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Effective vs. Ineffective Preservation Using Water Activity*

By: David Steinberg, Steinberg & Associates

Posted: January 4, 2011

Important to understanding preservation in cosmetics and personal care is the discussion of water activity, as was discussed last week in Tony O'Lenick's Comparatively Speaking column, titled "Water Content vs. Water Activity." The present column takes this concept a step further with expert David Steinberg's explanation of how to use water activity for effective preservation.

*Editor's note: This column was adapted with permission from chapter 8 of Preservatives for Cosmetics, Steinberg's book published by Allured Books.

The concept of water activity to prevent the growth of microorganisms is as old as time and as new as the 1960s. All microorganisms need sufficient water and nutrients to grow. The oldest use of water activity to prevent spoilage goes back to honey.

Honey would appear to be the ideal growth medium for microorganisms. It contains water and sugar. Although honey is produced and used everywhere in the world, it does not need to be preserved. The reason lies in the sugar, which makes the water unavailable for microorganisms to utilize for growth. Therefore, there is an important difference between water content and the available water. This is known as water activity.

Water activity (a_w) is defined as the ratio of a product's water vapor pressure compared to pure water at the same temperature, as shown in the following formula: