Calorimetric characterization of the micellar calcium phosphate in cows' milk.

Cows’ milk contains inorganic calcium phosphate as a tightly-bound component of the casein micelles. The composition and chemical properties of this calcium phosphate have been studied for many years. Although the Ca/P ratio is similar to that found in hydroxyapatite, the use of other methods of characterization is desirable, since the various crystalline and amorphous forms of inorganic calcium phosphate tend to vary considerably in composition. The heat of solution should be of use in this respect, since it would be expected to be different for each crystal form. This has now been tested experimentally, by using an LKB model 10700 batch microcalorimeter (LKB instruments Ltd., Croydon CR2 8YD, Surrey, U.K.). To measure the heats of solution of inorganic calcium phosphates, weighed amounts were placed in the calorimeter cells, and, after thermal equilibrium had been reached, mixed with an excess of IM-HC~O.,. The heat evolved was measured and the results are shown in Table 1, under the heading A H (acid). These heats are for reactions of the type: Ca3(P0& (s) + 6H+(aq.) = 3Caz+(aq.) + 2H3P04(aq.) but have not been corrected for the heat of dilution of the reactants or products. Because of the problem of variable composition (shown by the values for actual Ca/P ratios in Table I), the heats have been expressed in kJ/mol of Ca. The nature of the solid phases (except for the acid phosphates) was determined by X-ray diffraction. The hydroxyapatite gel was a commercially available sample of gel prepared by the method of Keilin & Hartree (1938), obtained from BDH Chemicals, Poole, Dorset BH12 4NN, U.K.) and could be unambiguously identified as hydroxyapatite from its X-ray diffraction pattern. The amorphous gel was prepared in a simulated milk salt solution at pH 6.60, containing lmmpyrophosphate to inhibit crystallization (Francis, 1969). Most of the heats are large and exothermic, and, in one case, agree well with a value given by other workers (Meadowcroft & Richardson, 1963). It is not convenient to measure the heat of solution of milk calcium phosphate by mixing with acid; instead, a buffered solution of EDTA was used, to ensure that all components of the final reaction mixture were soluble. A standard pH value of 6.60 was chosen, and all results were corrected to this pH. The heat of solution of a calcium phosphate at any pH may be defined by an equation of the type