Methods: Dissolution rates of hydroxyapatite discs were measured using a pH stat technique. Dissolution was measured in citric acid solution (0.3% w/v, pH 3.2). 12 different common food additives were tested, all at 0.02%w/v, to determine whether they had any effect on hydroxyapatite dissolution rate.
Results: Pyrophosphate, tripolyphosphate and a linear polyphosphate (average 25 phosphate units) significantly reduced hydroxyapatite dissolution rate by 35, 46 and 64%, respectively. Xanthan gum and carboxymethylcellulose significantly reduced hydroxyapatite dissolution rate by 29 and 16%, respectively. 25-unit polyphosphate exhibited considerable persistence of action, causing a reduction in dissolution rate for 8 x 30 min exposures to citric acid after treatment, but xanthan gum did not exhibit substantial persistence of action. The combination of polyphosphate and xanthan gum at the levels used enhanced the effect, reducing hydroxyapatite dissolution rate by 70% and exhibiting similar persistence of action to the solution containing only polyphosphate.
Conclusion: The protective effect is ascribed to the binding of condensed phosphate or to the formation of an adsorbed layer of gum at the hydroxyapatite surface. These compounds are suggested to have potential as erosion-reducing agents in soft drinks.
This research was supported by GSK Consumer Healthcare.