Structure and Synthesis of Milk Fat. VI. Unity of the Phospholipids in Milk

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Abstract
The same classes of P-lipids (mainly cephalin, lecithin and sphingo-myelin) were demonstrated 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 thawing tubes of milk or buttermilk centrifuged (25,000 X g for 6 hr.) so as 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. This is consistent with the concept of a membrane of fairly homogeneous lipid composition in milk serum and on the surface of milk fat globules. This membrane is probably derived at least in part from the cytoplasmic membrane in the apical region of the secreting cell. Some affinity of unsaturated lipids for the protein phase may be relevant to lipid oxidation and enzymatic phenomena.