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Characteristics of Protein Fractions Isolated from the Fat/Plasma Interface of Homogenized Milk¹

Department of Food Science, Michigan Agricultural Experiment Station, East Lansing

ABSTRACT

The technique employed to isolate the proteins of the fat-globule membrane of homogenized milk was based on a procedure described by Herald and Brunner (4) for isolating the membrane proteins of nonhomogenized milk. Since homogenization results in the alteration of the components constituting the fat/plasma interface in milk, their methods were modified and extended to include a fractionation for milk proteins other than the normal membrane-proteins.

The insoluble membrane-protein, constituting a portion of the normal membrane, was isolated from the homogenized fat-globule membrane. This reddish-brown fraction remained, at least in part, with the fat globule following homogenization.

The soluble membrane-protein of the nonhomogenized fat globule was recovered from preparations of the membrane of homogenized milk. A portion of this protein fraction appeared to be associated electrophoretically with the -casein oriented at the fat/plasma interface. The remaining portion was recovered from the membrane as an unassociated protein fraction.

An atypical casein fraction, in which the alpha component appeared to be associated with the normally occurring soluble membrane-protein (glycoprotein) wasisolated from the resurfaced fat-globule. The association occurred when milk washomogenized or when artificial systems containing whole casein, or -casein, solublemembrane-protein, and milk fat were homogenized. This phenomenon was characterizedby a trailing peak on the -casein peak in the electrophoretic patterns. When consideredas a part of the alpha peak, unusually high -ß-casein ratios were obtained.An accompanying decrease in the electrophoretic mobility of -casein was noted.

The presence of a heat-coagulable protein fraction, presumed to be whey proteins but not identified conclusively, was also noted.

¹ Journal Article 2575, Michigan Agricultural Experiment Station, East Lansing.