DAIRY CHEMISTRY AND BIOCHEMISTRY Table 1. 1 Composition (%)of milks of some species Total solids Fat Protein Donkey Domestic rabbit 18.3 Indian elephant Polar b Grey seal 11.2 production is seasonal. Not only do the concentrations of the principal and minor constituents vary with the above factors, the actual chemistry of some of the constituents also varies, e.g. the fatty acid profile is strongly influenced by diet. Some of the variability in the composition and constituents of milk can be adjusted or counteracted by processing technology but some differen ces may still persist. The variability of milk and the consequent problems will become apparent in subsequent chapters From a physicochemical viewpoint, milk is a very complex fluid. The constituents of milk occur in three phases. Quantitatively, most of the ma of milk is a true solution of lactose, organic and inorganic salts, vitamins and other small molecules in water. In this aqueous solution are dispersed proteins, some at the molecular level(whey proteins), others as large colloidal aggregates, ranging in diameter from 50 to 600 nm(the caseins), and lipids which exist in an emulsified state, as globules ranging in diameter from 0.1 to 20 um. Thus, colloidal chemistry is very important in the stud of milk, e.g. surface chemistry, light scattering and rheological properties Milk is a dynamic system owing to: the instability of many of its structures, e.g, the milk fat globule membrane; changes in the solubility of many constituents with temperature and pH, especially of the inorganic salts but also of proteins; the presence of various enzymes which can modify constituents through lipolysis, proteolysis or oxidation/reduction; the growth of micro-organisms, which can cause major changes either directly through their in pH or redox potential(,)or through enzymes they excrete; and the interchange of gases with the atmosphere, e.g carbon dioxide. Milk was intended to be consumed directly from the mammary gland and to be expressed from the gland at frequent intervals However, in dairying operations, milk is stored for various periods, ranging from a few hours to several days, during which it is cooled (and perhaps2 DAIRY CHEMISTRY AND BIOCHEMISTRY Table 1.1 Composition (%) of milks of some species Species Total solids Fat Protein Lactose Ash Human 12.2 3.8 1 .o 7.0 0.2 cow 12.7 3.7 3.4 4.8 0.7 Goat 12.3 4.5 2.9 4.1 0.8 Sheep 19.3 1.4 4.5 4.8 1.0 Pig 18.8 6.8 4.8 5.5 - Horse 11.2 1.9 2.5 6.2 0.5 Donkey 11.7 1.4 2.0 7.4 0.5 Reindeer 33.1 16.9 11.5 2.8 - Domestic rabbit 32.8 18.3 11.9 2.1 1.8 Bison 14.6 3.5 4.5 5.1 0.8 Indian elephant 31.9 11.6 4.9 4.1 0.7 Polar bear 47.6 33.1 10.9 0.3 1.4 Grey seal 67.7 53.1 11.2 0.7 - production is seasonal. Not only do the concentrations of the principal and minor constituents vary with the above factors, the actual chemistry of some of the constituents also varies, e.g. the fatty acid profile is strongly influenced by diet. Some of the variability in the composition and constituents of milk can be adjusted or counteracted by processing technology but some differen￾ces may still persist. The variability of milk and the consequent problems will become apparent in subsequent chapters. From a physicochemical viewpoint, milk is a very complex fluid. The constituents of milk occur in three phases. Quantitatively, most of the mass of milk is a true solution of lactose, organic and inorganic salts, vitamins and other small molecules in water. In this aqueous solution are dispersed proteins, some at the molecular level (whey proteins), others as large colloidal aggregates, ranging in diameter from 50 to 600nm (the caseins), and lipids which exist in an emulsified state, as globules ranging in diameter from 0.1 to 20 pm. Thus, colloidal chemistry is very important in the study of milk, e.g. surface chemistry, light scattering and rheological properties. Milk is a dynamic system owing to: the instability of many of its structures, e.g., the milk fat globule membrane; changes in the solubility of many constituents with temperature and pH, especially of the inorganic salts but also of proteins; the presence of various enzymes which can modify constituents through lipolysis, proteolysis or oxidation/reduction; the growth of micro-organisms, which can cause major changes either directly through their growth, e.g. changes in pH or redox potential (EJ or through enzymes they excrete; and the interchange of gases with the atmosphere, e.g. carbon dioxide. Milk was intended to be consumed directly from the mammary gland and to be expressed from the gland at frequent intervals. However, in dairying operations, milk is stored for various periods, ranging from a few hours to several days, during which it is cooled (and perhaps