3 Milk lipids 3.1 Introduction The milks of all mammals contain lipids but the concentration varies widely between species from c. 2% to greater than 50%(Table 3. 1). The principal function of dietary lipids is to serve as a source of energy for the neonate and the fat content in milk largely reflects the energy requirements of the species, e.g. land animals indigenous to cold environments and marine mammals secrete high levels of lipids in their milks Milk lipids are also important 1. as a source of essential fatty acids (i.e. fatty acids which cannot be synthesized by higher animals, especially linoleic acid, C18: 2) and fat soluble vitamins(A, D, E, K); and 2. for the favour and rheological properties of dairy products and foods in which they are used Because of its wide range of fatty acids, the favour of milk fat is superior to that of other fats. In certain products and after certain processes, fatty acids serve as precursors of very flavourful compounds such as methyl ketones and lactones. Unfortunately, lipids also serve as precursors of compounds Table 3. 1 The fat content of milks from various species (g1-) Species Fat content S Fat conter Marmoset Musk-ox 32-206 Mink ed kangaroo Lemurs g From Christie(1995)
3 Milk lipids 3.1 Introduction The milks of all mammals contain lipids but the concentration varies widely between species from c. 2% to greater than 50% (Table 3.1). The principal function of dietary lipids is to serve as a source of energy for the neonate and the fat content in milk largely reflects the energy requirements of the species, e.g. land animals indigenous to cold environments and marine mammals secrete high levels of lipids in their milks. Milk lipids are also important: 1. as a source of essential fatty acids (i.e. fatty acids which cannot be synthesized by higher animals, especially linoleic acid, &) and fatsoluble vitamins (A, D, E, K); and 2. for the flavour and rheological properties of dairy products and foods in which they are used. Because of its wide range of fatty acids, the flavour of milk fat is superior to that of other fats. In certain products and after certain processes, fatty acids serve as precursors of very flavourful compounds such as methyl ketones and lactones. Unfortunately, lipids also serve as precursors of compounds Table 3.1 The fat content of milks from various species (g I-') Species Fat content Species Fat content cow Buffalo Sheep Goat Dall-sheep Moose Antelope Elephant Human Horse Monkeys Lemurs Pig Musk-ox 33-47 47 40-99 41 -45 109 32-206 39-105 93 85-190 38 19 10-51 8-33 68 Marmoset Rabbit Guinea-pig Snowshoe hare Muskrat Mink Chinchilla Rat Red kangaroo Dolphin Manatee Pygmy sperm whale Harp seal Bear (four species) 77 183 39 71 110 134 117 103 9-119 62-330 55-215 502- 5 32 108-331 153 From Christie (1995)
DAIRY CHEMISTRY AND BIOCHEMISTRY that cause off-flavour defects(hydrolytic and oxidative rancidity) and as solvents for compounds in the environment which may cause off-flavour For many years, the economic value of milk was based mainly or totally on its fat content, which is still true in some cases. This practice was satisfactory when milk was used mainly or solely for butter production Possibly, the origin of paying for milk on the basis of its fat content, apart from its value for butter production, lies in the fact that relatively simple quantitative analytical methods were developed for fat earlier than for protein or lactose. Because of its economic value, there has long been commercial pressure to increase the yield of milk fat per cow by nutritional or genetic means. To facilitate the reader, the nomenclature, structure and properties of t principal fatty acids and of the principal lipid classes are summarized in Appendices 3A, 3B and 3C. The structure and properties of the fat-soluble vitamins, A, D, E and K, are discussed in Chapter 6 3.2 Factors that affect the fat content of bovine milk Bovine milk typically contains c. 3.5% fat but the level varies widely, depending on several factors, including: breed, individuality of the animal, stage of lactation, season, nutritional status, type of feed, health and age of the animal, interval between milkings and the point during milking when the sample is taken Of the common European breeds, milk from Jersey cows contains the ighest level of fat and that from Holstein/ Friesians the lowest( Figure 3.1 The data in Figure 3.1 also show the very wide range of fat content in individual-cow samples The fat content of milk decreases during the first 4-6 weeks after parturition and then increases steadily throughout the remainder of lacta- tion,especially toward the end( Figure 3. 2). For any particular population, fat content is highest in winter and lowest in summer, due partly to the effect of environmental temperature. Production of creamery(manufacturing) milk in Ireland, New Zealand and parts of Australia is very seasonal lactational, seasonal and possibly nutritional effects coincide, leading to large seasonal changes in the fat content of milk(Figure 3. 3), and also in he levels of protein and lactose For any individual animal, fat content decreases slightly dr success- ive lactations, by c. 0.2% over a typical productive lifetime(about five lactations). In practice, this factor usually has no overall effect on the fat content of a bulk milk supply because herds normally include cows of various ages. the concentration of fat (and of all other milk-specific constituents)decreases markedly on mastitic infection due to impaired
