The effect of pH and concentration on the rates of kill of benzoic acid solutions against E. coli.

Mathematical models were developed which related benzoic acid concentrations to their D-values at room temperature. Linear regression was used to determine D-values (i.e. the time required for a particular concentration of preservative at specified pH, temperature and medium to cause a 90% reduction in the number of viable organisms, e.g. Escherichia coli. A number of concentrations of benzoic acid at either pH 3 or pH 4 were used. Linear regression of the log D-values versus the log of the concentration was used to derive power curves (a minimum of four points per pH were examined). Concentration exponents, eta-values (the logarithmic values relating changes in rates of kill to specified changes in concentrations) and A-values (extrapolated D-values at 1% concentration), were determined. The effect of pH on the eta and A-values of benzoic acid on E. coli was investigated. Benzoic acid was found to be more sensitive to pH changes than was anticipated from the Henderson-Hasselbalch equation which relates dissociated and undissociated fractions at the two pH levels investigated. However, the sensitivity of preservative activity to dilution at both pH values (3 and 4), as given by the eta-value, remained similar.