Hydration of Native and Rennin-Treated, Cold-Solubilized Caprine Caseins as Determined by Oxygen-17 Nuclear Magnetic Resonance

Abstract

The hydration of native and rennin-coagulated bovine and caprine caseins was investigated by oxygen-17 nuclear magnetic resonance (NMR) and fitted by nonlinear regression analysis. A charge-charge interaction model (B0) was employed to analyze the transverse relaxation (1/T2) data. Relaxation differences between reconstituted native micelles at 10°C and rennin-coagulated, cold-solubilized micelles of caseins strongly suggest that important structural dissimilarities exist between these milk proteins that are due to differences in the ratios of αs1- to β-CN. Variants of αs1-CN had significant effects on caprine casein hydration. The differences were more pronounced in rennin-coagulated than in native casein micelles. All rennin-coagulated, cold-solubilized casein micelles demonstrated significant decreases in hydration. Micelles containing low αs1-CN retain an open, highly hydrated structure, in comparison with similarly treated bovine and caprine casein micelles containing high αs1-CN. Second virial coefficients (B0 values) derived from oxygen-17 NMR data suggest that reconstituted bovine and caprine casein micelles containing high αs1-CN exhibit strong interactions, which at first decrease then increase during clotting. In contrast, the native caprine casein micelles containing low αs1-CN exhibit strong charge repulsions, which increase with clotting. The compositional differences are reflected in differences in the extent of protein-protein aggregation during casein clotting by rennin. Our results demonstrate that alteration in casein composition can dramatically alter cheese-making properties.