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.)
DAIRY CHEMISTRY AND BIOCHEMISTRY Weaning 10 0 100 Figure 1.3 Time-course of mammary development in rats(from Tucker, 1969) The substrates for milk synthesis enter the secretory cell across the basal membrane(outside), are utilized, converted and interchanged as they pass inwards through the cell and the finished milk constituents are excreted into the lumen across the lumenal or apical membrane. Myoepithelial cells (spindle shaped)form abasket around each alveolus and are capable of contracting on receiving an electrical, hormonally mediated, stimulus, there- by causing ejection of milk from the lumen into the ducts. Development of mammary tissue commences before birth, but at birth he gland is still rudimentary. It remains rudimentary until puberty when rery significant growth occurs in some species; much less growth occurs in other species, but in all species the mammary gland is fully developed at puberty. In most species, the most rapid phase of mammary gland develop- ment occurs at pregnancy and continues through pregnancy and partur ition, to reach peak milk production at weaning. The data in Figure 1.3 the development pattern of the mammary gland in the rat, the species has been thoroughly studied in this regard Mammary development is under the regulation of a complex set of hormones. Studies ng endocrinectomy(removal of different endocrine organs) show that the principal hormones are oestrogen, progesterone growth hormone, prolactin and corticosteroids( Figure 1.4)
6 DAIRY CHEMISTRY AND BIOCHEMISTRY 3 t-” 10 0 0 100 200 Days Figure 1.3 Time-course of mammary development in rats (from Tucker, 1969). The substrates for milk synthesis enter the secretory cell across the basal membrane (outside), are utilized, converted and interchanged as they pass inwards through the cell and the finished milk constituents are excreted into the lumen across the lumenal or apical membrane. Myoepithelial cells (spindle shaped) form a ‘basket’ around each alveolus and are capable of contracting on receiving an electrical, hormonally mediated, stimulus, thereby causing ejection of milk from the lumen into the ducts. Development of mammary tissue commences before birth, but at birth the gland is still rudimentary. It remains rudimentary until puberty when very significant growth occurs in some species; much less growth occurs in other species, but in all species the mammary gland is fully developed at puberty. In most species, the most rapid phase of mammary gland development occurs at pregnancy and continues through pregnancy and parturition, to reach peak milk production at weaning. The data in Figure 1.3 show the development pattern of the mammary gland in the rat, the species that has been thoroughly studied in this regard. Mammary development is under the regulation of a complex set of hormones. Studies involving endocrinectomy (removal of different endocrine organs) show that the principal hormones are oestrogen, progesterone, growth hormone, prolactin and corticosteroids (Figure 1.4)
PRODUCTION AND UTILIZATION OF MILK ATROPHIC GLAND Oest +GH+C DUCT GROWTH Ocst +Prog+Pl+ LOBULO-ALVEOLAR GROWTH PL +C MILK SECRETION Figure 1.4 The hormonal control of mammary development in rats. Oest, Oestrogen: Prog, progesterone; GH, growth hormone; PL, prolactin; C, corticosteroid 1.5 Ultrastructure of the secretory cell The structure of the secretory cell is essentially similar to that of other eukaryotic cells. In their normal state, the cells are roughly cubical, c. 10 um in cross-section It is estimated that there are c. 5 x 10 2 cells in the udder of the lactating cow. a diagrammatic representation of the cell is shown in igure 1.2d. It contains a large nucleus towards the base of the cell and is surrounded by a cell membrane, the plasmalemma. The cytoplasm contains the usual range of organelles mitochondria: principally involved in energy metabolism (tricarboxylic acid(Krebs) cycle) endoplasmic reticulum: located towards the base of the cell and to which are attached ribosomes, giving it a rough appearance(hence the term, rough endoplasmic reticulum, RER). Many of the biosynthetic reaction of the cell occur in the rer: Golgi apparatus: a smooth membrane system located toward the apical gion of the cell, where much of the assembly and 'packaging of synthesized material for excretion occur
