Table Of ContentDairy Chemistry
and Biochemistry
P.F. FOX and P.L.H. McSWEENEY
Department of Food Chemistry
University College
Cork, Ireland
BLACKIE ACADEMIC & PROFESSIONAL
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First edition 1998
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Preface
Milk has been the subject of scientific study for about 150years and,
consequently, is probably the best characterized, in chemical terms, of our
major foods. It is probably also the most complicated and serves as the raw
material for a very large and diverse family of food products. Dairy science
has existed as a university discipline for more than 100 years; it is the oldest
sector of food science (and technology), with the exception of brewery
science. Since dairy chemistry is a major facet of dairy science, it might be
expected to have been the subject of numerous books. This is, in fact, not
so. During the past 40years, as far as we are aware, only six books or series
on dairy chemistry have been published in English, i.e. Principles of Dairy
Chemistry (Jenness and Paton, 1959), Dairy Chemistry and Physics (Walstra
and Jenness, 1984), Fundamentals of Dairy Chemistry (Webb and Johnson,
1964; Webb, Johnson and Alford, 1974; Wong et al., 19SS), Developments in
Dairy Chemistry (Fox, four volumes, 1982, 1983, 1985, 1989), Advanced
Dairy Chemistry (Fox, three volumes, 1992, 1995, 1997) and Handbook of
Milk Composition (Jensen, 1995). Of these, Principles of Dairy Chemistry and
Dairy Chemistry and Physics were written essentially for senior undergrad-
uate students. The other four books/series were focused principally on
lecturers, researchers, senior postgraduate students and senior production
management. Thus, at present there is a lack of books written at senior
undergraduate/junior postgraduate level specializing in dairy chemistry/
science. This book is intended to fill that gap and should be as useful to
graduates working in the dairy industry as it is to those still studying.
The book assumes a knowledge of chemistry and biochemistry but not
of dairy chemistry. As the title suggests, the book has a stronger biochemical
orientation than either Principles of Dairy Chemistry or Dairy Chemistry and
Physics. In addition to a fairly in-depth treatment of the chemistry of the
principal constituents of milk, i.e. water, lactose, lipids, proteins (including
enzymes), salts and vitamins, various more applied aspects are also covered,
e.g. heat-induced changes, cheese, protein-rich products and the applications
of enzymes in dairy technology. The principal physical properties are also
described.
To facilitate the reader, the structure of various molecules mentioned
frequently in the text are given in appendices but we emphasize that a good
general knowledge of chemistry and biochemistry is assumed. The chemical
composition of the principal dairy products is also included.
xiv PREFACE
The book does not cover the technology of the various dairy products,
although brief manufacturing protocols for some products are included to
facilitate discussion; however, a number of textbooks on various aspects of
dairy technology are referenced. Neither are the chemical analyses, microbi-
ology and nutritional aspects of dairy products covered, except in a very
incidental manner. The effects of dairy husbandry on the composition and
properties of milk are discussed briefly, as is the biosynthesis of milk
constituents; in both cases, some major textbooks are referenced.
We hope that the book will answer some of your questions on the
chemistry and biochemistry of milk and milk products and encourage you
to undertake more extensive study of these topics.
The highly skilled and enthusiastic assistance of Ms Anne Cahalane and
Ms Brid Considine in the preparation of the manuscript and of Professor
D.M. Mulvihill and Dr N. O’Brien for critically and constructively review-
ing the manuscript are gratefully acknowledged and very much appreciated.
P.F. Fox
P.L.H. McSweeney
General references on dairy chemistry
Alais, C. (1974) Science du Lait. Principes des Techniques Laitieres, 3rd edn,
SEP Editions, Paris.
Fox, P.F. (ed.) (1982-1989) Developments in Dairy Chemistry, Volumes 1, 2,
3 and 4, Elsevier Applied Science Publishers, London.
