Other Ways of Working...........................................................................295Suggested Literature .........................................................................................296Chapter 10 Concentration Processes......................................................29710.1 General Aspects .......................................................................................297 10.1.1 Concentration of Solutes .............................................................297 10.1.2 Water Activity..............................................................................300 10.1.3 Changes Caused by Concentrating .............................................302 10.1.4 The Glassy State..........................................................................303 10.1.5 Reaction Rates .............................................................................30410.2 Evaporating..............................................................................................307© 2006 by Taylor & Francis Group, LLC
  15. 15. 10.3 Drying: General Aspects .........................................................................314 10.3.1 Objectives ....................................................................................314 10.3.2 Drying Methods...........................................................................31610.4 Spray Drying ...........................................................................................318 10.4.1 Drier Configuration .....................................................................318 10.4.2 Atomization .................................................................................319 10.4.3 Change of State of the Drying Air..............................................322 10.4.4 Changes of State of the Drying Droplets ...................................326 10.4.5 Two-Stage Drying........................................................................332Suggested Literature .........................................................................................335Chapter 11 Cooling and Freezing ..........................................................33711.1 Cooling ....................................................................................................33711.2 Freezing ...................................................................................................338Suggested Literature .........................................................................................340Chapter 12 Membrane Processes ...........................................................34112.1 General Aspects .......................................................................................341 12.1.1 Types of Processes ......................................................................341 12.1.2 Efficiency .....................................................................................343 12.1.3 Technical Operation.....................................................................34512.2 Ultrafiltration ...........................................................................................346 12.2.1 Composition of the Retentate......................................................346 12.2.2 Permeate Flux..............................................................................34912.3 Reverse Osmosis......................................................................................35112.4 Desalting ..................................................................................................354Suggested Literature .........................................................................................356Chapter 13 Lactic Fermentations ...........................................................35713.1 Lactic Acid Bacteria ................................................................................357 13.1.1 Taxonomy ....................................................................................357 13.1.2 Metabolism ..................................................................................360 13.1.3 Genetics .......................................................................................373 13.1.4 Bacteriocins .................................................................................37413.2 Acid Production.......................................................................................37413.3 Bacteriophages.........................................................................................377 13.3.1 Phage Composition and Structure...............................................377 13.3.2 Phage Multiplication ...................................................................377 13.3.3 Phage Resistance Mechanisms....................................................382 13.3.4 Inactivation ..................................................................................38313.4 Ecological Aspects...................................................................................38413.5 Starters .....................................................................................................385© 2006 by Taylor & Francis Group, LLC
  16. 16. 13.5.1 Composition.................................................................................385 13.5.2 Properties .....................................................................................388 13.5.3 Shifts in Flora ..............................................................................388 13.5.4 Traditional Starter Manufacture ..................................................390 13.5.5 Modern Starter Manufacture .......................................................394Suggested Literature .........................................................................................396Chapter 14 Fouling and Sanitizing ........................................................39914.1 Deposit Formation ...................................................................................39914.2 Cleaning...................................................................................................40514.3 Disinfection..............................................................................................408Suggested Literature .........................................................................................410Chapter 15 Packaging ............................................................................41115.1 Distribution Systems ...............................................................................41115.2 Packaging Materials ................................................................................41215.3 Filling Operation .....................................................................................415Suggested Literature .........................................................................................417Part IIIProductsChapter 16 Milk for Liquid Consumption.............................................42116.1 Pasteurized Milk......................................................................................421 16.1.1 Manufacture .................................................................................422 16.1.2 Shelf Life .....................................................................................427 16.1.3 Extended-Shelf-Life Milk ...........................................................43016.2 Sterilized Milk .........................................................................................431 16.2.1 Description...................................................................................431 16.2.2 Methods of Manufacture .............................................................432 16.2.3 Shelf Life .....................................................................................43616.3 Reconstituted Milks.................................................................................43716.4 Flavor .......................................................................................................43716.5 Nutritive Value.........................................................................................439 16.5.1 Modification of Composition ......................................................439 16.5.2 Loss of Nutrients .........................................................................44016.6 Infant Formulas .......................................................................................441 16.6.1 Human Milk ................................................................................441 16.6.2 Formula Composition and Manufacture .....................................444Suggested Literature .........................................................................................444© 2006 by Taylor & Francis Group, LLC
  17. 17. Chapter 17 Cream Products ...................................................................44717.1 Sterilized Cream ......................................................................................447 17.1.1 Manufacture .................................................................................448 17.1.2 Heat Stability ...............................................................................448 17.1.3 Stability in Coffee .......................................................................448 17.1.4 Clustering.....................................................................................45017.2 Whipping Cream .....................................................................................452 17.2.1 Desirable Properties.....................................................................452 17.2.2 Manufacture .................................................................................453 17.2.3 The Whipping Process ................................................................45417.3 Ice Cream.................................................................................................458 17.3.1 Manufacture .................................................................................459 17.3.2 Physical Structure: Formation and Stability ...............................462 17.3.3 Role of the Various Components ................................................465Suggested Literature .........................................................................................466Chapter 18 Butter ...................................................................................46718.1 Description...............................................................................................46718.2 Manufacture .............................................................................................468 18.2.1 Processing Scheme ......................................................................468 18.2.2 The Churning Process .................................................................471 18.2.3 Working .......................................................................................47418.3 Properties .................................................................................................478 18.3.1 Microstructure..............................................................................478 18.3.2 Consistency..................................................................................480 18.3.3 Cold Storage Defects...................................................................48518.4 Cultured Butter from Sweet Cream ........................................................48618.5 High-Fat Products....................................................................................489 18.5.1 Anhydrous Milk Fat ....................................................................489 18.5.2 Modification of Milk Fat.............................................................490 18.5.3 Recombined Butter......................................................................492 18.5.4 Low-Fat Butter Products .............................................................494Suggested Literature .........................................................................................495Chapter 19 Concentrated Milks .............................................................49719.1 Evaporated Milk ......................................................................................497 19.1.1 Manufacture .................................................................................497 19.1.2 Product Properties .......................................................................501 19.1.3 Heat Stability ...............................................................................502 19.1.4 Creaming......................................................................................504 19.1.5 Age Thickening and Gelation .....................................................50519.2 Sweetened Condensed Milk ....................................................................507© 2006 by Taylor & Francis Group, LLC
