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Why does acid exert an effect?
Tooth erosion is something that affects enamel and dentin. Enamel is mainly composed of crystalline calcium phosphate, in the form of calcium hydroxyapatite.23 In addition there are a number of other elements present, including fluoride. Dentin is slightly less mineralized and also contains collagen.24
There doesn't seem to be a consensus at which pH erosion occurs, with a guide figure for enamel between 5.2-5.5 and dentin 6.0, below which demineralization will occur in the absence of saliva.25
Saliva pH is around neutral - in the range pH 6.5-7, although there are variations in the population and throughout the day. Saliva contains proteins and phosphates, both of which can buffer the pH. In addition, bicarbonate ions can help neutralize acids, produced for example by plaque or as a result of food. Saliva also contains minerals, including calcium and phosphate, aiding remineralization.27 Another benefit of saliva is that it aids swallowing, thus removing much of the acid.
Saliva also replenishes the pellicle, the organic film covering the surface of the tooth. The pellicle exerts a protective action on the enamel with anionic proteins in the saliva being adsorbed onto the tooth surface, strongly attracted to the hydroxyapatite in the enamel.28-29
However, this protective action of the pellicle is limited to less severe erosive challenges and will not entirely prevent enamel from demineralization.29 Should exposure to the demineralization challenge be longer than 10 minutes, the pellicle may be washed away.29