Fox, P.F. (ed.) (1992-1997) Advanced Dairy Chemistry, Volumes 1, 2 and 3,
Elsevier Applied Science Publishers and Chapman & Hall, London.
Jenness, R. and Patton, S. (1959) Principles of Dairy Chemistry, John Wiley
& Sons, New York.
Jensen, R.G. (ed.) (1995) Handbook of Milk Composition, Academic Press,
San Diego.
Walstra, P. and Jenness, R. (1984) Dairy Chemistry and Physics, John Wiley
& Sons, New York.
Webb, B.H. and Johnson, A.H. (eds) (1964) Fundamentals of Dairy Chemis-
try, AVI, Westport, CT, USA.
Webb, B.H., Johnson, A.H. and Alford, J.A. (eds) (1974) Fundamentals of
Dairy Chemistry, 2nd edn, AVI, Westport, CT, USA.
Wong, N.P., Jenness, R., Keeney, M. and Marth, E.H. (eds) (1988) Funda-
mentals of Dairy Chemistry, 3rd edn, Van Norstrand Reinhold, New York.
Contents
Preface xiii
General references on dairy chemistry xv
1 Production and utilization of milk 1
1.1 Introduction 1
1.2 Composition and variability of milk 1
1.3 Classification of mammals 3
1.4 Structure and development of mammary tissue 4
1.5 Ultrastructure of the secretory cell 7
1.6 Techniques used to study milk synthesis 8
1.6.1 Arteriovenous concentration differences 8
1.6.2 Isotope studies 9
1.6.3 Perfusion of isolated gland 9
1.6.4 Tissue slices 10
1.6.5 Cell homogenates 10
1.6.6 Tissue culture 10
1.7 Biosynthesis of milk constituents 11
1.8 Production and utilization of milk 11
1.9 Trade in milk products 18
References 20
Suggested reading 20
2 Lactose 21
2.1 Introduction 21
2.2 Chemical and physical properties of lactose 23
2.2.1 Structure of lactose 23
2.2.2 Biosynthesis of lactose 23
2.2.3 Lactose equilibrium in solution 25
2.2.4 Significance of mutarotation 27
2.2.5 Solubility of lactose 27
2.2.6 Crystallization of lactose 28
2.2.7 Problems related to lactose crystallization 31
2.3 Production of lactose 39
2.4 Derivatives of lactose 42
2.4.1 Enzymatic modification of lactose 42
2.4.2 Chemical modifications 43
2.4.3 Fermentation products 50
2.5 Lactose and the Maillard reaction 54
2.6 Nutritional aspects of lactose 56
2.6.1 Lactose intolerance 58
2.6.2 Galactosaemia 61
vi CONTENTS
2.7 Determination of lactose concentration 62
2.7.1 Polarimetry 62
2.7.2 Oxidation and reduction titration 63
2.7.3 Colorimetric methods 64
2.7.4 Chromatographic methods 65
2.7.5 Enzymatic methods 65
References 65
Suggested reading 66
3 Milk lipids 67
3.1 Introduction 67
3.1 Factors that affect the fat content of bovine milk 68
3.3 Classes of lipids in milk 71
3.4 Fatty acid profile of milk lipids 75
3.5 Synthesis of fatty acids in milk fat 81
3.6 Structure of milk lipids 87
3.7 Milk fat as an emulsion 90
3.8 Milk fat globule membrane 92
3.8.1 Isolation of the fat globule membrane 93
3.8.2 Gross chemical compositlion of FGM 94
3.8.3 The protein fraction 94
3.8.4 The lipid fraction 95
3.8.5 Other membrane components 97
