NMR Water Relaxation and Self-Diffusion in Highly Concentrated Casein Systems
Résumé
In a variety of dairy processing operations (drying, membrane filtration...), milk is concentrated, leading to dense and ultimately solid dispersions of caseins. In this work, we explore how water mobility is affected in such conditions and if there is any relation between the properties of water as seen by NMR and the “macroscopic” phase behavior of the dispersions. We also investigate the effect of structure by comparing the NMR results obtained with dispersions of casein micelles (NPC, native phosphocaseinate) and dispersions of caseins which are not organized into micelles (SC, sodium caseinate dispersions). Casein dispersions were prepared using NPC and SC powders dissolved in the aqueous phase of a fresh skimmed milk. For concentrations C > ~150 g/L, the dispersions were made through equilibrium dialysis against a solution of high osmotic pressure (= osmotic stress technique). NMR measurements were performed with a 20 Mhz Bruker Minispec equipped with a pulsed field gradient probe. The spin-spin relaxation time T2 was measured using a CPMG sequence and the water diffusion coefficient Dw was measured using a Stejskal-Tanner spin echo sequence. . Our results first indicate that both 1/T2 and Dw vary in a monotonic fashion over the whole range of concentrations investigated, and for the two casein systems. This strongly suggests there is no relation between the macroscopic phase behavior (gel transition at ~200 g/l for NPC dispersions) and the water mobility at the molecular scale. . Second, no difference in Dw was observed for NPC and SC systems at the same concentrations. This indicates that Dw is not sensitive to the way the caseins are organized and only depends on the overall quantity of proteins. This behavior is discussed with respect to diffusion models that were recently developed. . Finally, and in opposite to Dw, we found that 1/T2 is strongly structure-dependent with much lower values for SC dispersions. This result is explained through the fact that caseins are “locally constrained” around mineral nanoclusters of calcium phosphate (CaP) in the casein micelle. Then the water protons, which are in fast chemical exchange with casein protons, have lower relaxation times in the micelle as compared to SC dispersions where the CaP nanoclusters are absent and the caseins are consequently less structured.