Addition of citrate to casein micelles: Modifications of their physico-chemistry and acid gelation.
Résumé
The organization and functional properties of casein micelles (CMs) are influenced by several factors such as temperature, pH, and ionic environment. In this study, different CMs suspensions were obtained by addition of citrate to milk at final added concentrations: 8.5, 14.5 and 18.7 mM and readjustment of pH to 6.75. Modified milks were concentrated by ultrafiltration at ~10°C and some samples were then diafiltered against milk ultrafiltrate and standardized at ~45 g.kg-1 total proteins. The analyses revealed that CMs were demineralized in Ca and P and dissociated. After acidification to pH 4.6 by HCl, set-style gels from samples containing 8.5 and 18.7 mM added citrate have a firmness reduced to 75 and 56% of that of the gel from the control sample, respectively (without citrate). In the opposite, the firmness obtained after addition of 14.5 mM of citrate increased to 116% of the value of the control sample. Same trends were observed with the viscosity values determined on the different stirred acid gels. For both rheological analyses, the comparison of diafiltered samples against undiafiltered samples did not show significant differences. These changes in the rheological properties of acid gels were probably related to the modifications of the micellar calcium phosphate responsible for the structure of CMs.