Assessment of the state of water in reconstituted milk protein dispersionsby 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 20g per 100g DM) with various micel lar casein concentrations (3-15 g per 100 g of dispersion), whey protein co ncentrations (9-3 g per 100 g of dispersion), lactose concentrations (0-7.5 g per 100g of dispersion), CaCl2 concentration (0-2 nM) and pH(6.2-6.6). R esults obtained with both techniques showed the predominant effect of micel lar 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 la ctose and 0.60 g/g for lactose containing dispersions) measured by DSC vari ed mainly in lactose and native phosphocaseinate (NPQ content. Furthermore, NMR water relaxation behaviour was consistent with a three site relaxation model. Because of fast diffusive exchange, the relaxation rate observed wa s the sum of the free water relaxation rate, the hydration water relaxation rate and the exchangeable protons relaxation rate, weighted by their relat ive proportions. Analysis of the variation in relaxation rate with the samp le 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.