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Determination of Soluble Calcium and Phosphorus in Commercial Milled Hydroxyapatite

Johns, Paul W., Dimler, Steven R., Watson, Julie J., Tigner, Maryann, Caskey, Paul F.
Food analytical methods 2016 v.9 no.6 pp. 1754-1762
calcium, centrifugation, colorimetry, health foods, hydroxyapatite, milk, models, pH, particle size, phosphorus, sodium caseinate
A method for the determination of soluble calcium and soluble phosphorus in commercial milled hydroxyapatite is described. The Ca and P residing in the supernatant after high-speed centrifugation of a 1.00 % (w/w) suspension in pH 6.8 HEPES buffer are quantified by cresolphthalein and phosphomolybdenum complex colorimetry, respectively. Method performance has been defined by assessments of linearity (r ² averages for Ca and P, respectively, were 0.998 ± 0.001, n = 15, and 1.000 ± 0.000, n = 15), intermediate precision (soluble Ca rsd = 12 % at 0.500 mg/g to 0.5 % at 8.04 mg/g; soluble P rsd = 18 % at 0.0526 mg/g to 0.5 % at 3.5 mg/g; soluble Ca/P molar ratio rsd = 18 % at 63 to 0.5 % at 1.88; n = 3 days), and accuracy (spike recovery average = 99.7 ± 3.6 % for soluble Ca, and 97.8 ± 0.6 % for soluble P; n = 4). A hydroxyapatite milled to successively smaller particle sizes exhibited increases in soluble Ca, P, and Ca/P as the particle size median decreased from 2.339 to 1.375 μm. Application of the method to seven commercial lots of milled hydroxyapatite found soluble Ca from 0.118 to 1.98 % of theoretical Ca, soluble P from 0.386 to 1.80 % of theoretical P, and soluble Ca/P (molar) from 0.258 to 3.54. The interaction of these lots with sodium caseinate in a simulated retort model was strongly correlated with their soluble Ca concentration (r ² > 0.9). The method provides a simple and inexpensive means for identifying potentially problematic lots of milled hydroxyapatite, i.e., lots with soluble Ca and P levels sufficiently high to adversely affect the physical stability of retort sterilized milk-based liquid nutritional products.