Objective: Ethanol, pH and water activity are three well-known parameters that can influence the preservation of cosmetic products. With the new constraints regarding the antimicrobial effectiveness and the restrictive use of preservatives, a D-optimal design was set up to evaluate the influence of these three parameters on the microbiological conservation.
Methods: To monitor the effectiveness of the different combination of these set parameters, a challenge test in compliance with the International standard ISO 11930: 2012 was implemented. The formulations established in our study could support wide variations of ethanol concentration, pH values and glycerin concentration without noticeable effects on the stability of the products.
Results: In the conditions of the study, determining the value of a single parameter, with the tested concentration, could not guarantee microbiological conservation. However, a high concentration of ethanol associated with an extreme pH could inhibit bacteria growth from the first day (D0). Besides, it appears that despite an aw above 0.6 (even 0.8) and without any preservatives incorporated in formulas, it was possible to guarantee the microbiological stability of the cosmetic product when maintaining the right combination of the selected parameters.
Conclusion: Following the analysis of the different values obtained during the experimentation, there seems to be a correlation between the aw and the selected parameters aforementioned. An application of this relationship could be to define the aw of cosmetic products by using the formula, thus avoiding the evaluation of this parameter with a measuring device.
Keywords: aw; challenge test; experimental design; formulation/stability; microbiology; safety testing.
© 2013 Society of Cosmetic Scientists and the Société Française de Cosmétologie.