326 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS after inoculation, the APC of the samples containing the parabens were lower than the APC in the control sample. This was not due to the order of sampling, because data for control samples were essentially the same when they were set up first or last. The D-values for the face cream control, the cream with the lower and the cream with the higher concentrations of parabens were 18, 4 and 0.6 hr, respectively. The times predicted for complete destruction of S. aureus in these samples were 63, 19 and 3 hr for the control, the face cream with the lower and the face cream with the higher concentration of parabens, respectively. This estimate was corroborated by APC of 10/ml at the 3-day sampling. The data shown in Figure 3 compare the antibacterial properties of hand lotion containing glyceryl dilaurate with lotion containing glyceryl monolaurate. The Hours Figure 3. Survivor curves showing the effects of substituting glyceryl monolaurate for glyceryl dilaurate on the rate of death of S. aureus in lotion. Symbols: ©--© = lotion with glyceryl dilaurate (control), and I--I = lotion with glyceryl monolaurate.

EVALUATION OF COSMETIC PRESERVATIVE EFFICACY 327 D-values for S. aureus were 2.5 and 4.2 hr for the lotion containing the mono- and di-esters, respectively. The predicted times for complete inactivation of the S. aureus populations in these samples were 14 and 22 hr for the lotion containing the mono- and the di-ester, respectively. No S. aureus were recovered at the 1-d sampling. Preservative efficacy testing with shampoos revealed that the test bacteria had similar D-values and that the molds had significantly larger D-values. The D-values for S. aureus, P. aeruginosa, Bacillus sp., A. niger and A. flavus were 0.67, 0.81, 0.81, 7.7 and 16.7 hr, respectively. The differences in rates of inactivation are readily apparent by examination of the survivor curves for S. aureus and A. flavus in Figure 4. The Hours Figure 4. Survivor curves showing the rates of inactivation of S. aureus and A. flavus in formaldehyde- containing shampoo. Symbols: ©--© = S. aureus and I--I = A. flavus.