1 Production and utilization of milk 1.1 Introduction Milk is a fluid secreted by the female of all mamalian species, of which there are more than 4000, for the primary function of meeting the complete nutritional requirements of the neonate of the species. In addition, milk serves several physiological functions for the neonate. Most of the non- nutritional functions of milk are served by proteins and peptides which include immunoglobulins, enzymes and enzyme inhibitors, binding or car rier proteins, growth factors and antibacterial agents. Because the nutri tional and physiological requirements of each species are more or less unique, the composition of milk shows very marked inter-species differences Of the more than 4000 species of mammal, the milks of only about 180 have been analysed and, of these, the data for only about 50 species are onsidered to be reliable (sufficient number of samples, representative sampling, adequate coverage of the lactation period). Not surprisingly, the milks of the principal dairying species, i. e cow, goat, sheep and buffalo, and the human are among those that are well characterized. The gross compo- sition of milks from selected species is summarized in Table 1.1; very extensive data on the composition of bovine and human milk are contained in Jensen(1995) 1.2 Composition and variability of milk In addition to the principal constituents listed in Table 1.1, milk contains several hundred minor constituents, many of which, e.g. vitamins, metal ions and favour compounds, have a major impact on the nutritional, technoloy cal and sensoric properties of milk and dairy products. Many of these effects will be discussed in subsequent chapters die tik is a very variable biological fluid. In addition to interspecies rences(Table 1.1), the milk of any particular species varies with the dividuality of the animal, the breed (in the case of commercial dairying species), health(mastitis and other diseases), nutritional status, stage of lactation, age, interval between milkings, etc. In a bulked factory milk supply, variability due to many of these factors is evened out, but some variability will persist and will be quite large in situations where milk
1 Production and utilization of milk 1.1 Introduction Milk is a fluid secreted by the female of all mamalian species, of which there are more than 4000, for the primary function of meeting the complete nutritional requirements of the neonate of the species. In addition, milk serves several physiological functions for the neonate. Most of the nonnutritional functions of milk are served by proteins and peptides which include immunoglobulins, enzymes and enzyme inhibitors, binding or carrier proteins, growth factors and antibacterial agents. Because the nutritional and physiological requirements of each species are more or less unique, the composition of milk shows very marked inter-species differences. Of the more than 4000 species of mammal, the milks of only about 180 have been analysed and, of these, the data for only about 50 species are considered to be reliable (sufficient number of samples, representative sampling, adequate coverage of the lactation period). Not surprisingly, the milks of the principal dairying species, i.e. cow, goat, sheep and buffalo, and the human are among those that are well characterized. The gross composition of milks from selected species is summarized in Table 1.1; very extensive data on the composition of bovine and human milk are contained in Jensen (1995). 1.2 Composition and variability of milk In addition to the principal constituents listed in Table 1.1, milk contains several hundred minor constituents, many of which, e.g. vitamins, metal ions and flavour compounds, have a major impact on the nutritional, technological and sensoric properties of milk and dairy products. Many of these effects will be discussed in subsequent chapters. Milk is a very variable biological fluid. In addition to interspecies differences (Table 1.1), the milk of any particular species varies with the individuality of the animal, the breed (in the case of commercial dairying species), health (mastitis and other diseases), nutritional status, stage of lactation, age, interval between milkings, etc. In a bulked factory milk supply, variability due to many of these factors is evened out, but some variability will persist and will be quite large in situations where milk
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 perhaps
2 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 differences 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
