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Characteristics of the Fat Globule Membrane of Cow's Milk^1,2,

Department of Food Science, Michigan State University, East Lansing 48823

ABSTRACT

Fragmented fat globule membrane material, separated from thrice-washed fat globules, was ultracentrifugally classified into three fractions of 7,500 S (> 7,500), 230 S (230 to 7,500), and 35 S (35 to 230) sedimentation characteristics. Their composition was from 82.5% protein and 17.5% lipid for the 7,500 S fraction to 44.7% protein and 55.3% lipid in the 35 S freaction. The proportion of polar lipids in the lipid moieties decreased with increasing lipid content.

The protein moieties contained approximately 14% nitrogen and varying amounts of hexose (2.82 to 4.15%), hexosamine (2.48 to 4.17%), and sialic acid (1.29 to 1.77%). These data indicate an increasing proportion of glycoproteins in successively lighter membrane fractions. No appreciable differences in amino acid composition were noted for the protein moieties.

Electron micrographs of the membrane fractions revealed differences in the size and structure of the particulate material. A proposed model for the fat globule membrane, as it exists in milk, provides for a protein or lipid-protein matrix to which lipoprotein micelles are adsorbed.

¹ Published with the approval of the Michigan Agricultural Experiment Station as Journal Article no. 4649.

² Supported in part by Grant no. R01-U1-00344-01, National Center for Urban and Industrial Health, USPHS, Cincinnati.

³ Present address: Department of Biology, Virginia Military Institute, Lexington.