Composition, Structure, and Integrity of Casein Micelles: A Review

Abstract

This review is an attempt to bring what is known about casein micelles together into a coherent summary. Some of the earlier models and theories no longer appear workable, and much is left to be concluded on this subject. It is hoped with what we now know that those who already have contributed and others will continue to pursue the under- standing of casein micelles. PHYSICAL PROPERTIES OF CASEIN MICELLES The biological function of bovine casein micelles is to provide efficient nutrition to the young calf. It does not require inherently a high degree of ordered structure but, rather, an effective mechanism for secretion of a highly concentrated solution of protein, calcium, and phosphate. During the past 20 yr research has been extensive to determine the composition and structure of casein micelles and to identify forces that maintain their integrity. The ap- proximate composition of bovine casein micelles is in Table 1. In cows' milk, casein micelles occur in colloidal dispersion. Structure and properties of the casein group of proteins, which comprise over 90% of the mass of casein micelles, have been reviewed (11, 13, 76, 89, 90, 98). Casein micelles are highly hydrated and spongelike colloidal particles containing about 3.7 g H20/g protein (6, 47, 48). Relatively little of this water (.5 g H20/g protein) is bound to the protein. The remainder is occluded within the micelle and moves with the micelle during hydrodynamic experiments. Numerous models describing casein micelles have been