Effect of Heat upon Electrophoresis and Ultracentrifugal Sedimentation Properties of Skimmilk Protein Fractions

Skimmilk was centrifuged at 11,700 × g, 46,900 × g, and 92,000 × g and the supernatant and sediment fractions were incorporated into phosphate buffer or treated with potassium oxalate to remove calcium and disperse the proteins.They were examined by free-bounda~a" electrophoresis and ultracentrifugal sedimentation.Heating skimmilk (88 C--10 rain) had little effect upon electrophoretie properties of the three protein sediment fractions, but reduced the magnitude of the differences in electrophoretic composition of the supernatant fractions.These results indicate that interaction of a-casein with fl-lactoglobulin is favored among the smaller skimmilk protein particles.Skimmilk and its sediment and supernatant fractions all exhibited two ultracentrifugal sedimentation components at pH 6.98 and 25 C. Oxalate treatment of unheated skimmilk increased Component I front 28 to 57% of the pattern area and reduced its sedimentation constant from 12.7 to 6.9 S. Heating skimmilk increased Component I from 28 to 83% of the pattern area and reduced its sedimentation constant to 6.7 S. The sedimentation patterns of heated skimmilk protein fractions were relatively unaffected by the oxalate treatment.No evidence of 25-48 S casein-serum protein complex particles was obtained in any of the heated skimmilk fractions.The protein complex formed by heating casein and serum protein mixtures in buffer has a reported sedimentation constant in the range of 25-48 S (6, 8, 14).There is reason to suspect that the presence of calcium or colloidal calcium phosphate, such as is in the natural milk salt system, would alter the effects of heat upon the properties of this complex species.Therefore, the technique of differential ultraeentrifugation followed by electrophoretie and ultraeentrifugal sedimentation analysis was employed to investigate the characteristics of the heat-altered milk proteins as a function of their particle size.In the present study, oxalate treatment of the protein fractions was utilized to: (a) reduce the turbidity of the solution by removing calcium and to accomplish more complete disaggregation and improve resolution of the various protein components, and (b) provide information on the role of ealcimn in the ag-