Optimizing Formula Preservation

By: Eric S. Abrutyn, TPC2 Advisors Ltd.

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

page 4 of 7

To determine efficacy against bacteria in a leave-on product, the test typically is conducted after less than 4 hr, and in a rinse-off formula, less than 8 hr. For yeast and molds, the test typically is conducted for leave-on products after less than 6 hr and for rinse-offs, less than 28 hr. It is important to remember that every change to the formula, such as the water content/ratio, ingredients, chelator, humectants level, fragrance or level of fragrance, during development requires a preservation re-evaluation.

Preservative Considerations
As noted, in order to choose an effective preservative system, one must consider interactions with other raw materials as well as when to process the preservative, sourcing microorganism-free raw materials, and choosing appropriate processing equipment and packaging. Following, and as shown in Table 1, are some suggestions from experienced formulators to ensure adequate preservation and a sterile environment.

Add preservatives to the water phase: Even when a preservative is oil-soluble, add it to the water phase of a formula to improve preservative contact with the aqueous phase, and so that it is not partitioned in the water-oil interface. Also, when adding preservatives to the water phase, the formulator should be careful of the aqueous phase pH, especially with parabens; often the water phase is alkaline, which can hydrolyze the parabens. Finally, if possible, add some preservative after emulsification for o/w emulsions to provide better aqueous phase preservation.

Consider combinations of preservatives: To develop a broad-spectrum microorganism kill, consider combinations of preservatives. Broad-spectrum activity is an important attribute of a preservative system, whether the combination preservative system is a single component or multi-component system. Also, it is better to add an antifungal compound with an antibacterial agent for improved broad-spectrum activity since each has a specific and complimentary function. In addition, using auxiliary ingredients such as pentylene glycol, hexylene glycol, glyceryl caprate and caprylate, sodium anisate or sodium levulinate can enhance preservative activity in the aqueous phase or at the aqueous-oil interface.

Test the raw materials: With regard to raw materials, test and reduce microorganism presence in them to eliminate a potential strain on the preservation system in a formula.