Role of Individual Milk Salt Constituents in Cross-linking by Colloidal Calcium Phosphate in Artificial Casein Micelles

Abstract

Artificial casein micelles were prepared at a casein concentration of 2.5% with various salt compositions, and the content of the casein aggregates cross-linked by colloidal calcium phosphate (CCP) was determined by high-performance gel chromatography on a TSK-GEL G4000SW column, using 6 M urea-simulated milk ultrafiltrate as the effluent. When phosphate was incorporated into calcium caseinate micelle systems, the casein aggregates cross-linked by CCP appeared. In these micelle systems, not all of the formed calcium phosphate seemed to participate in cross-linking, because part of the calcium phosphate was in the urea-insoluble form. When micelles were prepared at 30 mM calcium and 22 mM phosphate, the content of the casein aggregates crosslinked by CCP was higher in the presence of 10mM citrate than in the absence of citrate. The precipitate of calcium phosphate formed in the presence of citrate had cross-linking ability, whereas the precipitate formed in the absence of citrate did not. It is suggested that citrate plays an important role in cross-linking by CCP. Magnesium phosphate alone had no cross-linking ability. Magnesium increased the amount of calcium phosphate to promote cross-linking. 817Bovine casein micelles are highly hydrated and roughly spherical colloidal particles of 20 ~ 600 nm in diameter, which are heterogeneous in constititution. 1 ) They are composed of 93% casein and 7% inorganic constituents.The main casein constituents are iXs!-' iXs2 -' f3-and K-casein, in the proportions of about 3 : 0.8 : 3 : 1. The main constituent of inorganic matter is calcium phosphate, called colloidal calcium phosphate (CCP). CCP plays an important role in maintaining the integrity of casein micelles, because these micelles are disintegrated into submicelles when CCP is removed. 2 ) Although Schmidt!) has recently proposed a new model for the structure of casein micelles in which the micelles are composed of submicelles held together by means of CCP, no experimental evidence for the linkage between CCP and casein has been presented. In the previous study,3) we separated the casein aggregates cross-linked by CCP from reduced casein micelles by means of high-performance gel chromatography on a TSK-GEL G4000SW column, using 6 M urea-simulated milk ultrafiltrate (USMUF) as the effluent. In the present study, we have examined the crosslinking by CCP in artificial casein micelles which were prepared with various salt compositions in order to clarify the role of individual salt constituents. MATERIALS AND METHODSPreparation of artificial casein micelles. Artificial casein micelles were prepared according to the method of Knoop et a1 4 } with minor modifications to give a final casein concentration of 2.5%. Whole casein. which was prepared from bulk milk from the university herd by the acid precipitation method, was dissolved in water by the gradual addition of 1 N sodium hydroxide ensuring that the pH did not exceed 7, and then the pH was adjusted to 6.7. Tke salt solutions used were I M calcium chloride, 1 M magnesiu...

Phosphopeptides interacting with colloidal calcium phosphate isolated by tryptic hydrolysis of bovine casein micelles

“…This hypothesis was reinforced by the fact that part of the Ca (2 niM) is easily exchangeable (Yamauchi et al 1969;Pierre et al 1983). Furthermore, these results support the work of Aoki et al (1986Aoki et al ( , 1987a, which suggested that the casein-mineral crosslinking occurred as a function of the number of SerP residues present in casein sequences in the order a s2 -CN > a sl -CN > /?-CN. However, in our work we found only a small proportion of phosphopeptides derived from <x s2 -CN, probably because this protein is present in smaller amounts than a sl -CN and /5-CN and is therefore more difficult to detect.…”

The effect of commercial sterilization regimens on micellar casein concentrates

“…Whereas soluble Ca levels were significantly lower in the UHT-treated than in the retorted MCC, no difference was observed between the soluble Mg levels between the 2 treatments. It has been shown before that calcium phosphate can form cross-links in artificial casein micelle systems, whereas magnesium phosphate does not have this cross-linking ability (Kaminogawa et al, 1977;Aoki et al, 1987). Thus, it is reasonable to hypothesize that some of the calcium phosphate may have participated in the cross-linking and subsequent aggregation of casein micelles during the UHT treatment, which prevented it from returning into the soluble phase.…”

Dissociation during dialysis of casein aggregates cross-linked by colloidal calcium phosphate in bovine casein micelles

“…It has been proved that caseins are cross-linked through their ester phosphate groups by CCP (Aoki et al 19876). The role of the individual milk salt constituents in crosslinking by CCP has also been examined (Aoki et al 1987a).…”