Assessment of the State of Water in Reconstituted Milk Protein Dispersions by Nuclear Magnetic Resonance (NMR) and Differential Scanning Calorimetry (DSC)

DSC profiles and relaxation NMR 1H signals were used to characterise highly hydrated milk protein dispersion (3 to 20 g per 100 g DM) with various micellar casein concentrations (3-15 g per 100 g of dispersion), whey protein concentrations (0-3 g per 100 g of dispersion), lactose concentrations (0-7.5 g per 100 g of dispersion), CaCl2concentration (0-2 mM) and pH (6.2-6.6). Results obtained with both techniques showed the predominant effect of micellar casein and lactose concentrations on the water state. Heat of fusion of water and amount of unfreezable water (0.49 g/g for dispersions without lactose and 0.60 g/g for lactose containing dispersions) measured by DSC varied mainly in lactose and native phosphocaseinate (NPC) content. Furthermore, NMR water relaxation behaviour was consistent with a three site relaxation model. Because of fast diffusive exchange, the relaxation rate observed was the sum of the free water relaxation rate, the hydration water relaxation rate and the exchangeable protons relaxation rate, weighted by their relative proportions. Analysis of the variation in relaxation rate with the sample composition confirmed that the hydration water was mainly influenced by the casein concentration and that the chemical exchange effect, such as the lactose effect, could be attributed to soluble macromolecules. NMR versus DSC results were also discussed.