A-Tocopherol in milk

AOAC official method 992.03. Vitamin E activity (all-rac-a-tocopherol) in milk-based infant formula. Liquid chromatographic method. In MP Bueno, ed. Official Methods of Analysis of AOAC International. 16th ed. Arlington, VA AOAC International, 1995, p. 50-4-50-5. 

Vitamin E is an effective scavenger of lipid peroxy radicals and is efficient at protecting unsaturated fatty acids against lipid peroxidation. The chemistry of vitamin E is rather complex as there are eight compounds, four tocopherols and four tocotrienols, which exhibit vitamin E activity. The relative bioactivity of the various compounds varies considerably, from 1.0 for a-tocopherol to 0.03 for 8-tocopherol. In milk, a-tocopherol accounts for virtually all of vitamin E, although very small amounts of (3-tocopherols and y-tocopherols are present. Also, the concentration of a-tocopherol in milk fat varies widely, with the level in samples of commercial butter ranging from 18 to 35 pg/g fat (MAFF, 1999). These concentrations equate to a low level of vitamin E bioactivity, 0.025 to 0.05 IU/g fat. 

Clearly, more research is required to clarify the somewhat confused picture regarding the role of enzymes in the oxidation of milk lipids. However, the key factor affecting the susceptibility of milk to oxidation appears to be its relative content and distribution of pro-oxidants and antioxidants. Bruhn and Franke (1971) reported that spontaneous oxidation is directly proportional to the copper content and inversely proportional to the a-tocopherol content of milk. Charmley et al. (1991) showed that intramuscular injection of cows with a-tocopherol may overcome a spontaneous oxidized flavor problem caused by low levels of a-tocopherol in milk. In general, milk from pasture-fed cows is less susceptible to oxidation due to a higher content of tocopherols than milk from cows given dry feed (Bruhn and Franke 1971 Urbach, 1989, 1990). 

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