D-VALUE IN PRESERVATIVE EFFICACY 167 determine how well the data fit the linear regression. All analyses that had a correlation coefficient more positive than -0.97 were repeated. RESULTS The effect of methylparaben concentration on the rate of inactivation of S. aureus in lotion is illustrated in Figure 1. The D-values decreased with increasing concentrations O0 {i •O 1'5 • 2• HOURS Figure 1. Effect of methylparaben concentration on the rate of inactivation of S. aureus in lotion. Symbols: • lotion containing 0% methylparaben (control), • lotion containing 0.1% methylpa- raben, and ,•' =', lotion containing 0.2% methylparaben. of this preservative, and lotions containing 0% (control), 0.1% and 0.2% methylparaben had D-values for S. aureus of 19, 12 and 4 hr, respectively. Similar experiments with P. aeruginosa yielded D-values of 0.5 hr for the control and the samples containing methylparaben consequently, the lotion had adequate preservative efficacy for this organism without the addition of methylparaben.

168 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The D-values obtained with the different concentrations of methylparaben in lotion were used to construct the preservative death time curve for S. aureus (Figure 2). The correlation coefficient for the linear regression was --0.996. This demonstrates that there was an excellent fit of the data to the linear regression. The effect of formaldehyde concentration on the rate of killing of S. aureus in shampoo is illustrated in Figure 3. Here, the D-values ranged from 7.7 hr for the formaldehyde-free control to 0.98 hr for the sample containing 300 ppm formaldehyde. 0 0.'1 0'.2 0:3 CONCENTRATION (%) Figure 2. Preservative death time curve for S. aureus in lotion containing different concentrations of methylparaben. P. aeruginosa was inoculated into a parallel set of samples in this experiment, and D-values of 4 hr and 0.6 hr were obtained for the formaldehyde-free control and the 100-ppm formaldehyde sample, respectively. The D-values obtained with the different concentrations of formaldehyde in shampoo were used to calculate a preservative death time curve for S. aureus (Figure 4). The data gave an excellent fit to the linear regression when the curve was calculated using the D-values obtained with 100, 200 and 300 ppm formaldehyde, because the correlation coefficient was -1.00. No attempt was made to calculate a preservative death time