RESUMO
Dissolution of amorphous calcium phosphate (ACP) in aqueous citrate at varying pH has been studied with perspective of increasing availability of calcium from sidestreams of whey protein, lactose and/or cheese production or on development of new functional foods. ACP formed as an initial precipitate in 0.10â¯molâ¯L-1 equimolar aqueous calcium chloride, sodium citrate, and sodium hydrogenphosphate was used as model for mineral residues formed during milk processing. Upon acidification of the ACP suspension by hydrochloric acid decreasing pH from 6.5 to 4.5, the transformations of ACP occurred through an 8â¯h period of supersaturation prior to a slow precipitation of calcium citrate tetrahydrate. This robust supersaturation, which may explain increased availability of calcium phosphates in presence of citrate, presented a degree of supersaturation of 7.1 and was characterized by precipitation rates for 0.10â¯molâ¯L-1 equimolar aqueous calcium chloride, sodium hydrogencitrate, and sodium hydrogenphosphate with pHâ¯5.5, and for 0.10â¯molâ¯L-1 equimolar aqueous calcium chloride, sodium hydrogencitrate, and sodium dihydrogenphosphate with pHâ¯4.1, with a degree of supersaturation of 2.7. The crystallization processes were similar according to Avrami's model with a half-life for precipitation of approximately 5â¯h independent of the degree of supersaturation. Ion speciation based on measurement of pH, and total concentrations of calcium, phosphate and citrate, and of conductivity and calcium ion activity during precipitation indicates a low driving force for precipitation with calcium citrate complex dominating at pHâ¯5.5 and calcium hydrogencitrate complex dominating at pHâ¯4.1. Calcium hydrogencitrate is suggested to be the species involved in the crystal growth followed by solid state transformation to calcium citrate tetrahydrate.