Effects of emulsifying salts on the turbidity and calcium-phosphate-protein interactions in casein micelles.

Influence of emulsifying salts (ES) on some physical properties of casein micelles was investigated. A reconstituted milk protein concentrate (MPC) solution (5% wt/wt) was used as the protein source and the effects of ES [0 to 2.0% (wt/wt)] were estimated by measuring turbidity, acid-base titration curves and amount of casein-bound Ca and inorganic P (P(i)). Various ES, trisodium citrate (TSC), or sodium phosphates (ortho-, pyro-, or hexameta-) were added to MPC solution, and all samples were adjusted to pH 5.8. Acid-base buffering curves were used to observe changes in the amount and type of insoluble Ca phosphates. An increase in the concentration of TSC added to MPC solution decreased turbidity, buffering at pH approximately 5 (contributed by colloidal Ca phosphate), and amount of casein-bound Ca and P(i). Addition of up to 0.7% disodium orthophosphate (DSP) did not significantly influence turbidity, buffering curves, or amount of casein-bound Ca and P(i). When higher concentrations (i.e., > or =1.0%) of DSP were added, there was a slow decrease in turbidity. With increasing concentration of added tetrasodium pyrophosphate (TSPP), turbidity and buffering at pH approximately 5 decreased, and amount of casein-bound Ca and P(i) increased. When small concentrations (i.e., 0.1%) of sodium hexameta-phosphate were added, effects were similar to those when TSPP were added but when higher concentrations (i.e., > or =0.5%) were added, the buffering peak shifted to a higher pH value, and amount of casein-bound Ca and P(i) decreased. These results suggested that each type of ES influenced casein micelles by different mechanisms.