MILK LIPIDS dairy products, they impart strong flavours which are undesirable in milk or butter (they cause hydrolytic rancidity) but they contribute to the desirable favour of some cheeses, e. g. blue, Romano, Parmesan. 2. Ruminant milk fats contain low levels of polyunsaturated fatty acid (PUFAs)in comparison with monogastric milk fats. This is because a high proportion of the fatty acids in monogastric milk fats are derived from dietary lipids (following digestion and absorption) via blood Unsaturated fatty acids in the diet of ruminants (grass contains con- siderable levels of PUFAs )are hydrogenated by rumen micro-organisms unless protected by encapsulation (section 3. 16. 1 ). The low level of PUFAS in bovine milk fat is considered to be nutritionally undesir ble 3. The milk fats from marine mammals contain high levels of long-chain highly unsaturated fatty acids, presumably reflecting the requirement that the lipids of these species remain liquid at the low temperatures of their environments 4. Ruminant milk fats are also rich in medium-chain fatty acids. These are synthesized in the mammary gland via the usual malonyl CoA pathway (section 3. 5)and are released from the synthesizing enzyme complex by thioacylases; presumably, the higher levels of medium chain acids in ruminant milk fats compared with those of monogastric animals refect higher thioacylase activity in the mammary tissue of the former 5. The fatty acid profile of bovine milk fat shows a marked seasonal pattern, especially when cows are fed on pasture in summer. Data for Irish milk fat are shown in Figure 3. 5; the changes are particularly marked for C4:0 C1s:0 and C18:. These changes affect the Reichert Meissl, Polenski and iodine(a measure of unsaturation)(Figure 3. 6)numbers and the melting point and hardness (spreadability) of butter made from these milks winter butter, with low levels of C4: o and Ci8: and a high level of C16- is much harder than summer butter( Figure 3.7) 6. Unsaturated fatty acids may occur as cis or trans isomers; trans isomers which have higher melting points than the corresponding cis isomers, are considered to be nutritionally undesirable. bovine milk fat contains a low level (5%)of trans fatty acids in comparison with chemically hydro- genated(hardened) vegetable oils, in which the value may be 50% due to non-stereospecific hydrogenation Bovine milk fat contains low concentrations of keto and hydroxy acids (each at c. 0.3% of total fatty acids ) The keto acids may have the carbonyl group(C=O) at various positions. The 3-keto acids give rise to methyl ketones(R-C-CH3)on heating(high concentrations of methyl ketones are produced in blue cheeses through the oxidative activity of Penicillium roqueforti). The position of the hydroxy group on the hydroxy acids also
MILK LIPIDS 77 dairy products, they impart strong flavours which are undesirable in milk or butter (they cause hydrolytic rancidity) but they contribute to the desirable flavour of some cheeses, e.g. Blue, Romano, Parmesan. 2. Ruminant milk fats contain low levels of polyunsaturated fatty acids (PUFAs) in comparison with monogastric milk fats. This is because a high proportion of the fatty acids in monogastric milk fats are derived from dietary lipids (following digestion and absorption) via blood. Unsaturated fatty acids in the diet of ruminants (grass contains considerable levels of PUFAs) are hydrogenated by rumen micro-organisms unless protected by encapsulation (section 3.16.1). The low level of PUFAs in bovine milk fat is considered to be nutritionally undesirable. 3. The milk fats from marine mammals contain high levels of long-chain, highly unsaturated fatty acids, presumably reflecting the requirement that the lipids of these species remain liquid at the low temperatures of their environments. 4. Ruminant milk fats are also rich in medium-chain fatty acids. These are synthesized in the mammary gland via the usual malonyl CoA pathway (section 3.5) and are released from the synthesizing enzyme complex by thioacylases; presumably, the higher levels of medium chain acids in ruminant milk fats compared with those of monogastric animals reflect higher thioacylase activity in the mammary tissue of the former. 5. The fatty acid profile of bovine milk fat shows a marked seasonal pattern, especially when cows are fed on pasture in summer. Data for Irish milk fat are shown in Figure 3.5; the changes are particularly marked for C,:, C,,:, and c18:I. These changes affect the Reichert Meissl, Polenski and iodine (a measure of unsaturation) (Figure 3.6) numbers and the melting point and hardness (spreadability) of butter made from these milks: winter butter, with low levels of C4:o and c18:] and a high level of C,,:, is much harder than summer butter (Figure 3.7). 6. Unsaturated fatty acids may occur as cis or trans isomers; trans isomers, which have higher melting points than the corresponding cis isomers, are considered to be nutritionally undesirable. Bovine milk fat contains a low level (5%) of trans fatty acids in comparison with chemically hydrogenated (hardened) vegetable oils, in which the value may be 50% due to non-stereospecific hydrogenation. Bovine milk fat contains low concentrations of keto and hydroxy acids (each at c. 0.3% of total fatty acids). The keto acids may have the carbonyl group (C=O) at various positions. The 3-keto acids give rise to methyl ketones (R-C-CH,) on heating (high concentrations of methyl ketones are produced in blue cheeses through the oxidative activity of Penicilliurn roqueforti). The position of the hydroxy group on the hydroxy acids also 0 //
AIRY CHEMISTRY AND BIOCHEMISTRY D a121 圣@品 SONDJ FMAMJ A Month 200 OND J F ASO Figure 3.5 Seasonal changes in the concentration of individual fatty acids in Irish bovine milk fat.(a)C40(▲),C6(,C8a(口,C●,C120(oh(b)C40O,C1so(●;(c)C16o(● CIs1(O). From Cullinane et al., 1984a)
78 DAIRY CHEMISTRY AND BIOCHEMISTRY 4- 3- 2- . . . I I . I-,.,.,.,. MJJASONDJFMA 13 i 12- 0 0 8 3 11- g loW .- cd LL 9- MJJASONDJFMAMJJA Month (b)
MILK LIPIDS Figure 3.6 Seasonal changes in the iodine number of Irish bovine milk fat(from Cullinane et a,1984a) 」 A SO J F MA MJ J A S C Figure 3.7 Seasonal variations in the mean firmness of irish butter at 4C(O or 15C(o) rom Cullinane et al., 1984b)
MILK LIPIDS 79 42 - h Y d? 40- 2 M 0 !l 38- - on v 8 36- $ 2 5 0 34 - MJJASONDJF Month Figure 3.6 Seasonal changes in the iodine number of Irish bovine milk fat (from Cullinane et al., 1984a). MJJASONDJFMAMJJASO Month Figure 3.7 Seasonal variations in the mean firmness of Irish butter at 4°C (@) or 15°C (0) (from Cullinane et al., 1984b)