Composition and Properties of Submicellar Casein Complexes in Colloidal Phosphate-Free Skimmilk

Abstract

Colloidal phosphate-free milks prepared from raw or heated (80 C for 30 min) skimmilk by adjustment to pH 4.9 or addition of ethylene- diaminetetraacetate followed by dialysis were similar in total and dialyzable nitrogen, cal- cium, and phosphate. A small amount of inorganic phosphate was not dialyzable and 20 to 25% of the original skimmilk nondialyz- able calcium was retained. Centrifugation of colloidal phosphate-free skimnfilk at 150,000 × g for one hour deposited a fraction (..- 4% of the total casein) enriched in as-casein~ Analytical ultraeentrifugation revealed one principal component with sed- imentation constant of 7.5 S (20 C) but also showed much size heterogeneity. Electron microscopy confirmed the size heterogeneity and also revealed that the submieellar casein complexes in colloidal phosphate-free skimmilk are smaller, irregularly shaped, and less electron dense than casein micelles in skimmilk. Gel filtration likewise confirmed the size heter- ogeneity of the snbmicellar casein particles. Sepharose 4B provided better resolution of submicellar casein particles than either Sep- hadex G-200 or Sepharose 2B. The successive fractions from filtration on Sepharose 4B were shown by starch gel electrophoresis to increase progressively in fl-casein and decrease in as-casein.