Study of calcium role in colloidal stability of reconstituted skim milk under rennet coagulation conditions

The role of calcium in rennet coagulation of milk is unquestionable in production technology of many cheeses. Therefore, understanding the possible mechanism of calcium influence on the colloidal stability of casein micelles may be the key to control the process of milk coagulation. It is evident that calcium ions are involved in maintenanceof milk coagulation stability, but the molecular mechanism of how these ions influence micellar caseins system is not fully known. Thus, the role of calcium in maintenance of the colloidal stability of milk is quite an urgent problem. Methodologically, the research was based on analysis of coagulation process of reconstituted skim milk, enriched with ions of calcium, magnesium and sodium. Milk whey separated from the clot after coagulation was investigated for sodium, magnesium, calcium and phosphorus. A simple quantitative model, which includes kinetic description of the proteolysis process and the thermodynamics of the dissociation process of the functional groups of micellar caseins, was worked out to analyze experimental results. Kinetic and thermodynamic methods of describing the process of stability loss in micellar system were combined in one model, using the concept of solvent quality which is defined by the second osmotic virial coefficient. The experiments showed that calcium and magnesium ions chemically connect to casein micelles. Using reasonable assessments for thermodynamic and kinetic parameters, we managed to get quite adequate description of the experimental data on the coagulation of reconstituted skimmed milk enriched with calcium and magnesium ions. It was stated that the equilibrium constants for the dissociation of magnesium and calcium caseinates should differ by more than two orders of magnitude. The authors demonstrated principal possibility of using the model to describe the rennet, acid and mixed acid-rennet clotting of milk.