Potassium Sorbate

Description

Potassium sorbate is the inactive potassium salt of sorbic acid used as preservative in cosmetic formulations.  It readily dissolves in water where it converts to sorbic acid, its active form, at a low pH. Sorbic acid generation is very pH dependent. While it shows some activity up to pH 6 (about 6%), it is most active at pH 4.4 (70%). At pH 5.0 it is 37% active. As sorbic acid, it is considered to be active against mold, fair against yeast and poor against most bacteria. Sorbic acid is an unsaturated fatty acid and as such is subject to oxidation (use of an antioxidant like Mixed Tocopherols T50 is recommended). It is also sensitive to UV light and may turn yellow in solution.  Gluconolactone is reported to stabilize potassium sorbate against discoloration and darkening in aqueous solutions and may be useful in stabilizing sorbic acid in the water phase of a product.

While sorbic acid is naturally occurring in some fruits (like the berries of the mountain ash), virtually all production of sorbic acid, from which potassium sorbate is derived, is manufactured synthetically, and is a nature-identical compound chemically equivalent to the molecule found in nature.

Sorbic acid can cause contact dermatitis at concentrations above or below 0.5%. Studies show that if it is used at a concentration of no more than 0.2% it is unlikely to constitute a safety hazard.

Potassium sorbate is not a broad spectrum preservative for cosmetic use and should be combined with other preservatives. If potassium sorbate is used as a preservative, the pH of the finished product may need to be reduced for potassium sorbate to be effective.  This is because potassium sorbate is the inactive salt form of sorbic acid.  To be useful, the pH of the formulation must be low enough to release the free acid for useful activity.