MULTINUCLEAR NMR-STUDY OF THE PH DEPENDENT WATER STATE IN SKIM MILK AND CASEINATE SOLUTIONS

The effects of decreasing pH and micellar calcium concentrations of reconstituted skim milk and caseinate solution were studied by H-1 and O-17 NMR spectroscopy. The proton transverse relaxation rate 1/T2 of skim milk decreased as the pH decreased, reaching a minimum at pH 5.3. However, as the pH fell sodium caseinate solution showed a continuous increase in 1/T2, with no minimum. Analysis of proton relaxation as a function of the interpulse time in the CPMG (Carr-Purcell-Meiboom-Gill) sequence demonstrated that both the proton exchange mechanism and 'bound' water contributed to proton relaxation in skim milk. The study of O-17 relaxation rate as a function of pH confirmed the change in protein hydration upon acidification. Increasing the amount of EDTA showed that the proton transverse relaxation rate of skim milk decreased until a plateau was reached when the micellar calcium was totally solubilized. With excess EDTA the relaxation rates of skim milk and caseinate solution were identical. A strong correlation was also found between the pH dependent relaxation rate and the solubilization of micellar phosphorus as detected by P-31 NMR. Together, these results suggested that aggregation of caseins by calcium and colloidal calcium phosphate is mainly responsible for the excess hydration in skim milk micelles.