The Milk Fat Globule Emulsion Science As Applied to Milk Products and Comparable Foods
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The milk fat globule Emulsion science as applied to milk products and comparable foods H. Mulder and P. Walstra vCAB/ Commonwealth Agricultural Bureaux " Farnham Royal, Bucks., England, 1974 Centre for Agricultural Publishing and Documentation Wageningen, the Netherlands, 1974 Preface In a sense, this book is a revision of 'Zuivelonderzoek : Problemen en Resultaten bij Wetenschappelijk Zuivelonderzoek' (Problems and Results in Dairy Research), Volumes I (The fat in milk) and II (Butter), by H. Mulder, published by Koninklijke Nederlandse Zuivelbond FNZ in 1947. After 'Zuivelonderzoek' went out of print, a revised edition was considered for some time but proved impractical. The book was written at a time that scientific methods were gaining impetus in dairy problems. In this book, Mulder comprehensively reviewed progress in research on certain problems, and discussed implications for the dairy industry of results then available. But since the development of funda mental sciences has so profoundly influenced dairy science, and the scope of our knowledge has so much widened and deepened, such a plan could not now be carried through. Hence we came to write a completely new book. Accumulation of systematic knowledge and changes in the dairy and food industries diverted us from making the classical milk products the central themes, around which the material could be grouped. The basis is rather the raw material, milk, and its properties and the processes involved in manufacture into products, either traditional or re- combined dairy products or other foods. The key to the book is the milk fat globule. By suitable presentation we hope we have made the book useful to workers with other products too. We have tried to proceed as much as possible from theory, particularly physical chemistry and colloid science, and we have considered understanding of processes more important than practical details. Crucial then is how to apply these fundamentals in food processing. Hence we have refrained from long theoretical arguments, or complicated mathematical treatment. Naturally, we have laid some stress on our own preferences and on our own work. Several internal reports, mostly by students, have been used. We have not tried to list all the literature in the references: there is too much. Literature appearing after 1972 has not been digested. A book like this cannot be produced without the help of many, and we gratefully acknowledge their assistance. The staff of the Dairying Laboratory helped in many ways: with criticism, in experimental work to fill gaps and to remove doubts, in making figures and plates, and in typewriting and correcting the manuscript. Several colleagues, from the Netherlands Institute for Dairy 5 Research among others, helped with critical discussions or let us use unpublished results. We want to mention Dr M. van den Tempel (Unilever Research, Vlaardingen) and Dr J. Lyklema (Department of Physical and Colloid Chemistry of the University, Wageningen), who scrutinized drafts of Chapters 3 and 6, respectively. In congenial cooperation, Mr J. C. Rigg (Pudoc) not only made English out of our attempts, but improved the text in other ways too. Mr J. M. Davies (CBDST) made the last careful checks in text and references. Finally we are indebted to the publishers, if only for their patience. H. Mulder Wageningen, April 1973 P. Walstra Writing a book like this one is almost too arduous a task for one author. When a mere revision of 'Zuivelonderzoek' proved impractical, I was therefore very fortunate in having a co-author who did the majority of the work. Mainly through his drive and initiative, the book has been knocked into its final shape. H. Mulder 6 Table of contents Preface Terms, symbols and units 11 1 Structure of milk 13 1.1 Composition of cows'milk 13 1.2 Synthesis of milk 14 1.3 Structural elements 16 1.4 Changes 22 1.5 Conclusions 23 References 23 2 The fat in milk 25 2.1 Component fatty acids 25 2.2 The different lipids 26 2.3 The fat in milk products 30 2.4 Physical properties of milk fat 30 References 32 3 Crystallization behaviour of milk fat 33 3.1 Melting range 33 3.2 Nucleation and growth of crystals 35 3.3 Polymorphism 37 3.4 Mixed crystals 40 3.5 Proportion of solid fat 45 3.6 State of dispersion of the fat 46 3.7 Crystal habit and consistency 49 3.8 Conclusions 51 References 52 4 The fat dispersion 54 4.1 Globule size distribution 54 4.2 Optical properties 62 4.3 Viscosity 62 4.4 Composition of individual globules 64 References 65 5 The fat-globule membrane 67 5.1 Defining the membrane 67 5.2 Origin of the membrane 68 5.3 Methods of analysis 72 5.4 Composition 76 5.5 Structure 84 5.6 Changes caused by processing 88 5.7 Lipase action and the membrane 93 5.8 Fat oxidation, copper and the membrane 95 5.9 Conclusions 97 References 98 6 Stability of the fat emulsion 101 6.1 What is instability ? 