Determining Preservative Efficacy in W/O Formulas, Part I: Test Performance and Limitations via Cosmetics & Toiletries

Cosmetic products based on w/o emulsions are widespread and have a long history of use. For this type of emulsion, water droplets are distributed in a lipid phase. Unlike water-miscible o/w emulsions, w/o emulsions do not mix with added water without mechanical action. This has a significant impact on microbiological stability, the performance of microbiological testing and interpretation of results.As microbiological stability is an essential aspect of product quality and plays a central role in safety assessment, questions on microbiological test methodology and corresponding result-interpretation are increasingly relevant. The present article addresses these.