Molecular mechanism of the renneting process of casein micelles in skim milk, examined by viscosity and light-scattering experiments and simulated bymodel SCF calculations

The properties of the outer hairy layer of casein micelles have been studie d using several methods. Viscosity and light-scattering measurements have b een performed on rennet-destabilized skim milk at different concentrations of calcium. We have reproduced the known result that calcium promotes desta bilization at relatively high kappa-casein surface coverages (even at only 50% of the kappa-casein molecules cut off). This suggests that calcium brid ges contribute to the casein micelle attraction. The effect may be direct b y calcium-mediated bridging of beta-casein or alpha(s1)-casein (phosphate/c arboxylate) or kappa-casein (carboxylate) or indirect by altering the inter play between several types of caseins in the micelle. In the analysis of th e experiments, we make use of an adhesive hard sphere approximation. The ca sein hairy layer has further been modeled by a self-consistent field (SCF) theory in which coarse-grained molecular details were included. In these ca lculations the effect of calcium bridges cannot directly be accounted for. It is shown that a kappa-casein layer induces repulsive interactions of mai nly steric origin. Cutting the N-terminus part results in reduced( shorter- ranged) repulsive pair potentials, which still dominate over the van der Wa als attraction. As a result of the altered interplay between the several ty pes of casein molecules during the renneting process, the alpha(s1)-casein concentration in the outer hairy layer may increase. Again using the SCF mo del, but now using a less-detailed description of the system, we examine a possible result of this. We confirm that diblock copolymers (simple model f or kappa-casein) and diblock copolymer with shorter soluble block (simple m odel for para-kappa-casein) give repulsion and triblock copolymers (simple model for alpha(s1)-casein) give attraction by means of bridging (adsorptio n). The renneting process is simulated by increasing the ratio of model par a-kappa-casein to kappa-casein at a fixed alpha(s1)-casein concentration. A s a result,the potential shifts from repulsive to attractive. We show that by reducing the range of repulsive interactions of the diblock copolymers, we can make room available to express the attractive contributions of the m odel alpha(s1)-casein. This comprises a novel molecular-based mechanism for the renneting process.