Milk Ultracentrifugal Opalescent Layer. 3. Yield and Casein and Lipid Composition as a Function of Centrifugation Time

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 × g. 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 from 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 αs-casein and β-casein and more κ-casein, β-lactoglobulin, and slow-migrat’ng proteins that streaked frcm the starting slots. © 1973, American Dairy Science Association. All rights reserved.