Tooth decay (caries) is a common problem affecting teeth, and affects people of all ages. There are bacteria which live in plaque, and make it very sticky. These bacteria feed on sugars which come from food and drink (such as sucrose, glucose and fructose), and produce acids. The acids they produce can damage the tooth’s surface by softening it (dissolving away the minerals in the tooth’s outer surface which is called enamel).
Once all the sugar has been used up, the acid gradually disappears and the tooth’s surface starts to repair itself. Each time something sweet is eaten or drunk, plaque bacteria produce more acids, which attack the tooth’s enamel again and softens it.
So the acidity of plaque (its pH) goes up and down periodically. The pH of plaque drops (becomes more acid) each time sugar is eaten and plaque bacteria produce acid.
The pH then gradually returns back to normal (with the help of saliva) until the next time plaque bacteria feed on sugars.