PRODUCTION AND UTILIZATION OF MILK heated) and agitated to various degrees. These treatments will cause at some physical changes and permit some enzymatic and microbiolo changes hich may alter the processing properties of milk. Again, it may possible to counteract some of these changes 1.3 Classification of mammals The essential characteristic distinguishing mammals from other animal species is the ability de of specialized organs(mammary glands) for the nutrition of its newborn The class Mammalia is divided into three subclasses 1. Prototheria. This subclass contains only one order, Monotremes, species of which are egg-laying mammals, e.g. duck-billed platypus echidna, and are indigenous only to Australasia. They possess many (perhaps 200)mammary glands grouped in two areas of the abdomen the glands do not terminate in a teat and the secretion(milk )is licked by the young from the surface of the gland 2. Marsupials. The young of marsupials are born live(viviparous)after short gestation and are premature at birth to a greater or lesser degree, depending on the species. After birth, the young are transferred to a pouch where they reach maturity, e.g. kangaroo and wallaby. In marsu- pials, the mammary glands, which vary in number, are located within the pouch and terminate in a teat. The mother may nurse two offspring, differing widely in age, simultaneously from different mammary glands that secrete milk of very different composition, designed to meet the different specific requirements of each offspring 3. Eutherians. About 95% of all mammals belong to this subclass. The developing embryo in utero receives nourishment via the placental blood supply (they are referred to as placental mammals ) and is born at a high, but variable, species-related state of maturity. All eutherians secrete milk, which, depending on the species, is more or less essential for the development of the young; the young of some species are born sufficiently mature to survive and develop without milk The number and location of mammary glands varies with species from two, e.g. human, goat and sheep, to 14-16 for the pig. Each gland is anatomically and physiologically separate and is emptied via a teat. The wide interspecies variation in the composition(Table 1. 1)and the chemistry of the constituents of milk, as discussed elsewhere, renders milk species-specific, i.e., designed to meet the requirements of the young of that species. There is also a surprisingly good relationship between milk yield and maternal body weight( Figure 1. 1); species bred for commercial milk production, e.g. dairy cow and goat, fall above the line
PRODUCTION AND UTILIZATION OF MILK 3 heated) and agitated to various degrees. These treatments will cause at least some physical changes and permit some enzymatic and microbiological changes which may alter the processing properties of milk. Again, it may be possible to counteract some of these changes. 1.3 Classification of mammals The essential characteristic distinguishing mammals from other animal species is the ability of the female of the species to produce milk in specialized organs (mammary glands) for the nutrition of its newborn. 1. Prototheria. This subclass contains only one order, Monotremes, the species of which are egg-laying mammals, e.g. duck-billed platypus and echidna, and are indigenous only to Australasia. They possess many (perhaps 200) mammary glands grouped in two areas of the abdomen; the glands do not terminate in a teat and the secretion (milk) is licked by the young from the surface of the gland. 2. Marsupials. The young of marsupials are born live (viviparous) after a short gestation and are ‘premature’ at birth to a greater or lesser degree, depending on the species. After birth, the young are transferred to a pouch where they reach maturity, e.g. kangaroo and wallaby. In marsupials, the mammary glands, which vary in number, are located within the pouch and terminate in a teat. The mother may nurse two offspring, differing widely in age, simultaneously from different mammary glands that secrete milk of very different composition, designed to meet the different specific requirements of each offspring. 3. Eutherians. About 95% of all mammals belong to this subclass. The developing embryo in utero receives nourishment via the placental blood supply (they are referred to as placental mammals) and is born at a high, but variable, species-related state of maturity. All eutherians secrete milk, which, depending on the species, is more or less essential for the development of the young; the young of some species are born sufficiently mature to survive and develop without milk. The number and location of mammary glands varies with species from two, e.g. human, goat and sheep, to 14-16 for the pig. Each gland is anatomically and physiologically separate and is emptied via a teat. The wide interspecies variation in the composition (Table 1.1) and the chemistry of the constituents of milk, as discussed elsewhere, renders milk species-specific, i.e., designed to meet the requirements of the young of that species. There is also a surprisingly good relationship between milk yield and maternal body weight (Figure 1.1); species bred for commercial milk production, e.g. dairy cow and goat, fall above the line. The class Mammalia is divided into three subclasses:
