Supra-aggregates of Casein Micelles as a Prelude to Coagulation

Abstract

Native casein micelles (i.e., the micelles in fresh milk) can be treated as a collection of polydisperse hard spheres. This follows from small angle neutron scattering, viscosity, diffusivity, and other measurements. Therefore, the equilibrium and transport properties of native casein micelles in an ultrafiltration permeate solvent can be described by theories developed for colloidal hard sphere dispersions because native casein micelles are sterically stabilized by a brush of κ-casein (CN) molecules (and perhaps β-CN). The κ-CN brush induces a very short-ranged or steep repulsion (it rises from zero to a large value) when two micelles meet each other. Electrostatic interactions are highly screened because of the high ionic strength (0.08 M) of skim milk. The colloidal properties of casein micelles change with the technological treatment applied to skim milk. This paper describes the consequences for the casein micelle properties of heating, renneting, and acidification.It appears that the properties of the micelles can be described generally by adopting the adhesive hard sphere model. In that model, the steep repulsive interaction of two micelles is preceded by a short-ranged Van der Waals attraction. By relating the strength of the attraction to the degree of technological treatment (e.g., renneting time or pH changes), the colloidal properties of the micelles can be described simply by using adhesive hard sphere theory. This theory also predicts the phase behavior of such a system. For instance, it predicts correctly the gelation of casein micelles under various conditions.The adhesive hard sphere model allows a general and consistent understanding of the colloidal properties of casein micelles caused by technological treatments. The practical relevance of the models is illustrated with a few examples.