3.8.6 Membrane structure 97
3.8.7 Secretion of milk lipid globules 100
3.9 Stability of the milk fat emulsion 10 4
3.9.1 Emulsion stability in general 104
3.9.2 The creaming process in milk 106
3.10 Influence of processing operations on the fat globule
membrane 108
3.10.1 Milk supply: hydrolytic rancidity 108
3.10.2 Mechanical separation of milk 111
3.10.3 Homogenization 113
3.10.4 Heating 116
3.1 1 Physical defects in milk and cream 117
3.11.1 Free fat 118
3.12 Churning 118
3.13 Freezing 126
3.14 Dehydration 126
3.15 Lipid oxidation 127
3.15.1 Pro-oxidants in milk and milk products 130
3.15.2 Antioxidants in milk 132
3.15.3 Spontaneous oxidation 133
3.15.4 Other factors that affect lipid oxidation in milk and
dairy products 133
3.15.5 Measurement of lipid oxidation 134
3.16 Rheology of milk fat 134
3.16.1 Fatty acid profile and distribution 134
3.16.2 Process parameters 137
References 140
Suggested reading 141
Appendices 141
vii
CONTENTS
4 Milk proteins 146
4.1 Introduction 146
4.2 Heterogeneity of milk proteins 149
4.2.1 Other protein fractions 150
4.3 Preparation of casein and whey proteins 152
4.3.1 Acid (isoelectric) precipitation 152
4.3.2 Centrifugation 153
4.3.3 Centrifugation of calcium-supplemented milk 153
4.3.4 Salting-out methods 153
4.3.5 Ultrafiltration 153
4.3.6 Gel filtration (gel permeation chromatography) 154
4.3.7 Precipitation with ethanol 154
4.3.8 Cryoprecipitation 154
4.3.9 Rennet coagulation 154
4.3.10 Other methods for the preparation of whey proteins 155
4.4 Heterogeneity and fractionation of casein 155
4.4.1 Resolution of caseins by electrophoresis 159
4.4.2 Microheterogeneity of the caseins 160
4.4.3 Nomenclature of the caseins 162
4.5 Some important properties of the caseins 163
4.5.1 Chemical composition 163
4.5.2 Secondary and tertiary structures 175
4.5.3 Molecular size 178
4.5.4 Hydrophobicity 178
4.5.5 Influence of Ca2+ on caseins 179
4.5.6 Action of rennets on casein 179
4.5.7 Casein association 180
4.5.8 Casein micelle structure 180
4.6 Whey proteins 186
4.6.1 Preparation 186
4.6.2 Heterogentity of whey proteins 186
4.7 P-Lactoglobulin 187
4.7.1 Occurrence and microheterogeneity 188
4.7.2 Amino acid composition 188
4.7.3 Primary structure 189
4.7.4 Secondary structure 189
4.7.5 Tertiary structure 189
4.7.6 Quaternary structure 190
4.7.7 Physiological function 191
4.7.8 Denaturation 192
4.8 a-Lactal bumin 192
4.8.1 Amino acid composition 192
4.8.2 Genetic variants 192
4.8.3 Primary structure 192
4.8.4 Secondary and tertiary structure 193
4.8.5 Quaternary structure 193
4.8.6 Other species 193
4.8.7 Biological function 194
4.8.8 Metal binding and heat stability 194
4.9 Blood serum albumin 195
4.10 Immunoglobulins (Ig) 195
4.11 Minor milk proteins 199
4.12 Non-protein nitrogen 199
4.13 Comparison of human and bovine milks 200
...