DAIRY CHEMISTRY AND BIOCHEMISTRY Friesian Cow a Morse Buffalo e Beef Co ·3 ubon Rabbit e Rats· Guinea-Pig lamster Tree shrew Body Weight (kg) Figure 1.1 Relation between daily milk yield and maternal body weight for some species (modified from Linzell, 1972) 1.4 Structure and development of mammary tissue The mammary glands of all species have the same basic structure and all are located external to the body cavity(which greatly facilitates research on milk biosynthesis). Milk constituents are synthesized in specialized epithelial cells(secretory cells or mammocytes, Figure 1.2d)from molecules absorbed from the blood. The secretory cells are grouped as a single layer around a central space, the lumen, to form more or less spherical or pear-shaped bodies, known as alveoli( Figure 1.2c). The milk is secreted from these calls into the lumen of the alveoli. When the lumen is full, the myoepithelial cells surrounding each alveolus contract under the influence of oxytocin and the milk is drained via a system of arborizing ducts towards sinuses or cisterns (Figure 1. 2a)which are the main collecting points between suckling or milking. The cisterns lead to the outside via the teat canal. Groups of alveoli, which are drained by a common duct, constitute a lobule: neighbouring lobules are separated by connective tissue(Figure 1.2b). The secretory elements are termed the " lobule-alveolar systemto distinguish them from the duct system. The whole gland is shown in Figure 1.2a Milk constituents are synthesized from components obtained from blood; consequently, the mammary gland has a plentiful blood supply also an elaborate nervous system to regulate excretion
4 DAIRY CHEMISTRY AND BIOCHEMISTRY R,~, . . Oumea-Pig Il.,lll*lcr . 1khidii:i 3 3 10.' Body Wcight (kg) Figure 1.1 Relation between daily milk yield and maternal body weight for some species (modified from Linzell, 1972). 1.4 Structure and development of mammary tissue The mammary glands of all species have the same basic structure and all are located external to the body cavity (which greatly facilitates research on milk biosynthesis). Milk constituents are synthesized in specialized epithelial cells (secretory cells or mammocytes, Figure 1.2d) from molecules absorbed from the blood. The secretory cells are grouped as a single layer around a central space, the lumen, to form more or less spherical or pear-shaped bodies, known as alveoli (Figure 1.2~). The milk is secreted from these calls into the lumen of the alveoli. When the lumen is full, the rnyoepithelial cells surrounding each alveolus contract under the influence of oxytocin and the milk is drained via a system of arborizing ducts towards sinuses or cisterns (Figure 1.2a) which are the main collecting points between suckling or milking. The cisterns lead to the outside via the teat canal. Groups of alveoli, which are drained by a common duct, constitute a lobule; neighbouring lobules are separated by connective tissue (Figure 1.2b). The secretory elements are termed the 'lobule-alveolar system' to distinguish them from the duct system. The whole gland is shown in Figure 1.2a. Milk constituents are synthesized from components obtained from the blood; consequently, the mammary gland has a plentiful blood supply and also an elaborate nervous system to regulate excretion
PRODUCTION AND UTILIZATION OF MILK (b) 题 61000 VESSE ALVEOLUS CSTFRN MILKFAT ORODLETS RTFRLAL BLOO CAHLLARES MITCCHONDRION Figure 1.2 Milk-producing tissue of a cow, shown at progressively larger scale longitudinal section of one of the four quarters of a mammary gland;(b) arrangement of alveoli and the duct system that drains them; (c)single alveolus consisting of an mmary gland; (d)a lactating cell; part of the cell membrane becomes the membrane at droplets; dark circular bodies in the va Golgi apparatus are protein particles, which are discharged into the lumen. (From Patton, 1969)
PRODUCTION AND UTILIZATION OF MILK 5 WPI.LAR1ES C0:NECTIbE ISSUE N LK PRVEIN GOLGi \ PPAHATUS Figure 1.2 Milk-producing tissue of a cow, shown at progressively larger scale. (a) A longitudinal section of one of the four quarters of a mammary gland; (b) arrangement of the alveoli and the duct system that drains them; (c) single alveolus consisting of an elliptical arrangement of lactating cells surrounding the lumen, which is linked to the duct system of the mammary gland; (d) a lactating cell; part of the cell membrane becomes the membrane covering fat droplets; dark circular bodies in the vacuoles of Golgi apparatus are protein particles, which are discharged into the lumen. (From Patton, 1969.)