68 DAIRY CHEMISTRY AND BIOCHEMISTRY that cause off-flavour defects (hydrolytic and oxidative rancidity) and as solvents for compounds in the environment which may cause off-flavours. For many years, the economic value of milk was based mainly or totally on its fat content, which is still true in some cases. This practice was satisfactory when milk was used mainly or solely for butter production. Possibly, the origin of paying for milk on the basis of its fat content, apart from its value for butter production, lies in the fact that relatively simple quantitative analytical methods were developed for fat earlier than for protein or lactose. Because of its economic value, there has long been commercial pressure to increase the yield of milk fat per cow by nutritional or genetic means. To facilitate the reader, the nomenclature, structure and properties of the principal fatty acids and of the principal lipid classes are summarized in Appendices 3A, 3B and 3C. The structure and properties of the fat-soluble vitamins, A, D, E and K, are discussed in Chapter 6. 3.2 Factors that affect the fat content of bovine milk Bovine milk typically contains c. 3.5% fat but the level varies widely, depending on several factors. including: breed, individuality of the animal, stage of lactation, season, nutritional status, type of feed, health and age of the animal, interval between milkings and the point during milking when the sample is taken. Of the common European breeds, milk from Jersey cows contains the highest level of fat and that from Holstein/Friesians the lowest (Figure 3.1). The data in Figure 3.1 also show the very wide range of fat content in individual-cow samples. The fat content of milk decreases during the first 4-6 weeks after parturition and then increases steadily throughout the remainder of lactation, especially toward the end (Figure 3.2). For any particular population, fat content is highest in winter and lowest in summer, due partly to the effect of environmental temperature. Production of creamery (manufacturing) milk in Ireland, New Zealand and parts of Australia is very seasonal; lactational, seasonal and possibly nutritional effects coincide, leading to large seasonal changes in the fat content of milk (Figure 3.3), and also in the levels of protein and lactose. For any individual animal, fat content decreases slightly during successive lactations, by c. 0.2% over a typical productive lifetime (about five lactations). In practice, this factor usually has no overall effect on the fat content of a bulk milk supply because herds normally include cows of various ages. The concentration of fat (and of all other milk-specific constituents) decreases markedly on mastitic infection due to impaired
MILK LIPIDS Holstein Ayrshire Figure 3. 1 Range of fat content in the milk of individual cows of four breeds( from Jenness and Patton, 1959) synthesizing ability of the mammary tissue; the effect is clear-cut in the case of clinical mastitis but is less so for subclinical infection Milk yield is reduced by underfeeding but the concentration of fat usuall increases, with little effect on the amount of fat produced. Diets low in roughage have a marked depressing effect on the fat content of milk, with little effect on milk yield. Ruminants synthesize milk fat mainly from carbohydrate-derived precursors; addition of fat to the diet usually cause slight increases in the yield of both milk and fat, with little effect on fat content of milk. Feeding of some fish oils (e.g. cod liver oil, in an effort to ease the concentrations of vitamins A and d in milk) has a very marked (c. 25%)depressing effect on the fat content of milk, apparently due to the high level of polyunsaturated fatty acids( the effect is eliminated by hydr genation), although oils from some fish species do not cause this effect
I MILK LIPIDS 35 - 30 - B f - 25- 0 % 3 - S s 20- r 0 15- 2 10 - 5- 69 Percentage fat Figure 3.1 Range of fat content in the milk of individual cows of four breeds (from Jenness and Patton, 1959). synthesizing ability of the mammary tissue; the effect is clear-cut in the case of clinical mastitis but is less so for subclinical infection. Milk yield is reduced by underfeeding but the concentration of fat usually increases, with little effect on the amount of fat produced. Diets low in roughage have a marked depressing effect on the fat content of milk, with little effect on milk yield. Ruminants synthesize milk fat mainly from carbohydrate-derived precursors; addition of fat to the diet usually causes slight increases in the yield of both milk and fat, with little effect on fat content of milk. Feeding of some fish oils (e.g. cod liver oil, in an effort to increase the concentrations of vitamins A and D in milk) has a very marked (c. 25%) depressing effect on the fat content of milk, apparently due to the high level of polyunsaturated fatty acids (the effect is eliminated by hydrogenation), although oils from some fish species do not cause this effect
DAIRY CHEMISTRY AND BIOCHEMISTRY 50 10 Week of lactation Figure 3.2 Typical changes in the concentrations of fat(O), protein() and lactose(O)in bovine milk during lactation 4.6 兰E8 3.4 S O N D Ire 3.3 Seaso in the fat content of bovine milk in some Eu s(●, United Kingdom(口, france, Germany(△), reland(▲) om an foras tal 1981)
70 DAIRY CHEMISTRY AND BIOCHEMISTRY 5.0 - 0 L. 2 4.0 3.0 0 10 20 30 40 50 Week of lactation Figure 3.2 Typical changes in the concentrations of fat (O), protein (m) and bovine milk during lactation. 4.6 - 4.4 - 4.2 - 4.0 - 3.8 - 3.6 - 3.4 - lactose (0) in JFMAMJJASOND Month Figure 3.3 Seasonal changes in the fat content of bovine milk in some European countries: (Denmark (O), Netherlands (O), United Kingdom (O), France (U), Germany (A), Ireland (A) (From An Foras Taluntais, 1981.)
