Composition of milk fat globules with increased linoleic acid

Comparisons were made of the composition and distribution of the lipids in fat globules from conventional milks and polyunsaturated milks produced by cows fed protected lipid supplement. Washed creams were prepared from the milks of three individual cows fed a conventional ration, and three fed a protected sunflower-soybean supplement rich in linoleic acid. The washed creams were fractionated by treatment with sodium deoxycholate and centrifugation. Each washed cream and four fractions (designated as outer globule membrane, inner membrane, pellet, and globule core) were analyzed for protein, lipid, phospholipid, cholesterol, tocopherols, carotenoids, and fatty acid composition. The outer and inner membrane fractions were further fractionated into neutral and polar (phospholipid) lipid classes by thin layer chromatography. For both types of washed cream the approximate weight distribution of total solids was: outer membrane, 1%; inner membrane, 2%; pellet, 0.1%; and core, 96%. The percentages of protein, phospholipid, cholesterol, and carotenoids were all lower in the polyunsaturated than in the conventional creams. In the polyunsaturated creams, the percentages of both saturated and unsaturated C₁₈ acids were higher, and of acids of C₁₆ and shorter chain length lower, than in the conventional creams. The phospholipids in the outer and inner membranes from the polyunsaturated milks had larger proportions of linoleic acid than did the phospholipids from the conventional milks. However, this increase in unsaturation was less than that of the core neutral lipids. Pancreatic lipase hydrolysis of the core fractions showed that the increased linoleic acid was introduced preferentially at the 2-position of the triglycerides. In general, the observed changes in physical properties and in susceptibility of polyunsaturated milk to the development of oxidized flavor are consistent with the differences in the relative proportions of the various classes of lipids in the conventional and polyunsaturated milks. Data from thesis of D.H. Bianco submitted in partial fulfillment of the requirements for the M.S. Degree in Food Science, University of California, Davis. Presented in part at the AOCS Meeting, New Orleans, April, 1976.