kappa-casein as a polyelectrolyte brush on the surface of casein micelles

Casein micelles in milk are stabilized by a polymer brush made up mainly of the protein kappa-casein. This protein carries several carboxylic acid groups which are hydrolysed under physiological conditions, pH 6.7. Since the ionic strength is -0.1 mol, we treat the brush as a so-called salted brush and apply the scaling theory for the stability of this weak-polyelectrolyte-type brush. The stability of the brush, i.e. the stretched-to-collapsed transition, depends on the chain density and the charge density along the chain. On collapse of the brush either at low charge density or at low chain density (or both) the casein micelles lose their stability. By partially renneting the kappa-casein brush and gradually lowering the pH (charge density) we were able for the first time to test a scaling equation for the stretched-collapsed transition of a salted polymer brush.