The Solubility of Calcium Phosphate in Fresh Milk

Abstract

A knowledge of the calcium and phosphate equilibria in milk is fundamentally important, especially to investigators of nutritional and dairy manufacturing problems. No satisfactory method of determining calcium ion concentration in milk has been found and, largely for this reason, the state of the phosphate phosphorus has not been definitely determined. Recent work by Holt, La Mer and Chown (1), by Hastings, Murray and Sendroy (2), by Sendroy and Hastings (3), and by Shear and Kramer (4) has done much toward clarifying the phys- ico-chemical phase of the deposition of calcium salts in bone. The present article attempts to transfer to milk equilibria the results of these investigations on the behavior of the phosphate- calcium system, and to calculate therefrom the calcium ion con- centration and the ealcillm-phosphate ion product of milk. It is realized that some of the assumptions made are open to criticism and it is hoped that both the assumptions and the calculations will be examined critically, but with constructive purpose. A structure built on any assumption regarding the degree to which the calcium of mill~ is ionized is rather insecure on account of the presence in milk of considerable quantities of citrate and phosphate and on account of the comparatively large quantity of calcium itself. The assumption of complete ionization of can cium in blood serum, on the other hand, seems justified for mathe- matical purposes and has been made in the calculations of the investigators cited above. That, however, at least a part of the calcium of blood ser~zm exists as a slightly, or entirely, unionized but diffusible compound has been indicated by work of Green-