Tooth decay: how long teeth sit in acid

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Fluoride
Fluoride reacts with enamel to form fluorapatite, which dissolves less readily, lowering the critical pH at which the tooth starts to dissolve, and it makes early lesions remineralize faster and more fully; it blunts the harm of each acid spell but does not make sugar irrelevant.

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Plaque turns sugar into acid on the tooth

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The surface of a tooth carries a thin film made of bacteria and sticky material, called dental plaque, a kind of biofilm. After you eat something sugary, the bacteria in this film break the sugar down for energy and give off organic acids as waste; the main one is thought to be lactic acid.

The acid collects in the liquid inside the plaque (plaque fluid), right against the tooth surface. Normally saliva and plaque fluid hold enough dissolved calcium and phosphate to be supersaturated with respect to tooth enamel, so the tooth does not dissolve; only when the pH falls below a certain line does the fluid start pulling mineral out of the tooth surface, which is called demineralization. That line is the critical pH. The often-quoted 5.5 is only an approximation: the more calcium and phosphate around, the lower the line sits, and it may be as low as about 5.1 or as high as 6.5.

Once the sugar has been swallowed and diluted by saliva, the buffering action of saliva can neutralize the acid, the pH climbs back toward neutral, and redepositing mineral onto the tooth surface gains the upper hand; this is called remineralization. Dental caries is these two processes alternating over and over through the day; damage shows only when demineralization wins out and enough mineral has been lost.

Educational content, not medical advice. See a clinician about your own case.

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