Behavior of Calcium and Phosphate in Artificial Casein Micelles

Abstract

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 40mM Ca, 22 to 27mM phosphate, and 10mM 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 30mM Ca and 22mM phosphate were exchanged after dialysis of artificial casein micelles against a simulated milk ultrafiltrate at 4°C for 72h. 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°C for 72h. The proportion of exchanged colloidal 45Ca and 32P in artificial casein micelles during dialysis for 72h were higher at 25°C than at 4°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.