Optimizing Formula Preservation

By: Eric S. Abrutyn, TPC2 Advisors Ltd.

Posted: February 26, 2010, from the March 2010 issue of Cosmetics & Toiletries.

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The preservative system must also consider the potential for microbial growth in the water content of a formulation and any nutrients imparted by the preservative itself. This can be accomplished by understanding the raw material composition of the formula, i.e., the presence of natural ingredients, proteins, etc., and by using preserved surfactants. Further considerations include the formula’s stability under process, handling, and storage conditions; color and odor contributions of the preservative to the formula; and the preservative’s overall compatibility with the intended system and incompatibilities with other ingredients that may deactivate or weaken its performance. Finally, the shelf life, safety, ease of handling and cost effectiveness of the material are additional considerations.²

In most cases, a robust microorganism-free product starts with a sound formulation strategy and should include, as a key component, the elimination of all microorganisms. Therefore, it is important for formulators to understand the interaction of each raw material in the formulation with the various preservative options available. An optimum preservation concentration will ensure sufficient preservation levels, i.e., the MIC, yet not cause skin sensitization or reactions with other ingredients in the formula.

Water Activity (a_w) as a Control Strategy
One way to control microorganism contamination in personal care cosmetic formulations is through the determination and control of the available water in a formula, known as the water activity,^{3, 4, 5} which is expressed as the ratio of vapor pressure of the formula against pure water at 20°C. Formula vapor pressure is considered the relative humidity of the head space over a product. The rationale is that microorganisms need water and nutrients to survive and grow. Reducing the amount of bioavailable water will thus retard microbial growth.

Using water activity as a strategy, it is possible to limit the access of microorganisms to nutrients such as proteins through enhanced partitioning between the aqueous and oil phase. By calculating the amount of available water (a_w), one can predict the probability that the formula is preserved sufficiently to suppress the growth of microbes. This will improve the effectiveness of the preservative—i.e., less preservative is needed for improved efficacy—and provide a more robust formula that is able to inhibit the ability of microorganisms to multiple.

Pathogenic bacteria require an a_w value in the range of 0.86 to 0.99, with Gram negative bacteria being the most tolerant of a low a_w. Yeast and mold are resistant to even lower a_w values, at less than 0.70, and spores even more so. To decrease water activity in a formula, several materials can be used: glycerin, sorbitol, glycols including propylene, butylene and polyethylene, sugars, salts and dextrin.