CALCIUM-CONCENTRATION IN MILK IN RELATION TO HEAT-STABILITY AND FOULING

Some properties of skim milk with varying calcium concentrations were determined before and after heating, to study the effect of calcium on heat stability of the milk and on fouling caused by it. Either increa sing or decreasing the calcium concentration in the milk led to a lowe r heat stability and to more fouling in comparison with normal milk. F or normal milk the protein in the deposit consisted mainly of serum pr oteins. If the calcium concentration was changed there was a shift in the protein composition of the deposit from serum proteins to casein. In low-calcium milk the casein micelles were swollen and a large part of the kappa- and beta-casein was dissociated from the micelles. These so-called depleted micelles lack (part of) the stabilizing action of kappa-casein and are the cause of a lower heat stability and more case in deposition. In high-calcium milk the casein micelles had shrunk and hardly any dissociation was observed. Presumably, the increase in cal cium ion activity and in colloidal calcium phosphate reduces electrost atic and steric repulsions between the casein micelles, leading to inc reased interaction and consequently to a lower heat stability. This in stability of the casein micelles, with denatured beta-lactoglobulin as sociated at their surface, would then cause a large increase in protei n fouling.