Vlll CONTENTS
4.14 Synthesis and secretion of milk proteins 20 1
4.14.1 Sources of amino acids 20 1
4.14.2 Amino acid transport into the mammary cell 203
4.14.3 Synthesis of milk proteins 203
4.14.4 Modifications of the polypeptide chain 205
4.14.5 Structure and expression of milk protein genes 206
4.14.6 Secretion of milk-specific proteins 207
4.14.7 Secretion of immunoglobulins 209
4.15 Functional milk proteins 210
4.15.1 Industrial production of caseins 21 1
4.15.2 Novel methods for casein production 215
4.15.3 Fractionation of casein 216
4.1 5.4 Functional (physicochemical) properties of caseins 218
4.15.5 Applications of caseins 219
4.15.6 Whey proteins 219
4.15.7 Casein-whey protein co-precipitates 227
4.16 Biologically active proteins and peptides in milk 228
4.16.1 Lactoperoxidase 229
4.16.2 Lactotransferrin 229
4.16.3 Immunoglobulins 230
4.16.4 Vitamin-binding proteins 230
4.16.5 Growth factors 23 1
4.16.6 Bifidus factors 23 1
4.16.7 Milk protein hydrolysates 232
References 234
Suggested reading 236
Appendices 237
5 Salts of milk 239
5.1 Introduction 239
5.2 Method of analysis 239
5.3 Composition of milk salts 240
5.4 Secretion of milk salts 242
5.5 Factors influencing variation in salt composition 243
5.5.1 Breed of cow 24 3
5.5.2 Stage of lactation 244
5.5.3 Infection of the udder 247
5.5.4 Feed 247
5.6 Interrelations of milk salt constituents 247
5.7 Partition of milk salts between colloidal and soluble phases 249
5.7.1 Methods used to separate the colloidal and soluble phases 249
5.7.2 Soluble salts 250
5.7.3 Measurement of calcium and magnesium ions 254
5.7.4 Colloidal milk salts 256
5.8 Changes in milk salts equilibria induced by various treatments 260
5.8.1 Addition of acid or alkali 260
5.8.2 Addition of various salts 26 1
5.8.3 Effect of changes in temperature 26 1
5.8.4 Changes in pH induced by temperature 262
5.8.5 Etfect of dilution and concentration 262
5.8.6 Etfect of freezing 263
References 263
Suggested reading 264
CONTENTS ix
6 Vitamins in milk and dairy products 265
6.1 Introduction 265
6.2 Fat-soluble vitamins 266
6.2.1 Retinol (vitamin A) 266
6.2.2 Calciferols (vitamin D) 269
6.2.3 Tocopherols and related compounds (vitamin E) 272
6.2.4 Phylloquinone and related compounds (vitamin K) 274
6.3 B-group vitamins 275
6.3.1 Thiamin (vitamin B,) 275
6.3.2 Riboflavin (vitamin B2) 277
6.3.3 Niacin 279
6.3.4 Biotin 28 1
6.3.5 Panthothenic acid 281
6.3.6 Pyridoxine and related compounds (vitamin B6) 282
6.3.7 Folate 285
6.3.8 Cobalamin and its derivatives (vitamin B12) 287
6.4 Ascorbic acid (vitamin C) 289
References 291
Suggested reading 29 1
Appendices 29 1
7 Water in milk and dairy products 294
7.1 Introduction 294
7.2 General properties of water 294
7.3 Water activity 301
7.4 Water sorption 305
7.5 Glass transition and the role of water in plasticization 311
7.6 Non-equilibrium ice formation 312
7.7 Role of water in stickiness and caking of powders and
crystallization of lactose 313
7.8 Water and the stability of dairy products 313
References 316
Suggested reading 316
8 Enzymology of milk and milk products 317
8.1 Introduction 317
8.2 Indigenous enzymes of bovine milk 317
8.2.1 Introduction 317
8.2.2 Proteinases (EC 3.4~) 318
8.2.3 Lipases and esterases (EC 3.1.1.-) 322
8.2.4 Phosphatases 324
8.2.5 Lysozyme (EC 3.2.1.17) 327
8.2.6 N-Acetyl-P-D-glucosaminidase (EC 3.2.1.30) 328
8.2.7 y-Glutamyl transpeptidase (transferase) (EC 2.3.2.2) 328
8.2.8 Xanthine oxidase (EC 1.2.3.2) 328
8.2.9 Sulphydryl oxidase (EC 1.8.3.-) 330
8.2.10 Superoxide dismutase (EC 1.15.1.1) 330
8.2.11 Catalase (EC 1.11.1.6) 331
8.2.12 Lactoperoxidase (EC 1.1 1.1.7) 331
8.2.13 Other enzymes 333
