Studies on micellar calcium phosphate: composition and apparent solubility product in milk over a wide pH range

Abstract

Calcium and phosphate concentrations in milk ultrafiltrate prepared at 37 °C over the pH range 4-0-8 -0 have been measured and used to calculate the apparent solubility products for various calcium phosphates in milk. Micellar calcium phosphate had a constant Ca/Pj = 158±003 at all pH values and a near-invariant ion activity product for CaHP0 4 . Apparent solubility products have also been obtained for brushite, hydroxyapatite and amorphous tricalcium phosphate in milk salt solution over a similar pH range. No evidence was found for a hydroxyapatite or amorphous tricalcium phosphate phase in milk.Cow's milk contains inorganic calcium phosphate as a tightly bound component of the casein micelle which dissociates into submicelles when the calcium phosphate is removed. The nature of the micellar calcium phosphate has been reviewed recently (Schmidt, 1982) and the suggestion made that it is similar to amorphous tricalcium phosphate. The Ca/Pj ratio for micellar calcium phosphate is close to 1-5 and is in agreement with this suggestion. However, measurements of the apparent solubility product have suggested a structure more like that of dicalcium phosphate and a modified structure Ca(HPO 4 ) 0 . 7 (PO 4 ) 0 . 2 was proposed by Holt (1982). Extended X-ray absorption fine structure and near-edge structure measurements have shown that micellar calcium phosphate closely resembles brushite (CaHP0 4 . 2H 2 O) , in agreement with solubility product measurements. The discrepancy between the expected Ca/Pj = 1-0 for brushite and the higher measured value can be reconciled if it is assumed that some of the HP0 4~ ions in the brushite lattice arc replaced by serine phosphate groups of casein.The present work is aimed at extending the pH range over which the solubility product data of Holt (1982) were obtained, using a simpler method of sample preparation. Also, solubility product measurements have been made on suitable model systems in order to assess the validity of this method of determining the nature of the solid phase. EXPERIMENTAL All reagents used were Analar grade. Preparation of skim-milk ultrafiltrates al 37 °CUltrafiltrates were prepared from skim-milk after pH adjustment, in order to study the pH dependence of the apparent solubility products over a wide pH range.Fresh raw bulk milk (2 1) was collected from the NIRD Experimental Dairy. The

Physicochemical characterization of calcium-supplemented skim milk

“…These results suggest that, whatever the added calcium concentration, the colloidal calcium phosphate of calcium-enriched milks contains some dicalcium phosphate-like materials in near-equilibrium with the serum, like nonenriched milk. Such a hypothesis is in good accordance with Holt [16][17][18] and Chaplin [6] who found that colloidal calcium phosphate of skim milk closely resembled brushite. In the same way, acidification of calcium-supplemented milks showed the same solubilization of colloidal calcium phosphate as that observed for reference milk: the same linear relationship between colloidal calcium and colloidal inorganic phosphate was obtained (1.73x ± 5.01, Fig.…”

“…Relations between colloidal calcium concentration and colloidal inorganic phosphate concentration in the pH range 6.7-5.4 were also [19]. From the activities, and in the pH range 6.7-5.4, the ion activity products for dicalcium phosphate (CaHPO 4 ·2H 2 O and Ca(HPO 4 ) 0.7 (PO 4 ) 0.2 ) and tricalcium phosphate (Ca 3 (PO 4 ) 2 ) were calculated [6,19].…”

Effects of mineral content of bovine drinking water: Does iron content affect milk quality?

“…pH 6 and that iron has a tendency to lower pH (Chaplin, 1984;Gaucheron et al, 1996;Balintova and Petrilakova, 2011).…”

“…The addition of ferrous chloride (FeCl 2 ) to skim milk significantly decreased the pH of milk from 6.73 to 6.63. This, in turn, also displaces calcium and inorganic phosphate in the milk, which could potentially affect processes such as cheese making (Chaplin, 1984;Gaucheron et al, 1996).…”

“…Addition of ferrous chloride to skim milk has shown a decrease in pH and displacement of minerals such as calcium and inorganic phosphate from colloidal phase to the aqueous phase in the β-casein (Hegenauer et al, 1979a;Chaplin, 1984;Gaucheron et al, 1996;Gaucheron, 2000;Raouche et al, 2009). …”