MILK LIPIDS The quarters of a cows udder are anatomically separate and secrete milk of markedly different composition. The fat content of milk increas s continu ously throughout the milking process while the concentrations of the partially trapped in the alveoli and their passage is hinde co ppear to be various non-fat constituents show no change; fat globules a cow Is incompletely milked, the fat content of the milk obtained at that milking will be reduced; the trapped 'fat will be expressed at the subsequent milking, giving an artificially high value for fat content If the intervals between milkings are unequal (as they usually are in commercial farming), the yield of milk is higher and its fat content lower after the longer interval; the content of non-fat solids is not infuenced by milking interval 3.3 Classes of lipids in milk Triacylglycerols(triglycerides) represent 97-98% of the total lipids in the milks of most species(Table 3. 2). The diglycerides probably represent incompletely synthesized lipids in most cases, although the value for the ra probably also includes partially hydrolysed triglycerides, as indicated by the high concentration of free fatty acids, suggesting damage to the milk fat globule membrane (MFGM) during milking and storage. The very high level of phospholipids in mink milk probably indicates the presence of mammary cell membranes Although phospholipids represent less than 1% of total lipid, they play a particularly important role, being present mainly in the MFGM and other membraneous material in milk. The principal phospholipids are phos- phatidylcholine, phosphatidylethanolamine and sphingomyelin(Table 3. 3) Trace amounts of other polar lipids, including ceramides, cerobrosides and gangliosides, are also present. Phospholipids represent a considerable pro- portion of the total lipid of buttermilk and skim milk(table 3. 4), reflecting acids 3.2 Composition of individual simple lipids and total phospholipids in milks of some Lipid class Bufalo Human Rat Mink 986 98.2 875 81.3 Free fatty acids 00270 Phospholipids 0.5 0.26 16 07 15.3 From Christie(1995). T, Trace
MILK LIPIDS 71 The quarters of a cow’s udder are anatomically separate and secrete milk of markedly different composition. The fat content of milk increases continuously throughout the milking process while the concentrations of the various non-fat constituents show no change; fat globules appear to be partially trapped in the alveoli and their passage is hindered. If a cow is incompletely milked, the fat content of the milk obtained at that milking will be reduced; the ‘trapped’ fat will be expressed at the subsequent milking, giving an artificially high value for fat content. If the intervals between milkings are unequal (as they usually are in commercial farming), the yield of milk is higher and its fat content lower after the longer interval; the content of non-fat solids is not influenced by milking interval. 3.3 Classes of lipids in milk Triacylglycerols (triglycerides) represent 97-98% of the total lipids in the milks of most species (Table 3.2). The diglycerides probably represent incompletely synthesized lipids in most cases, although the value for the rat probably also includes partially hydrolysed triglycerides, as indicated by the high concentration of free fatty acids, suggesting damage to the milk fat globule membrane (MFGM) during milking and storage. The very high level of phospholipids in mink milk probably indicates the presence of mammary cell membranes. Although phospholipids represent less than 1% of total lipid, they play a particularly important role, being present mainly in the MFGM and other membraneous material in milk. The principal phospholipids are phosphatidylcholine, phosphatidylethanolamine and sphingomyelin (Table 3.3). Trace amounts of other polar lipids, including ceramides, cerobrosides and gangliosides, are also present. Phospholipids represent a considerable proportion of the total lipid of buttermilk and skim milk (Table 3.4), reflecting Table 3.2 species (weight YO of the total lipids) Composition of individual simple lipids and total phospholipids in milks of some Lipid class Triacylgl ycerols Diacylglycerols Monoacylgl ycerols Cholesteryl esters Cholesterol Free fatty acids Phospholipids cow Buffalo Human Pig Rat Mink 97.5 0.36 0.027 T 0.31 0.027 0.6 98.6 98.2 0.7 T 0.1 T 0.3 0.25 0.5 0.4 0.5 0.26 96.8 0.7 0.1 0.06 0.6 0.2 1.6 87.5 2.9 0.4 1.6 3.1 0.7 - 81.3 1.7 T T T 1.3 15.3 From Christie (1995). T, Trace