COMPARISON OF HYDRATION BEHAVIOR OF BOVINE AND CAPRINE CASEINS AS DETERMINED BY O-17 NUCLEAR-MAGNETIC-RESONANCE - TEMPERATURE-DEPENDENCE OFCOLLOIDAL STABILITY

Oxygen-17 nuclear magnetic resonance (NMR) transverse relaxation rates for bovine and caprine casein micelles at various temperatures were a nalyzed by nonlinear regression analysis and a protein activity model. The dependence of the NMR transverse relaxation rates was markedly no nlinear due to interactions between protein molecules. Temperature dep endences of the hydration parameters of the bovine and caprine casein micelles were in accordance with the hypothesis that hydrophobic inter actions are the predominant forces responsible for the self-associatio n of the caseins. Relaxation differences between reconstituted micelle s of bovine and caprine caseins strongly suggest that important struct ural dissimilarities exist between these milk proteins that are due to differences in the ratios of alpha(s1)- to beta-casein. A higher degr ee of hydration, characteristic of a more open and looser structure, i s observed for caprine casein micelles high in alpha(s1)-casein at 21 and 37 degrees C. The observed hydration behavior of bovine casein mic elles at all three temperatures is consistent with the hydration value s determined previously by deuterium NMR studies of bovine casein mice lles in D2O containing 1,4-piperazinediethanesulfonic acid. The corres pondence between the deuterium and oxygen-17 results suggests that bot h experiments detect exchangeable water ''trapped'' within the casein micelles. The dependence of the second virial coefficient B-0 on tempe rature was different for bovine and caprine casein micelles, suggestin g the importance of ''net'' electrostatic charges of these milk protei ns in their interactions with calcium and water.