Dental caries results from a demineralisation of the tooth enamel, usually due to acids lowering the pH below 5.5, the level at which tooth enamel will start to demineralise. The principle cause of dental caries is commensal bacteria, especially Streptococcus mutans, along with a range of lactobacilli that help lesions progress.

Bacteria accumulated in dental plaque, a biofilm adhering to the tooth, will digest fermentable carbohydrates such as sugars, producing acids. Low pH causes the tooth’s surface matrix to release hydroxyapatite, [Ca10(P04)6(0H)2] into the saliva, weakening the enamel’s integrity.⁷

Cariogenic activity tends to be localised under bacterial plaque that has adhered to the enamel. However, ingesting acidic foods (the popularity of sour sweets has worried dentists) or fluids such as still or carbonated soft drinks, fruit juices or vinegar, can reduce saliva pH levels to the point that hydroxyapatite starts to dissolve from across all tooth surfaces. Soft drinks containing citric acid and ascorbic acid are particularly acidic, with one study finding their pH was between 2.42 and 3.46.⁸

More prolonged exposure to an acidic pH increases the potential damage, with a more widespread dental erosion, and it becomes harder to restore the enamel matrix integrity. If damage to the enamel penetrates through to the dentine, the rate of decay increases.⁹