PRODUCTION AND UTILIZATION OF MILK 7 ATROPHIC GLAND Ocst + GH + C DUCT GROWTH LOBULO-ALVEOLAR GROWTH MILK SECRETION Figure 1.4 The hormonal control of mammary development in rats. Oest, Oestrogen; Prog, progesterone; GH, growth hormone; PL, prolactin; C, corticosteroids. 1.5 Ultrastructure of the secretory cell The structure of the secretory cell is essentially similar to that of other eukaryotic cells. In their normal state, the cells are roughly cubical, c. 10 pm in cross-section. It is estimated that there are c. 5 x 10’’ cells in the udder of the lactating cow. A diagrammatic representation of the cell is shown in Figure 1.2d. It contains a large nucleus towards the base of the cell and is surrounded by a cell membrane, the plasmalemma. The cytoplasm contains the usual range of organelles: 0 mitochondria: principally involved in energy metabolism (tricarboxylic acid (Krebs) cycle); 0 endoplasmic reticulum: located towards the base of the cell and to which are attached ribosomes, giving it a rough appearance (hence the term, rough endoplasmic reticulum, RER). Many of the biosynthetic reactions of the cell occur in the RER; 0 Golgi apparatus: a smooth membrane system located toward the apical region of the cell, where much of the assembly and ‘packaging’ of synthesized material for excretion occur;
IRY CHEMISTRY AND BIOCHEMISTRY lysosomes: capsules of enzymes (usually hydrolytic)distributed fairl uniformly throughout the cytoplasm Fat droplets and vesicles of material for excretion are usually apparent toward the apical region of the cell. The apical membrane possesses microvilli which serve to greatly increase its surface area 1.6 Techniques used to study milk synthesis 1. 6. Arteriovenous concentration differences The arterial and veinous systems supplying the mammary gland( Figure 1 for analysis. Differences in composition between arterial and venous blood give a measure of the constituents used in milk synthesis. The total amount of constituent used may be determined if the blood flow rate is known, which may be easily done by infusing a known volume of cold saline Figure 1.5 The blood vessel and nerve supply in the mammary glands of a cow Circulatory system(arteries, white: veins, stippled ) h, heart; a, abdominal aorta; pa, external pudic artery; Nerves: 1, first lumbar nerve; 2, second lumbar nerve: 3, external sper erve; 4, perineal nerve. A and v show blood sampling points for arteriovenous(Av)difference determinations (Mepham, 1987)
a DAIRY CHEMISTRY AND BIOCHEMISTRY 0 lysosomes: capsules of enzymes (usually hydrolytic) distributed fairly uniformly throughout the cytoplasm. Fat droplets and vesicles of material for excretion are usually apparent toward the apical region of the cell. The apical membrane possesses microvilli which serve to greatly increase its surface area. 1.6 Techniques used to study milk synthesis 1.6.1 Arteriovenous concentration diferences The arterial and veinous systems supplying the mammary gland (Figure 1.5) are readily accessible and may be easily cannulated to obtain blood samples for analysis. Differences in composition between arterial and venous blood give a measure of the constituents used in milk synthesis. The total amount of constituent used may be determined if the blood flow rate is known, which may be easily done by infusing a known volume of cold saline Figure 1.5 The blood vessel and nerve supply in the mammary glands of a cow. Circulatory system (arteries, white; veins, stippled): h, heart; a, abdominal aorta; pa, external pudic artery; pv, external pudic vein; s, subcutaneous abdominal vein; c, carotid artery; j, jugular vein. Nerves: 1, first lumbar nerve; 2, second lumbar nerve; 3, external spermatic nerve; 4, perineal nerve. A and V show blood sampling points for arteriovenous (AV) difference determinations (Mepham, 1987)
PRODUCTION AND UTILIZATION OF MILK solution into a vein and measuring the temperature of blood a little further downstream. The extent to which the blood temperature is reduced is inversely proportional to blood flow rate 1.6.2 Isotope studies Injection of radioactively labelled substrates, e.g. glucose, into the blood stream permits assessment of the milk constituents into which that substrate is incorporated. It may also be possible to study the intermediates through which biosynthesis proceedspo 1.6.3 Perfusion of isolated gland In many species, the entire gland is located such that it may be readily excised intact and undamaged. An artificial blood supply may be connected to cannulated veins and arteries(Figure 1.6): if desired, the blood supply may be passed through an artificial kidney. The entire mammary gland may Teat cannula Venous sa Oxygenator Figure 1.6 Diagram of circuit for perfusion of an isolated mammary gland of a guinea-pig, G, mammary gland; A, artery: v, veins( from Mepham, 1987)
PRODUCTION AND UTILIZATION OF MILK 9 solution into a vein and measuring the temperature of blood a little further downstream. The extent to which the blood temperature is reduced is inversely proportional to blood flow rate. 1.6.2 Isotope studies Injection of radioactively labelled substrates, e.g. glucose, into the bloodstream permits assessment of the milk constituents into which that substrate is incorporated. It may also be possible to study the intermediates through which biosynthesis proceeds. 1.6.3 Perfusion of isolated gland In many species, the entire gland is located such that it may be readily excised intact and undamaged. An artificial blood supply may be connected to cannulated veins and arteries (Figure 1.6); if desired, the blood supply may be passed through an artificial kidney. The entire mammary gland may thermometer Figure 1.6 Diagram of circuit for perfusion of an isolated mammary gland of a guinea-pig., G, mammary gland; A, artery; V, veins (from Mepham, 1987)
