Effective vs. Ineffective Preservation Using Water Activity*

By: David Steinberg, Steinberg & Associates

Posted: January 4, 2011

page 4 of 7

Friedel published one of the earliest papers on the use of water activity in determining the requirements of preservation in topical products.¹ This work showed how a formulator could measure water activity using a dew-point/chilled-mirror instrument^b and get reproducible and useful results. Among the examples given were creams, gels, anhydrous lip balms and ointments.

Water activity has become a valuable microbiologist tool for two purposes. First, it helps the microbiologist determine the potential for growth in raw materials. Secondly, as microbiological specifications are required for all ingredients in the EU, determining the a_w and finding this to be below 0.7 (the threshold for growth) allows for an intelligent method to assign the need for microbial testing of ingredients.

Testing for the water activity of the finished formulation before the addition of a preservative can benefit formulators since, if they find that only mold or yeast can grow, they do not need to add preservatives that are only active against bacteria.

There are certain ingredients that lower a_w;however, there appears to be no linear relationship between levels of these ingredients and a_w. There have been models published to predict the effects of addition of water binding ingredients to lower the water activity, but due to the complex nature of cosmetic formulations, these really do not work well. One must measure by instrumentation to determine water activity.

Ingredients that lower water activity include salts, which can lower the water activity below growth such as in the Dead Sea if they are high enough; and glycols and polyols such as glycerin, propylene glycol, polyethylene glycols, etc. There are commercially available mixtures on the market that claim to reduce the a_w  15-20%, making the cosmetic product “self-preserving.” There also have been attempts made to use high amounts of glycols to lower the water activity so no additional chemical preservative needs to be added; however, the aesthetics of the resulting products where unacceptable.