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Journal of Dairy Science Vol. 56 No. 11 1389-1395
© 1973 by American Dairy Science Association ®
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Milk Ultracentrifugal Opalescent Layer. 3. Yield and Casein and Lipid Composition as a Function of Centrifugation Time1

C. V. Morr2 and P. E. Swenson3

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

ABSTRACT

The yield and composition of opalescent layer, pellet and supernatant fractions from skim milk (0 to 5 C) were investigated as a function of ultracentrifugation time from 1 to 8 h at 147,000 xg.

Casein micelles in opalescent layer after 1 and 8 h accounted for 42 and 1% of total skim milk casein, respectively. The casein content of opalescent layer casein micelles decreased from 81 to 41% with increasing centrifugation time from 1 to 8 h. The lipoprotein and lipid content of opalescent layer casein micelles increased frcm 1 to 13% and from 1 to 19% by increase of centrifugation time from 1 to 8 h. Lipids extracted from opalescent layer pellet and supernatant contained 4.7 to 11.4% cholesterol. Lipids extracted from opalescent layer casein micelles contained greater amounts of phosphorus than lipids extracted from skim milk pellet and supernatant fractions.

Increasing centrifugation time to 8 h had no effect on the zonal electrophoretic properties of caseins recovered from skim milk pellet and supernatant and opalescent layer soluble casein. However, opalescent layer casein micelles obtained at longer centrifugation times contained progressively lower contents of {alpha}s-casein and ß-casein and more {kappa}-casein, ß-lactoglobulin, and slow-migrat'ng proteins that streaked frcm the starting slots.

FOOTNOTES

1 Scientific Journal Series Paper No. 8294, Minnesota Agricultural Experiment Station, St. Paul.

2 Present address: Ralston Purina Company, St. Louis, Missouri 63188.

3 Present address: Borden, Inc., Syracuse, New York 13204.






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