DAIRY CHEMISTRY AND BIOCHEMISTRY be maintained active and secreting milk for several hours; substrates may readily be added to the blood supply for study. 1. 6.4 Tissue slices The use of tissue slices is a standard technique in all aspects of metabolic biochemistry. The tissue is cut into slices, sufficiently thin to allow adequate rates of difusion in and out of the tissue. The slices are submerged in physiological saline to which substrates or other compounds may be added Changes in the composition of the slices and or incubation medium give some indication of metabolic activity, but extensive damage may be caused to the cells on slicing: the system is so artificial that data obtained by the tissue slice technique may not pertain to the physiological situation. How ever, the technique is widely used at least for introductory, exploratory experiments 1.6.5 Cell homogenates Cell homogenates are an extension of the tissue slice technique, in which the tissue is homogenized. As the tissue is completely disorganized, only individual biosynthetic reactions may be studied in such systems; usefu preliminary work may be done with homogenates 16.6 Tissue culture Tissue cultures are useful for preliminary or specific work but are in- complete In general, the specific constituents of milk are synthesized from molecules absorbed from the blood. These precursors are absorbed across the basal membrane but very little is known about the mechanism by which they are transported across the membrane. Since the membrane is rich in lipids, and precursors are mostly polar with poor solubility in lipid, it is unlikely that the precursors enter the cell by simple diffusion. It is likely, ommon with other tissues, that there are specialized carrier systems to transport small molecules across the membrane; such carriers are probably proteins The mammary gland of the mature lactating female of many species is by far the most metabolically active organ of the body. For many small mammals, the energy input required for the milk secreted in a single day may exceed that required to develop a whole litter in utero. a cow at peak lactation yielding 45 kg milk day- secretes approximately 2 kg lactose and 1.5 kg each of fat and protein per day. This compares with the daily weight gain for a beef animal of 1-1.5 kg day, 60-70% of which is water. In large
10 DAIRY CHEMISTRY AND BIOCHEMISTRY be maintained active and secreting milk for several hours; substrates may readily be added to the blood supply for study. 1.6.4 Tissue slices The use of tissue slices is a standard technique in all aspects of metabolic biochemistry. The tissue is cut into slices, sufficiently thin to allow adequate rates of diffusion in and out of the tissue. The slices are submerged in physiological saline to which substrates or other compounds may be added. Changes in the composition of the slices and/or incubation medium give some indication of metabolic activity, but extensive damage may be caused to the cells on slicing; the system is so artificial that data obtained by the tissue slice technique may not pertain to the physiological situation. However, the technique is widely used at least for introductory, exploratory experiments. 1.6.5 Cell homogenates Cell homogenates are an extension of the tissue slice technique, in which the tissue is homogenized. As the tissue is completely disorganized, only individual biosynthetic reactions may be studied in such systems; useful preliminary work may be done with homogenates. 1.6.6 Tissue culture Tissue cultures are useful for preliminary or specific work but are incomplete. In general, the specific constituents of milk are synthesized from small molecules absorbed from the blood. These precursors are absorbed across the basal membrane but very little is known about the mechanism by which they are transported across the membrane. Since the membrane is rich in lipids, and precursors are mostly polar with poor solubility in lipid, it is unlikely that the precursors enter the cell by simple diffusion. It is likely, in common with other tissues, that there are specialized carrier systems to transport small molecules across the membrane; such carriers are probably proteins. The mammary gland of the mature lactating female of many species is by far the most metabolically active organ of the body. For many small mammals, the energy input required for the milk secreted in a single day may exceed that required to develop a whole litter in utero. A cow at peak lactation yielding 45 kg milk day-' secretes approximately 2 kg lactose and 1.5 kg each of fat and protein per day. This compares with the daily weight gain for a beef animal of 1-1.5 kgday-', 60-70% of which is water. In large