Effects of mineral salts and calcium chelating agents on the gelation of renneted skim milk

The effects of adding CaCl2, orthophosphate, citrate, EDTA, or a mixture of these, to reconstituted skim milk (90 g of solids/kg solution) on the gelation of renneted milk were mediated by changes in Ca2+ activity and the casein micelle. At pH 6.65, the addition of citrate or EDTA, which removed more than 33% of the original colloidal calcium phosphate with the accompanying release of 20% casein from the micelle, completely inhibited gelation. Reformation of the depleted colloidal calcium phosphate and casein in the micelle, by the addition of CaCl2, removed this inhibition. When the minimum requirements for colloidal calcium phosphate and casein in the micelle were met, the coagulation time decreased with increasing Ca2+ activity, leveling off at high Ca2+ activity. The storage modulus of renneted gels, measured at 3 h, increased with increasing colloidal calcium phosphate content of micelles up to a level at which it was similar to 130% of the original colloidal calcium phosphate in the micelles. Further increases in colloidal calcium phosphate by the addition of CaCl2, orthophosphate, or mixtures of these, which did not change the proportion of casein in the micelle, decreased the storage modulus. The gelation of the renneted milk was influenced by Ca2+ activity, the amounts of colloidal calcium phosphate, and casein within the micelle, with the effects of colloidal calcium phosphate and casein within the micelle clearly dominating the storage modulus. These results are consistent with the model of Horne (Int. Dairy J. 8:171-177, 1998) which postulates that, following cleavage of the stabilizing kappa -casein hairs by rennet, the properties of the rennet gel are determined by the balance between the electrostatic and hydrophobic forces between casein micelles.