  18. 18. 19.2.1 Manufacture .................................................................................507 19.2.2 Keeping Quality...........................................................................509Suggested Literature .........................................................................................512Chapter 20 Milk Powder ........................................................................51320.1 Objectives ................................................................................................51320.2 Manufacture .............................................................................................51420.3 Hygienic Aspects .....................................................................................517 20.3.1 Bacteria in the Original Milk......................................................517 20.3.2 Growth during Manufacture........................................................519 20.3.3 Incidental Contamination ............................................................521 20.3.4 Sampling and Checking ..............................................................52120.4 Powder Characteristics ............................................................................522 20.4.1 The Particle..................................................................................522 20.4.2 Extractable Fat.............................................................................522 20.4.3 Free-Flowingness.........................................................................523 20.4.4 Specific Volume ...........................................................................525 20.4.5 Dissolution...................................................................................526 20.4.6 WPN Index ..................................................................................529 20.4.7 Flavor ...........................................................................................530 20.4.8 Conclusions..................................................................................53120.5 Deterioration ............................................................................................53120.6 Other Types of Milk Powder...................................................................535Suggested Literature .........................................................................................535Chapter 21 Protein Preparations ............................................................53721.1 Manufacture .............................................................................................538 21.1.1 Casein ..........................................................................................539 21.1.2 Whey Protein ...............................................................................540 21.1.3 Other Products .............................................................................54221.2 Functional Properties...............................................................................543 21.2.1 Solution Properties ......................................................................544 21.2.2 Gels ..............................................................................................546 21.2.3 Emulsions ....................................................................................548 21.2.4 Foams...........................................................................................549Suggested Literature .........................................................................................550Chapter 22 Fermented Milks .................................................................55122.1 General Aspects .......................................................................................55122.2 Types of Fermented Milks ......................................................................552 22.2.1 Mesophilic Fermentation.............................................................552 22.2.2 Thermophilic Fermentation .........................................................553© 2006 by Taylor & Francis Group, LLC
  19. 19. 22.2.3 Yeast–Lactic Fermentation ..........................................................555 22.2.4 Molds in Lactic Fermentation.....................................................55722.3 Cultured Buttermilk.................................................................................55722.4 Yogurt ......................................................................................................558 22.4.1 The Yogurt Bacteria.....................................................................559 22.4.2 Manufacture .................................................................................562 22.4.3 Physical Properties ......................................................................565 22.4.4 Flavor Defects and Shelf Life .....................................................56822.5 Nutritional Aspects ..................................................................................569 22.5.1 Composition.................................................................................569 22.5.2 Nutritional Value..........................................................................570 22.5.3 Probiotics .....................................................................................571 22.5.4 Prebiotics .....................................................................................572Suggested Literature .........................................................................................573Part IVCheeseChapter 23 Principles of Cheese Making ..............................................57723.1 Introduction..............................................................................................57723.2 Essential Process Steps ...........................................................................57923.3 Changes Occurring ..................................................................................580Suggested Literature .........................................................................................582Chapter 24 Cheese Manufacture............................................................58324.1 