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Journal of Dairy Science Vol. 54 No. 7 994-1000
© 1971 by American Dairy Science Association ®
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Fractionation of Skimmilk Casein Micelles by Rate-Zone and Isopycnic-Zone Ultracentrifugation in Sucrose Gradients1

C. V. Morr, S. H. C. Lin and R. V. Josephson2

Department of Food Science and Industries, University of Minnesota, St. Paul 55101

ABSTRACT

Rate-zone and isopycnic-zone ultracentrifugation techniques with sucrose gradients in a swinging-bucket rotor were utilized for fractionating skimmilk proteins with emphasis upon characterizing the different size casein micelles. Rate-zone ultracentrifugation separated skimmilk casein micelles from soluble casein and whey proteins, and further fractionated the different size micelles. Up to ~92% of skimmilk casein micelles was sedimented to the bottom of a 17.2 to 27.5% w/v sucrose gradient by centrifuging at 206,000 x g for two hours at 0 to 5 C. Zonal electrophoresis data, uncorrected for differences in dyebinding coefficients, revealed that the smallest casein micelles and soluble casein contained ~8.8% {alpha}s-casein, 89% ß-casein and 2.2% {kappa}-casein and the largest micelles recovered in the pellet fraction contained ~80% {alpha}s-casein, 19% ß-casein, and 0.8% {kappa}-casein. Isopycnic-zone ultracentrifugation banded whey proteins and soluble casein in ~8 to 10% w/v sucrose ({rho} - 1.02 to 1.04 g/ml), but failed to band casein micelles even in less double equals70% w/v sucrose gradients. It was concluded that the high concentrations of sucrose increased buoyant density of the casein micelles by removing their solvation and hydration layers, and thus, the isopycnic-zone technique is not applicable for studying casein micelles in their native state.

FOOTNOTES

1 Scientific Journal Series Paper 7485, Minnesota Agricultural Experiment Station.

2 Present address: Department of Dairy Technology, The Ohio State University, Columbus 43210.






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