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Gross Structure of the Fat Globule Membrane of Cow's Milk. II. Distribution of Amino Acids, Hexosamines, Ribonucleic Acid, Copper and Iron

Department of Food Science and Industries, University of Wisconsin, Madison

ABSTRACT

Washed cream was stirred at 25 C and centrifuged to separate the cream and aqueous layers. Water was added each time as the process was repeated until the cream phase inverted. The butter was melted and the aqueous portion isolated. All aqueous portions were centrifuged at 50,000 x g for 90 min at 4 C. Each supernatant pH was adjusted to aggregate the lipoproteins. The outer layer of the membrane desorbed from the cream consisted of reddish-brown pellets high in protein, low in lipid, and a supernatant aggregate low in protein, high in lipid.

The inner layer consisted of a pellet high in protein, low in lipid and a supernatant aggregate low in protein, high in lipid. An interfacial portion was high in protein, low in lipid. Amino acid compositions of the membrane fractions were uniform and indicated soluble and insoluble proteins in all fractions. Hexosamines were distributed throughout the membrane fractions, indicating glycoproteins in all fractions. Ribonucleic acid was distributed throughout the membrane, with the highest level in the inner layer supernatant aggregate. Iron was highest in the outer membrane layers, while copper was more evenly distributed.