The pH of plaque and concentration of lactic and volatile acids present before and after exposure in vivo to sugar has been investigated. Plaque was weighed and the acids quantitatively extracted, volatiles were estimated by GLC and the lactate isomers enzymically. All samples contained acetate, propionate, n-butyrate, L(+) and D(––) lactate. Total acid (mostly volatile) in plaque after overnight fasting and 2.5 h after breakfast was similar (3.0 ×10––5 mmol/mg wet weight). 5 min after exposure to one sugar lump (= 40% w/w sucrose/saliva), at the pH minimum, the concentration was 5 × 10––5 mmol/mg wet weight, the major change being an 8-fold rise in L(+), a 5-fold rise in D(––) lactate and a halving of volatiles. Increased sugar exposure (five successive lumps) did not immediately produce more acid than one lump, but, after 30 min, more L(+) lactate was present from the increased sugar and the pH recovery was slower. Plaque before or 60 min after sugar had more total volatiles than lactate. The mean values for total and individual acids produced from glucose and sucrose did not differ significantly (p > 0.5). Although the volatiles are the dominant acid radicals around neutral pH the ‘Stephan curve’ is due to production of L(+) and to a lesser extent D(––) lactic acid.

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