Effective vs. Ineffective Preservation Using Water Activity*

By: David Steinberg, Steinberg & Associates

Posted: January 4, 2011

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Measuring a_w is especially useful for formulations that are “atypical” such as w/o emulsions, low water content products, anhydrous products like lipsticks, powders, etc., and for water extracts. When using water extract, the addition of high levels of glycols (they can even be the extracting solvent) can be used to lower the water activity below 0.7 and avoid the need to add additional preservatives.

The typical water activity of some cosmetics is: 0.97 a_w for shampoos, 0.96 a_w for conditioners, 0.91 a_w for liquid soap, 0.86 a_w for hand cream, and 0.98 a_w for hand lotion.²

It is critical to understand that a_w is not cidal (it does not kill, it only prevents growth), and it can only be applied to products that are free of contamination. Hence cGMP’s and hazard analysis critical control point (HACCP) are critical to apply water activity preservation to raw materials and finished formulations. Remember that a_w can only be determined by measurement.

HACCP was originally developed for the space industry in conjunction with Pillsbury Company to assist in the production of food stuffs for astronaut consumption. The purpose was to have a high degree of quality and microbiologically safe foods. Hazard analysis identifies critical control points (CCP), as it relates to contamination. HAACP is now mandatory in the food industry and the US Food and Drug Administration (FDA) is working on this for drugs. It is useful for the production of cosmetics.

CCP can exist in every process and design in the manufacturing of cosmetics. It is used to identify locations where contamination could possibly occur and then establish procedures to monitor and control these specific areas. If one chooses to not incorporate traditional active preservatives or rely on water activity or other “non-preservative” methods, HACCP is a valuable tool to ensure quality.