We have studied the e!ect of ionic strength from about 0 to 0.12 M on physicochemical characteristics of micelles and on gelation by rennet. Native phosphocaseinate powder was used as it exhibits attractive technological properties. The powder was dissolved at 25 g l\ casein concentration with 0.2 g l\ sodium azide into deionised water, NaCl or CaCl solutions, at ionic strength from 0 to 0.12 M. The pH of micelle suspensions was corrected to pH 6.6 and, after overnight equilibration at 203C, pH values were checked and eventually corrected. After re-equilibration, calcium and phosphorus partition, micelle size, centrifuged pellet hydration were determined and-casein hydrolysis and gelation of the suspensions were followed after rennet addition. The increase in NaCl concentration of casein micelle suspensions resulted in calcium and phosphorus solubilisation from micelles, and increase in pellet hydration. In the same way, the "rst-order rate constant of the-casein hydrolysis with rennet was reduced, while CMP at the clotting time determined by Formagraph was unchanged. Rennet gelation was consequently retarded by NaCl increase. Suspensions of casein micelles into CaCl solutions did not exhibit calcium binding onto the micelles, as in milk, but a small reduction of di!usible phosphorus from 1.55 to 0.24 mM was shown. Pellet hydrations were unchanged. The increase in ionic strength with CaCl retarded-casein hydrolysis, with an enhanced e!ect compared to NaCl study. As the CMP at the rennet clotting time was strongly reduced by CaCl addition, the e!ect of CaCl increase is typically biphasic, with an initial decrease and a following increase in rennet clotting times. From a technological point of view, reduction of ionic strength of milk led to reduced rennet clotting time. Results are discussed in terms of physicochemical modi"cations of milk micelles and the e!ect of ionic strength on enzymic reaction.
A drying method by desorption in a water activity meter was used to simulate the conditions of spray drying and to determine the water transfer inside dairy concentrates towards the surface and from the surface to the drying air. The concentrates were also spray dried and solubility index of powders were determined during reconstitution. Whey protein concentrates (WPC) and native phosphocaseinate suspensions (NPCS) were used to study the effect of NaCl (420 mM), CaCI2 (222 mM), sodium phosphate (173 mM) and sodium citrate (238 mM) on the water transfers. The decrease in water transfer during drying was explained by the high hygroscopicity of added mineral salts to WPC. NaCl addition to NPCS decreased the water transfer during drying, but increased the solubility index. Citrate and phosphate addition to NPCS increased the water transfer during drying and reconstitution. CaC12 increased the water transfer during drying but the solubility index was always low. Results are discussed as a function of the aqueous environment, of the nature of mineral salts, of the structure of dairy proteins and of protein solvation.Copyright 0 1999 by Marcel Dckker. Ine.
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