Milk Properties and Pretreatment ...........................................................583 24.1.1 The Raw Milk..............................................................................583 24.1.2 Milk Treatment ............................................................................58424.2 Starters .....................................................................................................58624.3 Enzyme-Induced Clotting........................................................................588 24.3.1 Enzymes Used .............................................................................588 24.3.2 The Enzyme-Catalyzed Reaction ................................................590 24.3.3 Aggregation .................................................................................591 24.3.4 Gel Formation..............................................................................593 24.3.5 The Renneting Time ....................................................................594 24.3.6 Clotting of Heat-Treated Milk ....................................................59624.4 Curd Making............................................................................................596 24.4.1 Clotting ........................................................................................597 24.4.2 Accumulation of Various Components .......................................600 24.4.3 Concentrating before Clotting.....................................................601 24.4.4 Syneresis ......................................................................................603 24.4.5 Acid Production and Washing.....................................................608 24.4.6 Separation of Curd and Whey.....................................................610© 2006 by Taylor & Francis Group, LLC
  20. 20. 24.5 Shaping and Pressing ..............................................................................61324.6 Salting ......................................................................................................615 24.6.1 Mass Transport during Salting ....................................................616 24.6.2 Important Variables......................................................................622 24.6.3 Distribution of Salt and Water after Salting ...............................62424.7 Curing, Storage, and Handling................................................................625 24.7.1 Temperature .................................................................................626 24.7.2 Air Conditions .............................................................................627 24.7.3 Rind Treatment ............................................................................627 24.7.4 Packaging.....................................................................................63024.8 Cheese Composition and Yield ...............................................................631 24.8.1 Variables Involved .......................................................................632 24.8.2 Yield.............................................................................................636 24.8.3 Standardizing the Milk ................................................................638Suggested Literature .........................................................................................638Chapter 25 Cheese Ripening and Properties .........................................64125.1 Lactic Fermentation.................................................................................64125.2 Enzyme Sources ......................................................................................64225.3 Proteolysis................................................................................................644 25.3.1 Methods of Characterization .......................................................644 25.3.2 Milk Proteinases ..........................................................................645 25.3.3 Clotting Enzymes ........................................................................646 25.3.4 Enzymes of Lactic Acid Bacteria ...............................................648 25.3.5 Enzymes of Nonstarter Organisms .............................................650 25.3.6 Interaction between Enzyme Systems ........................................650 25.3.7 Ultrafiltration of Cheese Milk.....................................................65125.4 Lipolysis ..................................................................................................65325.5 Development of Flavor............................................................................654 25.5.1 Description...................................................................................654 25.5.2 Formation of Flavor Compounds ................................................65525.6 Development of Texture ..........................................................................659 25.6.1 Structure.......................................................................................659 25.6.2 Consistency..................................................................................66125.7 Accelerated Ripening ..............................................................................66925.8 Nutritive Value and Safety ......................................................................672Suggested Literature .........................................................................................675Chapter 26 Microbial Defects................................................................67726.1 Coliform Bacteria ....................................................................................67926.2 Butyric Acid Bacteria ..............................................................................68026.3 Lactobacilli ..............................................................................................68326.4 Heat-Resistant Streptococci.....................................................................684© 2006 by Taylor & Francis Group, LLC
