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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.