Saliva is a buffering substance whose pH is higher than that of the acids produced by oral microflora. It contains calcium phosphate, which aids in remineralization of the enamel on the outside of teeth. Saliva also contains amylase, which helps break down food in the oral cavity left after a meal. It also prevents caries through the mechanical flushing of food from tooth surfaces. The lack of saliva prevents remineralization, allowing cavity-causing substances to remain in the mouth for longer periods of time and causing a sustained low intraoral pH. The mouth becomes home to more dangerous cariogenic bacterial microflora. Together these factors cause rapid and widespread onset of caries. Caries due to xerostomia affect the roots and crown of virtually all teeth. In the worst case, a dentist is unable to save the teeth and the person must wear full dentures.