This will allow the processes of diffusion and neutralisation to exert a greater effect on plaque pH. Also, as before, plaque which is less dense and fully exposed to saliva flow will show a faster rate of pH recovery. It normally takes at least 20 minutes for the plaque pH to reach its resting value but it can take considerably longer depending on the factors described above. Saliva flow rate One of the most important factors governing the overall shape of a Stephan Curve, but particularly the pH recovery, is saliva flow rate. The effect of saliva on the Stephan Curve is easily demonstrated by isolating some dental plaque using cotton wool pads. This is shown in the diagram on the right.
Saliva exerts two effects.
First, it dilutes and carries away metabolites diffusing out of the plaque. Acid neutralisation by bicarbonate is accelerated by salivary carbonic anhydrase. This is secreted by acinar cells of the parotid and submandibular glands and is the only example of a secreted carbonic anhydrase in mammals. Find out more from this review article . In the in vitro method plaque is removed from the mouth at intervals and the pH measured using a micro-pH electrode.