A 1H-NMR study of bovine casein micelles; influence of pH, temperature and calcium ions on micellar structure.

The influence of pH, temperature and Ca depletion on bovine casein micelle suspensions in D2O containing simulated milk ultrafiltrate was studied by 1H-NMR spectroscopy. In the pH range of 5.8-7.5 the spectrum of the micelles showed very little pH dependence, indicating that no changes occurred in the dynamic behaviour of the proteins constituting the micelle. The NMR spectrum of casein micelles was strongly temperature dependent, particularly in the temperature range of 60-98 degrees C. Increase in temperature resulted in a strong increase in spectral intensity concomitant with changes in the spectral characteristics. In micelle suspensions these changes were reversible, and indicated that at elevated temperatures the rigid structure of the casein micelle started to melt, leading to an increased mobility of appreciable parts of the proteins in the micelle. Ca depletion of the casein micelles by addition of EDTA resulted in an increase in spectral intensity, which arose from the presence of casein components in the serum phase. The spectrum of these serum phase particles resembled closely the spectrum of a solution of total casein in simulated milk ultrafiltrate and was quite different from the spectrum of casein micelles. The implications of these results with respect to models of the structure of bovine casein micelles are discussed.