Casein micelle dispersion into water, NaCl and CaCl2 : physicochemical characteristics of micelles and rennet coagulation

We have studied the effect of ionic strength from about 0 to 0.12 M on physicochemical characteristics of micelles and on gelation by rennet. Native phosphocaseinate powder was used as it exhibits attractive technological properties. The powder was dissolved at 25 g l(-1) casein concentration with 0.2 g l(-1) sodium azide into deionised water, NaCl or CaCl(2) solutions, at ionic strength from 0 to 0.12 M. The pH of micelle suspensions was corrected to pH 6.6 and, after overnight equilibration at 20 degrees C, pH values were checked and eventually corrected. After re-equilibration, calcium and phosphorus partition, micelle size, centrifuged pellet hydration were determined and kappa-casein hydrolysis and gelation of the suspensions were followed after rennet addition. The increase in NaCl concentration of casein micelle suspensions resulted in calcium and phosphorus solubilisation from micelles, and increase in pellet hydration. In the same way, the first-order rate constant of the kappa-casein hydrolysis with rennet was reduced, while CMP at the clotting time determined by Formagraph was unchanged. Rennet gelation was consequently retarded by NaCl increase. Suspensions of casein micelles into CaCl(2) solutions did not exhibit calcium binding onto the micelles, as in milk, but a small reduction of diffusible phosphorus from 1.55 to 0.24 mM was shown. Pellet hydrations were unchanged. The increase in ionic strength with CaCl(2) retarded kappa-casein hydrolysis, with an enhanced effect compared to NaCl study. As the CMP at the rennet clotting time was strongly reduced by CaCl(2) addition, the effect of CaCl(2) increase is typically biphasic, with an initial decrease and a following increase in rennet clotting times. From a technological point of view, reduction of ionic strength of milk led to reduced rennet clotting time. Results are discussed in terms of physicochemical modifications of milk micelles and the effect of ionic strength on enzymic reaction.