The preservative efficacy testing method for powdered eye shadows

Preservative efficacy testing is based on a sample inoculation using a microbial suspension with a determined amount of colony-forming units (CFU). After that, the number of survivors is investigated by periodic evaluations, and the results are compared with specifications. When liquid cosmetics are evaluated, it is easy to obtain homogeneity between the inoculum and the sample, but for a powder sample it cannot be guaranteed. In this context, freeze-dried microorganisms could be used to help the homogenization. In this research, the preservative efficacy is evaluated using a powdered eye shadow. The microorganisms used were Staphylococcus aureus, Pseudomonas aeruginosa, Candida albicans, and Aspergillus niger. The challenge tests were performed in samples with (P) or without (NP) added preservative. The methods used to evaluate the results were the ones described in the official compendia and in the cosmetics guides of international associations, also using linear regression in calculating the D-value. The results showed that it is possible to use freeze-dried microorganisms instead of suspension to evaluate the preservative efficacy of cosmetic solids. The microorganism stability was verified by the determination of the microbial load and the minimum inhibitory concentration after freeze-drying and during the following six months.