A comparative study of the lipids of globule membrane and fat core and of the milk serum of cows.

Nine samples of fresh raw cow's milk were separated into fat globules and milk serum by centrifugation. After destabilization by freezing and thawing, the milk fat globules were resolved into membranes and fat cores. The lipid composition of these structures was compared to that of the surrounding milk serum. Of the total milk fat, 95-98% was in the fat cores, 0.5-1% in the globule membranes and the rest (1.5-4%) in the milk serum. The fat cores contained 88-93% triglyceride, 5.2-9.8% diglyceride, 1.5-7.3% free fatty acid and 0.2-0.4% cholesterol, but no phospholipid. The lipids of the membrane contained 21-44% phospholipid, made up of about equal proportions of phosphatidyl ethanolamine, phosphatidyl choline, and sphingomyelin. The other lipids of the membrane (56-79%) consisted of 83-88% triglyceride, 5.1-10.7% diglyceride, 1-5.1% free fatty acid and 0.4-1.9% cholesterol. The milk serum contained 30-45% phospholipid divided about equally among phosphatidyl ethanolamine, phosphatidyl choline and sphingomyelin. The rest (55-70%) of the milk serum lipids was made up of 71-83% triglycerides, 4.3-10.1% diglycerides, 8.7-15.7% free fatty acids, and 1.2-8.4% cholesterol. Corresponding phospholipid classes of milk serum and globule membranes had identical fatty acid compositions. The triglycerides and diglycerides of the globule membranes possessed increased proportions of palmitic and stearic acids in comparison to the glycerides of the fat cores.