MILK LIPIDS peroxides, e. g. H2O2 or benzoyl peroxide, or masked, e.g. with chlorophyll or titanium oxide Milk contains significant concentrations of fat-soluble vitamins(Table 3.5, Figure 3. 4)and milk and dairy products make a significant contribution to the dietary requirements for these vitamins in Western countries. The actual form of the fat-soluble vitamins in milk appears to be uncertain and their concentration varies widely with breed of animal, feed and stage of lactation, e.g. the vitamin A activity of colostrum is c. 30 times higher than that of mature milk Several prostaglandins occur in milk but it is not known whether they play a physiological role; they may not survive storage and processing in a biologically active form. Human milk contains prostaglandins E and F at concentrations 100-fold higher than human plasma and these may have a hysiological function, e.g. gut motility. 3.4 Fatty acid profile of milk lipids Milk fats, especially ruminant fats, contain a very wide range of fatty acids more than 400 and 184 distinct acids have been detected in bovine and human milk fats, respectively( Christie, 1995). However, the vast majority of these occur at only trace concentrations. The concentrations of the principal fatty acids in milk fats from a range of species are shown in table 3.6 Notable features of the fatty acid profiles of milk lipids include 1. Ruminant milk fats contain a high level of butanoic acid (C4: o)and other short-chain fatty acids. The method of expressing the results in table 3. 6 (% w/w)under-represents the proportion of short-chain acids-if ex- pressed as mol % butanoic acid represents c. 10% of all fatty acids(up to 15% in some samples), i. e. there could be a butyrate residue in c. 30% of all triglyceride molecules. The high concentration of butyric(butanoic) acid in ruminant milk fats arises from the direct incorporation of B-hydroxybutyrate (which is produced by micro-organisms in the rumen from carbohydrate and transported via the blood to the mammary gland where it is reduced to butanoic acid). Non-ruminant milk fats contain no butanoic or other short-chain acids the low concentrations of butyrate in milk fats of some monkeys and the brown bear require confirmation The concentration of butanoic acid in milk fat is the principle of the of butter with other fats i.e. Reichert Meissl and Polenski numbers which are measures of the volatile water-soluble and volatile water- insoluble fatty acids, respectively Short-chain fatty acids have strong, characteristic flavours and aromas. When these acids are released by the action of lipases in milk orMILK LIPIDS 75 peroxides, e.g. H,O, or benzoyl peroxide, or masked, e.g. with chlorophyll or titanium oxide). Milk contains significant concentrations of fat-soluble vitamins (Table 3.5, Figure 3.4) and milk and dairy products make a significant contribution to the dietary requirements for these vitamins in Western countries. The actual form of the fat-soluble vitamins in milk appears to be uncertain and their concentration varies widely with breed of animal, feed and stage of lactation, e.g. the vitamin A activity of colostrum is c. 30 times higher than that of mature milk. Several prostaglandins occur in milk but it is not known whether they play a physiological role; they may not survive storage and processing in a biologically active form. Human milk contains prostaglandins E and F at concentrations 100-fold higher than human plasma and these may have a physiological function, e.g. gut motility. 3.4 Fatty acid profile of milk lipids Milk fats, especially ruminant fats, contain a very wide range of fatty acids: more than 400 and 184 distinct acids have been detected in bovine and human milk fats, respectively (Christie, 1995). However, the vast majority of these occur at only trace concentrations. The concentrations of the principal fatty acids in milk fats from a range of species are shown in Table 3.6. Notable features of the fatty acid profiles of milk lipids include: 1. Ruminant milk fats contain a high level of butanoic acid (C4:o) and other short-chain fatty acids. The method of expressing the results in Table 3.6 (Yo, w/w) under-represents the proportion of short-chain acids - if ex￾pressed as mol %, butanoic acid represents c. 10% of all fatty acids (up to 15% in some samples), i.e. there could be a butyrate residue in c. 30% of all triglyceride molecules. The high concentration of butyric (butanoic) acid in ruminant milk fats arises from the direct incorporation of P-hydroxybutyrate (which is produced by micro-organisms in the rumen from carbohydrate and transported via the blood to the mammary gland where it is reduced to butanoic acid). Non-ruminant milk fats contain no butanoic or other short-chain acids; the low concentrations of butyrate in milk fats of some monkeys and the brown bear require confirmation. The concentration of butanoic acid in milk fat is the principle of the widely used criterion for the detection and quantitation of adulteration of butter with other fats, i.e. Reichert Meissl and Polenski numbers, which are measures of the volatile water-soluble and volatile water￾insoluble fatty acids, respectively. Short-chain fatty acids have strong, characteristic flavours and aromas. When these acids are released by the action of lipases in milk or