D-VALUE IN PRESERVATIVE EFFICACY 171 61 0 ' lO 5 HOURS Figure 5. Effect of glyceryl monolaurate concentration on the rate of inactivation of S. aureus in lotion. Symbols: • lotion containing 0% glyceryl monolaurate (control), • lotion containing 0.1% glyceryl monolaurate, and • lotion containing 0.2% g|yceryl monolaurate. linear regression. The correlation coefficients calculated for the studies described here ranged from -0.97 to -1.00. Thus, there was a very good fit of the data to the linear regressions. As was stated in an earlier report, $. aureus was used as the standard test organism in the preservative efficacy tests because this organism had greater resistance to the preservatives than other test bacteria in our culture collection (3). No attempts were made to calculate preservative death time curves for P. aeruginosa in these tests because the D-values obtained with the lowest concentrations of methylparaben, formaldehyde and glyceryl monolaurate were within the acceptance criteria (3). There was excellent agreement between predicted and actual preservative concentra- tions required to obtain a given D-value. For example, it was predicted that a D-value of 4 hr would be obtained by incorporating 0.2% methylparaben into lotion--this was found to be the case experimentally. These studies demonstrate the usefulness of D-value determinations and the preserva- tive death time curve in selecting the proper concentration of preservative to use in a cosmetic product.

172 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS O0 0:1 0".2 0:3 CO NCE NT R A TION (%) Figure 6. Preservative death time curve for S. aureus in lotion containing different concentrations of glyceryl monolaurate. ACKNOWLEDGEMENTS I thank Bill Dickman and Joan Barger for their assistance in preparing the test samples. REFERENCES (1) Anon, "Microbiological tests, antimicrobial preservatives--effectiveness," United States Pharmacopeia XIX, The United States Pharmacopeial Convention: Rockford, Md., 1975 pp 487-592. (2) Preservation Subcommittee of the CTFA Microbiological Committee, A guideline for the determina- tion of adequacy of preservation of cosmetics and toiletry formulations, TG/I Cosmet. Journal, 2, 20-23 (1970). (3) D. S. Orth, Linear regression method for rapid determination of cosmetic preservative efficacy,J. Soc. Cosmet. Chem., 30, 321-331 (1979).