Cholesteryl Esters of Cows’ Milk

Milk was obtained on postpartum d 3 (colostrum) and 7, 42, and 180 from 12 Holstein cows. Cholesteryl ester fatty acids were determined by gas-liquid chromatography after isolation by column and thin-layer chromatography. As lactation progressed, cholesteryl ester declined from 1.5 mg/dl in colostrum to .7 mg/dl in mature milk. Proportions of 14:0 and 18:1 fatty acids were higher in colostrum than in mature milk cholesteryl esters (28% of total fatty acids versus 19%), whereas 18:2 was significantly lower in colostrum than in mature milk. Fatty acids esterified with cholesterol in percent (mature milk, 42 d) were 12:0, .20; 14:0, 2.29; 16:0, 23.09; 16:1, 8.80; 18:0, 10.55; 18:1, 17.08; 18:2, 27.09; 18:3, 5.57; 20:3, .72; 20:4, 1.40. In mature milk cholesteryl esters, 62% of the fatty acids were unsaturated. The greatest difference occurred in 18:2 content, which was 27.1% in cholesteryl esters and only 2.6% of total fatty acids in milk. Results suggest that unsaturated fatty acids are associated preferentially with the cholesteryl ester fraction and that the fatty acid composition of cholesteryl esters of cows’ milk differs significantly from the composition of total milk lipid.