  21. 21. 26.5 Propionic Acid Bacteria ..........................................................................68426.6 Organisms on the Rind............................................................................68526.7 Other Aspects...........................................................................................686Suggested Literature .........................................................................................686Chapter 27 Cheese Varieties ..................................................................68727.1 Overview..................................................................................................687 27.1.1 Variations in Manufacture ...........................................................688 27.1.2 Types of Cheese ..........................................................................69427.2 Fresh Cheese............................................................................................696 27.2.1 Quarg ...........................................................................................697 27.2.2 Cottage Cheese ............................................................................69927.3 Gouda-Type Cheeses ...............................................................................702 27.3.1 Manufacture .................................................................................702 27.3.2 Properties and Defects.................................................................70927.4 Cheddar-Type Cheeses ............................................................................712 27.4.1 Manufacture .................................................................................712 27.4.2 Properties .....................................................................................71627.5 Swiss and Pasta-Filata Types ..................................................................718 27.5.1 Emmentaler..................................................................................719 27.5.2 Mozzarella ...................................................................................72227.6 Cheeses with a Specific Flora .................................................................724 27.6.1 Soft Cheese with Surface Flora ..................................................725 27.6.2 Blue-Veined Cheese.....................................................................73427.7 Processed Cheese.....................................................................................737Suggested Literature .........................................................................................739Part VAppendixAppendix A.1 Often-Used Symbols ...........................................................743 A.2 Abbreviations.......................................................................745 A.3 Conversion Factors..............................................................746 A.4 Physical Properties of Milk Fat ..........................................747 A.5 Amino Acid Composition of Milk Proteins .......................748 A.6 Amino Acid Sequences of Caseins.....................................750© 2006 by Taylor & Francis Group, LLC
  22. 22. A.7 Some Properties of Lactose ................................................754 A.8 Trace Elements in Cows’ Milk ...........................................755 A.9 Physical Properties of Milk and Milk Products .................756 A.10 Mass Density and Viscosity of Some Milk Fractions ......................................................................................757 A.11 Heat Transfer .......................................................................758 A.12 Data on Some Cheese Varieties ..........................................762© 2006 by Taylor & Francis Group, LLC
  23. 23. Part IMilk© 2006 by Taylor & Francis Group, LLC
  24. 24. 1 Milk: Main CharacteristicsMilk is defined as the secretion of the mammary glands of mammals, its primarynatural function being nutrition of the young. Milk of some animals, especiallycows, buffaloes, goats and sheep, is also used for human consumption, either assuch or in the form of a range of dairy products. In this book, the word milk willbe used for the ‘normal’ milk of healthy cows, unless stated otherwise. Occasion-ally, a com-parison will be made with human milk. This chapter is meant as a general introduction. Nearly all that is mentioned —with the exception of parts of Section 1.2 — is discussed in greater detail in otherchapters. However, for readers new to the field it is useful to have some idea of theformation, composition, structure, and properties of milk, as well as the variation —including natural variation and changes due to processing — that can occur inthese characteristics, before starting on the main text.1.1 COMPOSITION AND STRUCTURE1.1.1 PRINCIPAL COMPONENTSA classification of the principal constituents of milk is given in Table 1.1. Theprincipal chemical components or groups of chemical components are those presentin the largest quantities. Of course, the quantity (in grams) is not paramount in allrespects. For example, vitamins are important with respect to nutritive value; en-zymes are catalysts of reactions; and some minor components contribute markedlyto the taste of milk. More information on milk composition is given in Table 1.3. Lactose or milk sugar is the distinctive carbohydrate of milk. It is a disac-charide composed of glucose and galactose. Lactose is a reducing sugar. The fat is largely made up of triglycerides, constituting a very complicatedmixture. The component fatty acids vary widely in chain length (2 to 20 carbonatoms) and in saturation (0 to 4 double bonds). Other lipids that are present includephospholipids, cholesterol, free fatty acids, monoglycerides, and diglycerides. About four fifths of the protein consists of casein, actually a mixture of fourproteins: αS1-, αS2-, β-, and κ-casein. The caseins are typical for milk and have somerather specific properties: They are to some extent phosphorylated and have littleor no secondary structure. The remainder consists, for the most part, of the milkserum proteins, the main one being β-lactoglobulin. Moreover, milk contains nume-rous minor proteins, including a wide range of enzymes. The mineral substances — primarily K, Na, Ca, Mg, Cl, and phosphate —are not equi-valent to the salts. Milk contains numerous other elements in trace 3© 2006 by Taylor & Francis Group, LLC