101 6.2 Kinetic aspects 105 6.3 Repulsion 109 6.4 Barriers against coalescence 111 6.5 Disruption of fat globules 117 6.6 Other aspects 121 6.7 Conclusions 127 References 128 7 Changes in fat dispersion with processing 131 7.1 Agitation 131 7.2 Steam injection and flash boiling 135 7.3 Concentration and drying 135 7.4 Cream plug — 135 7.5 Freezing 136 References 138 8 Creaming and separation 139 8.1 Gravity creaming of single fat globules 139 8.2 Creaming in homogenized products 141 8.3 Centrifugal separation 144 8.4 Natural creaming 147 8.5 Properties of cream and skim-milk 156 8.6 Conclusions 159 References 160 9 Homogenization 163 9.1 Processes inside the homogenization valve 163 9.2 Globule size 169 9.3 Aspects of the newly formed membrane 175 9.4 Stability of homogenized milk 181 9.5 Effects on milk proteins 183 9.6 Miscellaneous effects 185 9.7 Other methods of homogenization 187 9.8 Conclusions 191 References 192 10 Cream 195 10.1 Properties affected by fat content 195 10.2 Rheological properties 198 10.3 Heat stability of protein in cream 205 References 208 11 Fat globules in foam products 210 11.1 Foam in general 210 11.2 Foaming of milk products 213 11.3 Whipping of cream 217 11.4 Ice cream 222 References 225 12 Isolation of milk fat 228 12.1 Mechanical clumping and phase inversion 228 12.2 Flotation churning 232 12.3 Other methods of isolating fat 237 12.4 Properties and use of milk fat 239 References 243 13 Structure and texture of butter 246 13.1 Butter structure 246 13.2 The fat globules of butter 250 13.3 Moisture 254 13.4 Air cells 262 13.5 Texture 263 13.6 Factors affecting consistency 266 13.7 Setting and work-softening 269 13.8 Temperature effects 272 13.9 Practical methods of controlling consistency 274 References 279 14 Synthetic fat-globules 14.1 Recombined milk 14.2 Comparison of fat globules References Subject index Plates Plate 1, opposite page Plate 2, opposite page Plate 3, opposite page Plate 4, opposite page Plate 5, opposite page Plate 6, opposite page Plate 7, opposite page Plate 8, opposite page Terms, symbols and units The following terms are ambiguous; we have used them in the sense indicated: casein: (usually) calcium caseinate/calcium phosphate complex as present in casein micelles clump: aggregate of fat globules with fat touching fat (Fig. 6.3) cluster: aggregate of fat globules partly sharing their surface layers (Fig. 6.3) floccule : aggregate of fat globules in which each globule has kept its identity (Fig. 6.3) plasma : milk or milk product minus fat globules serum : plasma minus casein micelles skim-milk : product obtained by gravity creaming separated milk : product obtained by centrifugal separation Some frequently used symbols are : A surface area (e.g. Eqn 4.4) cg relative width of size distribution (Section 9.2) d globule diameter (mostly in (xm) dvB volume surface average diameter (Eqn 4.3) g standard acceleration due to gravity (9.8 m s-2) G gravimetric fat percentage H creaming parameter of size distribution (Eqn 8.2) k Boltzmann constant (1.38 x 10~23 J K_1) P pressure R radius S shear rate (velocity gradient, s-1) T temperature (K, unless otherwise stated) v velocity Y surface or interfacial tension V viscosity V' apparent viscosity p mass density (g/ml) <p volume fraction 11 Units are mostly cm-g-s units. Conversions to SI units are given for some units often used : 1 atm = (1.03 kgf/cm2 = 14.7 p.s.i. = 106 dyne/cm2 =) 0.1 MPa 1 cal = 4.18 J 1 cP =1 mN s m~2 1 dyne/cm = 1 mN m_1 1 dyne/cm2 = 0.1 Pa 1 erg = 10~7 J 1 erg/cm2 = 1 mJ m~2 % = per cent by weight, unless otherwise stated 12 1 Structure of milk 1.1 Composition of cows' milk The composition of milk is itself not within the scope of this book, but since structure depends on composition, we include here some general remarks. In most texts on dairy chemistry, milk composition finds ample treatment (8,11,24). But, though literally thousands of investigations have been published, our knowledge of the composition of cows' milk is incomplete, partly through gaps in knowledge, partly through imperfections in methods, but largely to inherent variability. Although normal milk, produced by healthy cows, is remarkably constant in qualitative composition, amounts of constituents, structure and most physical properties are highly variable.