Standard preservative challenge test methods used for determining the preservative effectiveness of water-based products are not suitable for certain product formulations such as wax or oil based products, powders, lip balms, etc. For these types of products it is important to use methods that do not change the physical dynamics of the formula in order to accurately predict their microbial stability. The CTFA M-6 Guidelines offer methods for testing atypical cosmetic products such as these.
A summary of the CTFA M-6 method is described below:
- Enough samples are prepared such that there is a separate sample for each sampling time point and for each microorganism in the test.
- The microbial inoculum is prepared in such a way as not to change the physical characteristics of the formula. For example: the inoculum volume may be reduced or an oil based carrier system is used to inoculate oil based formulations when appropriate.
- The inoculum is introduced to the samples either by mixing in to the sample or by surface inoculation (for solid anhydrous products) in order to best imitate normal contamination during use.
- Separate samples are inoculated and used for determining the inoculum concentration and for the neutralization and recovery validations.
- The inoculated sample containers are held at room temperature for 14 to 28 days depending on the sample type.
- The inoculated samples are thoroughly mixed prior to sampling. For water-immiscible products (e.g. oils and emulsions), a suitable solubilizing agent is added to the neutralizing broth in order to recover microorganisms present in the test sample. Only the surface of the sample is tested for products where microorganisms would only be recovered from the product surface (e.g., sticks, pressed powders), .
- The product samples are evaluated at specific intervals within the 14 to 28 day period. Sampling is recommended immediately after inoculation and at least 2 of the following days depending on the product type: 2, 7, 14 and 28 days.
- Test sample colonies are counted at each specified interval to determine the amount of microorganisms remaining.
- The log reduction (when applicable) of each microorganism at each interval is reported.
1. Cosmetic, Toiletry, and Fragrance Association 2007. “CTFA Microbiology Guidelines"