Tocopherols, retinol, beta-carotene and fatty acids in fat globule membrane and fat globule core in cows' milk.

Milk fat globule membranes (MFGM) were isolated from milk from cows injected intraperitoneally with dl-alpha-tocopherol acetate. The fatty acid composition and content, and the contents of tocopherols, retinol and beta-carotene were determined and compared with the composition and content in the original cream sample. Intraperitoneal injection of 10 g dl-alpha-tocopherol acetate elevated the alpha-tocopherol content in the milk fat from 13-30 to 50-70 micrograms alpha-tocopherol/g milk fat 2-3 d after injection. The increase depended on the alpha-tocopherol status of the cow prior to injection. The concentrations of retinol and beta-carotene in the milk fat were unchanged after the alpha-tocopherol injections. MFGM fatty acids made up 18-27 g/kg total fatty acids in the milk fat. However, the proportions of monounsaturated and polyunsaturated fatty acids were higher in MFGM than in total milk fat, while the proportion of saturated fatty acids was lower in MFGM (P < 0.001). Thus, the long-chain polyunsaturated fatty acids in MFGM constituted approximately 40-70 g/kg total milk fat. alpha-Tocopherol was the only fat-soluble vitamin detected in MFGM; gamma-tocopherol, retinol and beta-carotene were detected only in the cream. A significant relationship between alpha-tocopherol contents in cream and MFGM was found: (alpha-tocopherol in MFGM fatty acids) = -1652 x (1/ln(alpha-tocopherol in cream fatty acids)3) + 97 (r = 0.857, P < 0.001) when the contents of alpha-tocopherol were expressed as microgram/g fatty acid. According to this equation the upper limit for incorporation of alpha-tocopherol into MFGM is 97 +/- 5 micrograms/g MFGM fatty acids. Thus, under normal farming conditions the alpha-tocopherol content will be highest in MFGM. However, in cream with an alpha-tocopherol concentration < 15 micrograms/g cream fatty acids the concentration in MFGM will be lower and furthermore will decrease rapidly. The possible importance of this relationship in relation to the oxidation of milk fat is discussed.