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Structure and Synthesis of Milk Fat. VI. Unity of the Phospholipids in Milk^{1, 2,}

Lipids Laboratory, Agricultural Experiment Station and Department of Dairy Science, The Pennsylvania State University, University Park

ABSTRACT

While recent findings have emphasized the complexity of milk phospholipids (P-lipids), as revealed by the classes present and their fatty acid compositions, results of this study indicate that these P-lipids occur in units of relatively homogeneous lipid composition. Disposition of the P-lipids was studied using cream separation, churning, and ultra-centrifugation to fractionate the lipids in milk. The classes of P-lipids present in the various milk fractions were revealed by thin-layer chromatography (TLC), and the fatty acid composition of total P-lipids in the fractions was determined by gas chromatography (GC).

The results demonstrated the same classes of P-lipids (mainly cephalin, lecithin, and sphingomyelin) in similar proportions with much the same total fatty acid composition in milk, skimmilk, cream, and buttermilk. This unity also was detected in fractions derived by freezing and sawing tubes of milk or buttermilk that had been centrifuged in a manner (25,000 x g for 6 hr) to yield differentiation of the P-lipids by sedimentation. In both milk and buttermilk the bulk of the P-lipids tended to deposit with a particle that settled mainly in the upper level of the casein layer.

The evidence is consistent with the concept of a membrane of fairly homogeneous lipid composition in milk serum and on the surface of milk fat globules. Current evidence suggests that this membrane is derived at least in part from the cytoplasmic membrane in the apical region of the secreting cell.

Some affinity of unsaturated lipids for protein phase was noted and may be relevant to lipid oxidation and enzymatic phenomena.

¹ Authorized for publication 3–9, 1964, as Paper No. 2879 in the Journal Series of the Pennsylvania Agricultural Experiment Station.

² Supported in part by the U. S. Public Health Service (H 3632).