Direct contact membrane method for evaluating preservative efficacy in solid cosmetics

Synopsis A method was developed for evaluating in-product preservative efficacy of solid cosmetics. Microbes (10(5)-10(6) colony-forming units) resting on membrane filters were placed in direct contact with the surface of pressed eye shadows at room temperature in a moist chamber. At daily intervals, or as appropriate, the filters were removed and viable counts were determined by pour plates of modified letheen agar. Linear regression analysis was used to evaluate preservative efficacy. Decimal reduction times (D values) of 1.2-1.7 days were obtained for Pseudomonas aeruginosa ATCC 9027, Staphylococcus aureus ATCC 6538 and Escherichia coli ATCC 8739 on in-house pressed eye shadows preserved with parabens and Germall 115. An average D value of 1.7 was also obtained for P. aeruginosa and S. aureus on a commercially pressed eye shadow containing the same preservatives as the in-house formulation. However, this commercial product was either totally ineffective or about 3-6 times less effective against E. coli. The predicted complete inactivation time corresponding to the above D value was about 9 days and was within the limits of current cosmetic guidelines for bacteria of 'less than 0.1% survival by the 14th day.'