Supra-aggregates of Casein Micelles as a Prelude to Coagulation

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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 measure- ments. Therefore, the equilibrium and transport properties of native casein micelles in an ultrafiltra- tion permeate solvent can be described by theories developed for colloidal hard sphere dispersions be- cause native casein micelles are sterically stabilized by a brush of k-casein (CN) molecules (and perhaps b-CN). The k-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 techno- logical 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 technologi- cal treatment (e.g., renneting time or pH changes), the colloidal properties of the micelles can be described simply by using adhesive hard sphere the- ory. 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 proper- ties of casein micelles caused by technological treat- ments. The practical relevance of the models is illus- trated with a few examples.