Behavior of calcium and phosphate in artificial casein micelles

This study investigated the behavior of Ca and phosphate in artificial casein micelles using 45Ca and 32P by means of UF, equilibrium dialysis, and gel permeation HPLC in the presence of 6 M urea. Artificial casein micelles were prepared at a casein concentration of 2.5% with 30 to 40 mM Ca, 22 to 27 mM phosphate, and 10 mM citrate using 45Ca or 32P. About 75 and 65% of colloidal 45Ca and 32P, respectively, in artificial casein micelles that were formed in the presence of 30 mM Ca and 22 mM phosphate were exchanged after dialysis of artificial casein micelles against a simulated milk ultrafiltrate at 4 degrees C for 72 h. The percentages of 45Ca and 32P in the fraction of casein aggregates that were crosslinked by micellar Ca phosphate were 26.9 and 27.6%, which decreased to about 10% after equilibrium dialysis against a simulated milk ultrafiltrate at 4 degrees C for 72 h. The proportion of exchanged colloidal 45Ca and 32P in artificial casein micelles during dialysis for 72 h were higher at 25 degrees C than at 4 degrees C, suggesting that the exchange of Ca and phosphate between the diffusible and colloidal phases depended on temperature. The results suggest that the exchange of Ca and phosphate between diffusible and colloidal phases proceeds slowly, and a portion of the Ca and phosphate in the artificial casein micelles is difficult to exchange.