Enamel dissolution in citric acid as a function of calcium and phosphate concentrations and degree of saturation with respect to hydroxyapatite

Abstract

The aim of this study was to investigate enamel dissolution in citric acid solutions as a function of solution calcium and phosphate concentrations and degree of saturation with respect to hydroxyapatite (DSHA). The primary relevance of the study is the development of soft drinks with reduced erosive potential. Nanoindentation was used to investigate changes in the hardness of polished human enamel surfaces after 120 s and 300 s exposure to solutions with pH 3.30 and a range of calcium and phosphate concentrations. All solutions were undersaturated with respect to hydroxyapatite, with 0.000 < or = DSHA < or = 0.295. A complex dependence of enamel softening on calcium concentration was observed. Substantial enamel softening occurred in solutions with calcium concentrations equal to or less than 120 mm (DSHA approximately 0.104), but there was little or no statistically significant softening of the enamel for calcium concentrations over 120 mm. This condition may be applicable to soft drink formulation. Furthermore, solutions with DSHA = 0.101 and different calcium/phosphate ratios resulted in different degrees of softening of the enamel. Hence, contrary to assumptions made in many models, enamel dissolution is not simply a function of DSHA, and individual calcium and phosphate concentrations are critical.