  25. 25. 4 Milk: Main Characteristics TABLE 1.1 Approximate Composition of Milk Average Content Rangea Average Content in Component in Milk (% w/w) (% w/w) Dry Matter (% w/w) Water 87.1 85.3−88.7 — Solids-not-fat 8.9 7.9−10.0 — Fat in dry matter 31 22−38 — Lactose 4.6 3.8−5.3 36 Fat 4.0 2.5−5.5 31 Proteinb 3.3 2.3−4.4 25 casein 2.6 1.7−3.5 20 Mineral substances 0.7 0.57−0.83 5.4 Organic acids 0.17 0.12−0.21 1.3 Miscellaneous 0.15 — 1.2 Note: Typical for milks of lowland breeds. a These values will rarely be exceeded, e.g., in 1 to 2% of samples of separate milkings of healthy individual cows, excluding colostrum and milk drawn shortly before parturition. b Nonprotein nitrogen compounds not included.quantities. The salts are only partly ionized. The organic acids occur largely asions or as salts; citrate is the principle one. Furthermore, milk has many miscel-laneous components, often in trace amounts. The total content of all substances except water is called the content of drymatter. Furthermore, one distinguishes solids-not-fat and the content of fat in thedry matter. The chemical composition of milk largely determines its nutritional value; theextent to which microorganisms can grow in it; its flavor; and the chemicalreactions that can occur in milk. The latter include reactions that cause off-flavours.1.1.2 STRUCTURAL ELEMENTSStructure can be defined as the geometrical distribution of the (chemical) compo-nents in a system. It may imply, as it does in milk, that the liquid contains particles.This can have important consequences for the properties of the system. Forinstance, (1) chemical components are present in separate compartments, whichcan greatly affect their reactivity; (2) the presence of particles greatly affects somephysical properties, like viscosity and optical appearance; (3) interaction forcesbetween particles generally determine the physical stability of the system; and (4)the separation of some components (fat and casein) is relatively easy. Figure 1.1 shows the main structural elements of milk. Of course, the figureis schematic and incomplete. Some properties of the structural elements are given© 2006 by Taylor & Francis Group, LLC
  26. 26. 1.1 Composition and Structure 5 A ×5 Milk B Plasma × 500 Fat globules Fat Membrane C Serum globule × 50000 Casein micellesFIGURE 1.1 Milk viewed at different magnifications, showing the relative size of struc-tural elements (A) Uniform liquid. However, the liquid is turbid and thus cannot behomogeneous. (B) Spherical droplets, consisting of fat. These globules float in a liquid(plasma), which is still turbid. (C) The plasma contains proteinaceous particles, which arecasein micelles. The remaining liquid (serum) is still opalescent, so it must contain otherparticles. The fat globules have a thin outer layer (membrane) of different constitution.(From H. Mulder and P. Walstra, The Milk Fat Globule, Pudoc, Wageningen, 1974.)in Table 1.2, again in a simplified form; the numerical data mentioned are meantonly to define orders of magnitude. The table clearly shows that aspects of colloidchemistry are essential for understanding the properties of milk and the manychanges that can occur in it. All particles exhibit Brownian motion; they have anelectrostatic charge, which is negative at the pH of milk. Their total surface areais large. Fat globules. To a certain extent, milk is an oil-in-water emulsion. But thefat globules are more complicated than emulsion droplets. In particular, thesurface layer or membrane of the fat globule is not an adsorption layer of one© 2006 by Taylor & Francis Group, LLC
  27. 27. 6 Milk: Main Characteristics TABLE 1.2 Properties of the Main Structural Elements of Milk Milk Plasma Serum Globular Lipoprotein Fat Globules Casein Micelles Proteins Particles Main components Fat Casein, water, Serum protein Lipids, proteins salts To be considered as Emulsion Fine dispersion Colloidal Colloidal solution dispersion Content 4 2.8 0.6 0.01 (% dry matter) Volume fraction 0.05 0.1 0.006 10−4 Particle diametera 0.1−10 µm 20−400 nm 3−6 nm 10 nm Number per ml 1010 1014 1017 1014 Surface area 700 40000 50000 100 (cm2/ml milk) Density 920 1100 1300 1100 (20°C; kg ⋅ m−3) Visible with Microscope Ultramicroscope Electron microscope Separable with Milk separator High-speed Ultrafiltration Ultrafiltration centrifuge Diffusion rate 0.0 0.1–0.3 0.6 0.4 (mm in 1h)a Isoelectric pH ∼3.8 ∼4.6 4−5 ∼4 Note: Numerical values are approximate averages. a For comparison, most molecules in solution are 0.4 to 1 nm diameter, and diffuse, say, 5 mm in 1 h. 1 mm = 103 µm = 106 nm = 107 Å.single substance but consists of many components; its structure is complicated.The dry mass of the membrane is about 2.5% of that of the fat. A small part ofthe lipids of milk is found outside the fat globules. At temperatures below 35°C,part of the fat in the globules can crystallize. Milk minus fat globules is calledmilk plasma, i.e., the liquid in which the fat globules float. Casein micelles consist of water, protein, and salts. The protein is casein.Casein is present as a caseinate, which means that it binds cations, primarilycalcium and magnesium. The other salts in the micelles occur as a calciumphosphate, varying somewhat in composition and also containing a small amountof citrate. This is often called colloidal phosphate. The whole may be calledcalcium-caseinate/calcium-phosphate complex. The casein micelles are notmicelles in the colloid-chemical sense but just ‘small particles.’ The micelles have© 2006 by Taylor & Francis Group, LLC
  28. 28. 1.2 Milk Formation 7an open structure and, accordingly, contain much water, a few grams per gramof casein. Milk serum, i.e., the liquid in which the micelles are dispersed, is milkminus fat globules and casein micelles. Serum proteins are largely present in milk in molecular form or as very smallaggregates. Lipoprotein particles, sometimes called milk microsomes, vary in quantityand shape. Presumably, they consist of remnants of mammary secretory cellmembranes. Few definitive data on lipoprotein particles have been published. Cells, i.e., leukocytes, are always present in milk. They account for about0.01% of the volume of milk of healthy cows. Of course, the cells contain allcytoplasmic components such as enzymes. They are rich in catalase. Table 1.3 gives a survey of the average composition and structure of milk.1.2 MILK FORMATIONMilk components are for the most part formed in the mammary gland (the udder)of a cow, from precursors that are the results of