Comparison of hydration behavior of bovine and caprine caseins as determined by oxygen-17 nuclear magnetic resonance: pH/pD dependence

The 17O NMR transverse relaxation rates (R2, s-1) of water in bovine and two caprine casein solutions were studied at 21°C with pH/pD values of 4.81, 6.95, and 8.16. The dependence of R2 was markedly nonlinear for all three proteins. Bovine and caprine caseins differ in the magnitude and the sign of the B0 virial coefficient, which is attributable to protein composition and structure. The composition of bovine casein and caprine casein high in αs1-casein solutions at pD 4.81 is reflected in the negative sign of the B0 virial coefficient and appears to be correlated with satisfactory aggregation (coagulation) properties. In acid-treated caprine casein solutions with low αs1-casein content, the positive sign of the B0 virial coefficient suggests structural environments which are resistant to self-association. The B0 values at pD 8.16 are also negative, indicative of unfolding leading to self-association. Analysis of the results also reveals marked differences in hydration behaviors, determined primarily by compositional and structural differences. The hydration of all three caseins is increased at acid and at alkaline pH/pD (pD 4.81 and 8.16), with minima at neutral pH/pD (pD 6.95). The hydration sensed by the 17O NMR experiments on casein, however, represents internal trapped water which serves to report on environmental changes. Enhanced self-association (prior to precipitation) of bovine and caprine caseins at acid conditions (pD 4.81) may account for the increased hydration. Alkali treatment (pD 8.16) may lead to swelling and possibly denaturation, all of which may enhance hydration. Our results demonstrate the feasibility of monitoring functional coagulation properties of bovine and caprine milks by NMR relaxation measurements