MODELING CALCIUM-INDUCED SOLUBILITY IN CAPRINE MILK CASEINS USING A THERMODYNAMIC LINKAGE APPROACH

The phenomena of calcium-induced precipitation of bovine and caprine w hole caseins (salting out) and the resolubilization of these proteins at higher calcium concentrations (salting in) are thermodynamically li nked with changes in protein solubility resulting from calcium binding . The differences in calcium sensitivities of caprine whole caseins un der various conditions of temperature and ionic strength (KCl) appear to be correlated with the content of the alpha(s1)-casein component. H owever, the solubility behavior of caprine whole caseins characterized by low content of alpha(s1)-casein (5% of total) is more closely rela ted to solubility properties displayed by bovine casein (38% of total) . The properties of whole caprine casein high in alpha(s1)-casein cont ent (17% of total) appear to be dominated by the binding of calcium to higher affinity sites phosphate groups), which results in less stabil ity. Decreasing the temperature to 1 degrees(C) dramatically altered t he salting out of both caprine caseins but not bovine casein. These re sults suggested that the solubility and calcium-binding properties of caprine whole caseins are in part determined by hydrophobic interactio ns. However, salting out of both of the caprine caseins is effected by competitive K+-Ca2+ binding at 1 degrees C, indicating a role for ion ic interactions as well. Because such KCl-dependent changes do not occ ur in whole bovine caseins, protein-protein interactions appear to be stronger in this case. These results show that alteration in casein co mposition can clearly effect the functionality of the whole casein and that thermodynamic linkage analysis can readily quantitate these diff erences that are linked to calcium binding.