Review: Casein Micelle Structure; An Examination of Models

The casein micelle system of bovine milk is unique in that protein aggregates of similar spherical shape but extreme variability of size are formed by the self-assembly of three major nonidentical subunits. The monomeric subunits appear to be approximately the same size and shape with similar amphiphilic natures, the chief difference in properties being in the carbohydrate-containing κ-casein which acts to stabilize the system against precipitation by calcium ion. Micelle models with κ-casein exclusively in the interior lack a stabilization mechanism and can be eliminated. Statistical considerations of a chain polymer model also lead to its rejection. Electron microscopy reveals spherical submicellar aggregates which at present can be accounted for by only three models. Of these three, the experimental data are predicted only by one in which α_s1-, β-, and κ-casein subunits are associated into spherical soap-micelle-like particles with the κ-casein segregated into one portion, giving these submicelles an amphiphilic nature. The α_s1- and β-caseins are hydrophobic while the κ-casein portion of the submicelle surface is hydrophilic. Of particular interest is the ability of this micelle model to explain the formation of a minimum micelle which is larger than a submicellar particle.