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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Lower water activity, water partitioning: Lower the water activity and water partitioning of a preservative to prevent the migration of it away from the aqueous phase. This can be accomplished by increasing glycerin and other polyols above 5%; reducing the surface tension between the oil phase and water phase with functional siloxanes, particularly dimethicone polyethers and flurosilicones; and minimizing sources of energy for microbial growth (e.g., carbohydrates, anionic surfactants, proteins, and natural gums).

Optimize the pH of the aqueous phase: This provides optimum preservative system activity. Also, use preservatives at the appropriate pH for the product. For example, organic acids or organic acid combinations should not be used over a pH of 6.0.

Enhance preservation system activity: Use EDTA to increase the microbial cell wall permeability of Gram negative bacteria cell walls by complexing it with the magnesium in the cell wall. This enhances the penetration of preservatives into the cell to capture essential micronutrient metals (e.g., iron) and more effectively eliminates Gram-negative bacteria, particularly with quaternary ammonium compounds, parabens, sorbic acid, imidazolidinyl urea and DMDM hydantoin. Other examples of possible chelating agents include citric acid and sodium citrate, etridonic acid, pentasodium triphosphate, sodium gluconate, phytic acid and sodium phytate. It should be noted that all the ingredients listed here do not necessarily enhance preservation in all formulas. It is important to choose based on the given application. For example, nonionics inactivate rather than enhance parabens. Also, there has been some discussion that amphoterics could have a negative effect on preservative enhancement.

Consider other factors when choosing a material: These can include the material’s odor; for example, DMDM hydantoin can have a fishy smell, and phenoxyethanol a somewhat unpleasant scent. Also consider the preservative’s discoloration potential. For instance, clear shampoos can turn yellow in the presence of urea. Some natural botanical oils will also discolor a formula over time in the presence of preservatives; the formulator should check their synergistic stability before using them in a formulation. Finally, safety and allergenicity are inherent concerns with all bioactive materials.

Consider ingredients that can enhance preservatives: Ingredients that can effectively enhance preservatives include cationics (quaterniums), due to their high antimicrobial activity, and anionics such as fatty acid soaps, alkyl benzene sulfonates and palmitic acid salts, for their weak antibacterial effects. Amphoterics including lauryl betaine and N-lauroyl-DL-phenylalanine may demonstrate similar activities to some cationic derivatives; however, these are not well-substantiated.