Reactive Crystallization Behaviour of Calcium Phosphate with and Without Whey Protein Addition

Abstract

The reactive crystallization of calcium phosphate was studied by reacting potassium phosphate and calcium nitrate aqueous solutions in the batch system. When the solution has high supersaturation at the beginning of reaction, amorphous tricalcium phosphate (ACP), Ca3(PO4)2·nH20 was precipitated instantly as the initial solid phase. The formation of ACP depends on the ion activity product of ACP regardless of initial pH. ACP transforms to calcium phosphates such as dicalcium phosphate dihydrate (DCPD), CaHPO4·2H2O, hydroxyapatite (HAP), Ca5OH(PO4)3 and octacalcium phosphate (OCP), Ca8H2(PO4)6·5H2O. Effects of solution pH, reaction temperature and addition of whey protein on the behaviour of reactive crystallization of calcium phosphates have been studied. The precipitation diagram of calcium phosphates was obtained as a function of reaction temperature and initial solution pH, whose ranges were 15–50°C and 5–9. By the addition of whey protein isolate, the nucleation and growth of calcium phosphate were retarded. The existence of proteins slows down the reaction of calcium ions and phosphate ions. By the addition of WPI, the shape modification of DCPD and HAP was observed. DCPD became thicker, gathered in bundles and its surface was uneven and cracked. The surface state of HAP particles does not change, whereas the size changes slightly. The cubic form of HAP was also observed by addition of WPI, which may be formed by the crush after natural drying because HAP obtained was caked very hard by the addition of WPI.