digestion. Digestion. Mammals digest their food by the use of enzymes to obtain simple,soluble, low-molar-mass components, especially monosaccharides; small pep-tides and amino acids; and fatty acids and monoglycerides. These are taken upin the blood, together with other nutrients, such as various salts, glycerol, organicacids, etc. The substances are transported to all the organs in the body, includingthe mammary gland, to provide energy and building blocks (precursors) formetabolism, including the synthesis of proteins, lipids, etc. In ruminants like the cow, considerable predigestion occurs by means ofmicrobial fermentation, which occurs for the most part in the first stomach orrumen. The latter may be considered as a large and very complex bio-fermenter.It contains numerous bacteria that can digest cellulose, thereby breaking downplant cell walls, providing energy and liberating the cell contents. From celluloseand other carbohydrates, acetic, propionic, butyric and lactic acid are formed, whichare taken up in the blood. The composition of the organic acid mixture dependson the composition of the feed. Proteins are broken down into amino acids. Therumen flora uses these to make proteins but can also synthesize amino acids fromlow-molar-mass nitrogenous components. Further on in the digestive tract themicrobes are digested, liberating amino acids. Also, food lipids are hydrolyzed inthe rumen and partly metabolized by the microorganisms. All these precursors canreach the mammary gland. Milk Synthesis. The synthesis of milk components occurs for the greaterpart in the secretory cells of the mammary gland. Figure 1.2 illustrates such acell. At the basal end precursors of milk components are taken up from the blood,and at the apical end milk components are secreted into the lumen. Proteins areformed in the endoplasmic reticulum and transported to the Golgi vesicles, inwhich most of the soluble milk components are collected. The vesicles grow insize while being transported through the cell and then open up to release theircontents in the lumen. Triglycerides are synthesized in the cytoplasm, forming© 2006 by Taylor & Francis Group, LLC
  29. 29. TABLE 1.3Composition and Structure of Milka 8 FAT GLOBULE CASEIN MICELLE SERUM Water 790 g Organic acids Proteins Protein citrate 1600 mg casein + Glycerides casein 26 g triglycerides 40 g proteose peptone + Carbohydrates formate 40 mg β-lactoglobulin 3.2 g diglycerides 0.1 g Salts 2g monoglycerides 10 mg 850 mg lactose 46 g acetate 30 mg α-lactalbumin 1.2 g Ca Fatty acids 60 mg phosphate 1000 mg glucose 70 mg lactate 20 mg serum albumin 0.4 g Sterols 100 mg citrate 150 mg Carotenoids 0.3 mg K, Mg, Na others oxalate 20 mg immunoglobulins 0.8 g Vitamins A, D, E, K Water ~80 g others 10 mg proteose peptone + Water 60 mg Enzymes Others Minerals others lipase plasmin Ca, bound 300 mg Gases Nonprotein nitrogenous MEMBRANE Ca, ions 90 mg oxygen 6 mg compounds water + Mg 70 mg nitrogen 16 mg peptides + protein 700 mg LEUKOCYTE phospholipids 250 mg K 1500 mg Lipids amino acids 50 mg cerebrosides 30 mg LIPOPROTEIN Na 450 mg glycerides + urea 250 mg glycerides + PARTICLE fatty acids 15 mg Cl 1100 mg fatty acids 20 mg ammonia 10 mg Many enzymes streols 15 mg other lipids lipids phosphate 1100 mg phospholipids 100 mg others 300 mg e.g., catalase protein enzymes sulfate 100 mg cerebrosides 10 mg Enzymes Nucleic acids alkaline phosphatase enzymes Water xanthine oxidase water bicarbonate 100 mg sterols 15 mg acid phosphatase Milk: Main Characteristics many others others peroxidase Cu 4 µg Fe 100 µg Trace elements many others Zn 3 mg Vitamins, e.g. Phosphoric esters ~300 mg Fe 120 µg riboflavin 2 mg Others Cu 20 µg ascorbic acid 20 mg many othersa Approximate average quantities in 1 kg milk. Note: The water in the casein micelles contains some small-molecule solutes.© 2006 by Taylor & Francis Group, LLC
  30. 30. 1.2 Milk Formation 9 LUMEN Microvillus Golgi vesicle with casein micelles Junctional complex Golgi apparatus Cytosol Nascent fat globule Lysosome Mitochondrion Outer cell membrane Nucleus (plasmalemma) Endoplasmic Ribosomes reticulum Basement membrane 5 µmFIGURE 1.2 Stylized diagram of a mammary secretory cell. Below is the basal part, ontop the apical part of the cell. The cell is bounded by other secretory cells to form theglandular epithelium. See text for further details. (From P. Walstra and R. Jenness, DairyChemistry and Physics, Wiley, New York, 1984. With permission.)small globules, which grow while they are transported to the apical end of thecell. They become enrobed by the outer cell membrane (or plasmalemma) whilebeing pinched off into the lumen. This type of secretion is called merocrine,which means that the cell remains intact. Table 1.4 gives some information about the synthesis of specific components.Most are synthesized in the cell. Others are taken up from the blood but, generally,not in the same proportion as in the blood; see, especially, the salts. This meansthat the cell membranes have mechanisms to reject, or allow passage of, specificcomponents. Some substances, notably water and small lipophilic molecules, can© 2006 by Taylor & Francis Group, LLC
  31. 31. TABLE 1.4 10Synthesis of Important Milk Components Milk Component Precursor in Blood Plasma Synthesis of Component Concentration Concentration In the Specific Specific forName (% w/w) Name (% w/w) Secretory Cell? for Milk? the Species?Water 86 Identical 91 No No NoLactose 4.7 Glucosea 0.05 Yes Yes NoProtein Caseins 2.6  Yes Yes Yesb β-lactoglobulin 0.32   Amino acids 0.04 Yes Yes Yes α-lactalbumin 0.12   Yes Yes Yes Lactoferrin  0.01  Yes No Yes Serum albumin 0.04 Identical 3.2 No No Yes Immunoglobulins 0.07 Most are identical 1.5 No No YesEnzymes Trace Various — Yesc Noc YesLipids Acetic acid 0.01   Triglycerides 4 β -Hydroxy butyric acid 0.006  Partly Partly Acylglycerols    Phospholipids 0.03 Some lipids 0.3 Milk: Main CharacteristicsCitric Acid 0.17 Glucosea 0.05 Yes No NoMinerals Identical No No No Ca 0.13 0.01 Pd 0.09 0.01 Na 0.04 0.34 K 0.15 0.03 Cl 0.11 0.35a Glucose can also be formed in the secretory cells from some amino acids.b All proteins are species specific, but comparable proteins occur in the milk of all ruminants.c Is not true for all enzymes.d In various phosphates.© 2006 by Taylor & Francis Group, LLC
