Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth after eating,which lowers the pH. Tooth decay starts when the pH of the mouth is lower than 5.5. Tooth enamel, made up of calcium phosphate gets corroded below this pH. The best way to prevent this is to use toothpastes, which are generally basic, for cleaning the teeth as they neutralize the excess acid and prevent tooth decay.