Diffusion of sugars and acids in human dental plaque in vitro

Using a clearance technique, diffusion coefficients, D, were measured in plaque samples in which metabolic activity was eliminated by fixation with glutaraldehyde. Diffusion coefficients for sucrose, sodium acetate and sodium lactate at 37 degrees C were 3.0, 5.0 and 4.8 x 10(-6) cm2 s-1, respectively. Diffusion rate in plaque was between 2- and 5-fold slower than in water with uncharged species retarded less than charged species. However, diffusion in thin layers of plaque (100 microns) was rapid with clearance-halving times. T1/2, of about 6 s for acetate and lactate and 9 s for sucrose. Acetic and lactic acids diffuse faster than sucrose in plaque. Thus, the prolonged lowering of plaque pH following exposure to dietary carbohydrate is due to continued acid production in the plaque rather than to a restriction of diffusion of acid.