Casein micelle size and composition related to the enzymatic coagulation process

Chymosin (EC 3.4.23.4) and rennet, the latter containing about 85% chymosin and 15% pepsin, have been compared according to their coagulation properties with native micelles of different sizes or monomeric caseins as substrate. The casein micelles were separated on columns of controlled-pore glass (CPG-10/3000), which fractionates particles of up to 300 nm diameter. The results show that the coagulation time varies with the micelle size. The effect, which is more pronounced with chymosin than with rennet, appears to be related to the availability of kappa-casein. Therefore the largest micelles, with a lower kappa-casein content, showed longer coagulation times than medium size micelles. In the region of the smallest micelles this time increases again, probably due to an increased beta-casein content. Addition of monomeric kappa-casein decreased the coagulation time with both rennet and chymosin, but alpha s1-and beta-casein had the opposite effect. When isolated monomeric caseins were treated alone with rennet or chymosin, kappa-casein caused turbidity, but alpha s1-and beta-casein did not. Centrifugation experiments with micelles after monomeric casein addition showed that a limited amount of the added casein was able to join the micelle. This was confirmed by chromatographic studies.