Effect of citric acid clearance on the saturation with respect to hydroxyapatite in saliva

Citric acid contained in beverages and foods is one of the main causes of enamel erosion. It was hypothesized that the clearance of citric acid from saliva would influence the degree of salivary saturation with respect to hydroxyapatite (OHAp). Ten subjects rinsed with 2% citric acid, pH 2.1, for 5 s. Before and at 1, 2, 5, 10 and 15 min after the rinse, a small saliva sample was collected. The sample pH, ionic strength and the concentrations of ionized calcium, inorganic phosphate and citric acid were determined. The pH of the saliva decreased from 7.21+/-0.40 (mean+/-SD) at time zero to a minimum of 6.46+/-0.88 at 1 min after the rinse. It was back to baseline value after 15 min. The saliva was supersaturated with respect to OHAp at time zero. After the rinse with the citric acid the saturation level shifted to undersaturation in all individuals except one. At 1 and 2 min after the rinse the saliva was, on average, undersaturated. After 5 min the average saturation level was back to supersaturation. The individual differences were large. The correlations between the rate of clearance of citric acid during the 1 minute and the mimimum degree of saturation with respect to OHAp at 1, 2 and 5 min after the rinse were significant (r = 0.84, r = 0.76 and r = 0.79, respectively). In conclusion, rinsing with citric acid will cause a decrease in the saturation level with respect to OHAp in a highly individual pattern.