  32. 32. 1.2 Milk Formation 11pass the cell more or less unhindered. Some other components, such as serumalbumin and chlorides, can ‘leak’ from the blood into the milk by passing throughthe spaces between secretory cells. Also, some leukocytes somehow reach thelumen. Finally, cell remnants, such as part of the microvilli depicted in Figure 1.2and tiny fragments of cytoplasm that occasionally adhere to a fat globule, aresecreted and form the lipoprotein particles of Table 1.2. Excretion. The glandular epithelium, consisting of layers of secretory cells,form spherical bodies called alveoli. Each of these has a central lumen into whichthe freshly formed milk is secreted. From there, the milk can flow through smallducts into larger and still larger ones until it reaches a cavity called the cistern.From the cistern, the milk can be released via the teat. A cow has four teats andhence four separate mammary glands, commonly called (udder) quarters. Excretion of the milk does not happen spontaneously. The alveoli have tocontract, which can be achieved by the contraction of muscle tissue around thealveoli. Contraction is induced by the hormone oxytocin. This is released intothe blood by stimulation of the teats of the animal, be it by the suckling youngor by the milker. The udder is not fully emptied. Lactation. When a calf is born, lactation — i.e., the formation and secretionof milk — starts. The first secretion greatly differs in composition from milk (seeSubsection 2.7.1.5). Within a few days the milk has become normal and milk yieldincreases for some months, after which it declines. The yield greatly varies amongcows and with the amount and the quality of the feed taken by the cow. For milchcows, milking is generally stopped after about 10 months, when yield has becomequite low. The duration from parturition to leaving the cow dry is called thelactation period, and the time elapsed after parturition is the stage of lactation.1.3 SOME PROPERTIES OF MILK Milk as a Solution. Milk is a dilute aqueous solution and behaves accord-ingly. Because the dielectric constant is almost as high as that of pure water, polarsubstances dissolve well in milk and salts tend to dissociate (although this dis-sociation is not complete). The ionic strength of the solution is about 0.073 M. ThepH of milk is about 6.7 at room temperature. The viscosity is low, about twice thatof water, which means that milk can readily be mixed, even by convection currentsresulting from small temperature fluctuations. The dissolved substances give milkan osmotic pressure of about 700 kPa (7 bar) and a freezing-point depressionclose to 0.53 K. The water activity is high, about 0.995. Milk density ( ρ 20) equalsabout 1029 kg⋅m−3 at 20°C; it varies especially with fat content. Milk as a Dispersion. Milk is also a dispersion; the particles involved aresummarized in Table 1.2. This has several consequences, such as milk beingwhite. The fat globules have a membrane, which acts as a kind of barrier betweenthe plasma and the core lipids. The membrane also protects the globules againstcoalescence. The various particles can be separated from the rest. The fat globules can be concentrated in a simple way by creaming, whicheither occurs due to gravity or — more efficiently — is induced by centrifugation.© 2006 by Taylor & Francis Group, LLC
  33. 33. 12 Milk: Main CharacteristicsIn this way cream and skim milk are obtained. Skim milk is not identical to milkplasma, though quite similar, because it still contains some small fat globules.Cream can be churned, leading to butter and buttermilk; the latter is rather similarin composition to skim milk. Likewise, casein micelles can be concentrated and separated from milk, forinstance, by membrane filtration. The solution passing through the membrane isthen quite similar to milk serum. If the pores in the membrane are very small, alsothe serum proteins are retained. When adding rennet enzyme to milk, as is donein cheese making, the casein micelles start to aggregate, forming a gel; whencutting the gel into pieces, these contract, expelling whey. Whey is also similarto milk serum but not quite, because it contains some of the fat globules and partof the κ-casein split off by the enzyme. Casein also aggregates and forms a gelwhen the pH of the milk is lowered to about 4.6. Moreover, water can be removed from milk by evaporation. Altogether, arange of liquid milk products of various compositions can be made. Some exam-ples are given in Table 1.5. Flavor. The flavor of fresh milk is fairly bland. The lactose produces somesweetness and the salts some saltiness. Several small molecules present in verysmall quantities also contribute to flavor. The fat globules are responsible for thecreaminess of whole milk. Nutritional value. Milk is a complete food for the young calf, and it canalso provide good nutrition to humans. It contains virtually all nutrients, most ofthese in significant quantities. However, it is poor in iron and the vitamin Ccontent is not high. It contains no antinutritional factors, but it lacks dietary fibre. Milk as a Substrate for Bacteria. Because it is rich in nutrients, manymicroorganisms, especially bacteria, can grow in milk. Not all bacteria that needsugar can grow in milk, some being unable to metabolize lactose. Milk is poorin iron, which is an essential nutrient for several bacteria, and contains someantibacterial factors, such as immunoglobulins and some enzyme systems. More-over, milk contains too much oxygen for strictly anaerobic bacteria. Altogether,the growth of several bacteria is more or less restricted in raw milk, but severalothers can proliferate, especially at high ambient temperatures.1.4 VARIABILITYFreshly drawn milk varies in composition, structure, and properties. Even within themilk from a single milking of one cow, variation can occur. The fat globules varyin size and, to some extent, in composition, and the same applies to casein micelles. Natural Variation. The main factors responsible for natural variation in milkare the following: • Genetic factors: Breed and individual. • The stage of lactation: This can have a significant effect. Especially the milk obtained within 2 or 3 d after parturition tends to have a very different composition; it is called colostrum or beestings.© 2006 by Taylor & Francis Group, LLC

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