There is no “official” protocol for evaluating products for preservation efficacy. Both the US Pharmacopoeia (USP) and the Personal Care Product Council (PCPC) (formerly known as Cosmetics Toiletries and Fragrance Association) (CTFA) have published “Guidelines for Determining the Adequacy of Preservation”. Most microbiological laboratories (Cosmetic, Industrial, Household, and Private Consulting) evaluate preservation efficacy in a manner similar to these protocols. Basically, this consists of the following:
The products that are desired to be protected (OTC pharmaceuticals, cosmetics, laundry detergents, dish detergents, etc.) are prepared with preservative and then inoculated separately with strains of bacteria and fungi obtained from the American Type Culture Collection (ATCC). The samples are stored, then examined for survivors, and these numbers recorded. The PCPC recommends at least 2 such challenges and that the test be carried out for a minimum of 28 days. The USP Method outlines only one such challenge.
In both instances, the recommended criteria for acceptability are as follows:
Bacteria: both methods must demonstrate a “Reduction” in the viable bacteria count within 7 days following each challenge and no increases for the duration of the test. For USP the number is 99.9%, for CTFA 99%.
Fungi:(Yeast + Molds): For PCPC at least a 90% “Reduction” of yeast and mold within 7 days following each challenge and no increase for the duration of the test period. For USP, no increase in levels over the initial inoculum levels.
The Surety Laboratories preservation test procedure is also a variation of this method, however, we employ modifications that we feel give results that are much closer to real world situations. These modifications are outlined below:
Surety Microorganisms (bacteria and fungi, molds + yeasts): all have been isolated from contaminated products (and are stored in these products between tests). We feel these organisms are more adaptable to this type of testing and closer to real world situation than those purchased from ATCC. Also, if desired by our customers, Surety will incorporate any so called “problem microorganisms” isolated from specific products. Surety Laboratories will include these “bugs” in the preservation test procedure in addition to our regular battery of test microorganisms.
The Surety Laboratories procedure incorporates six separate challenges of our battery of test microorganisms (one challenge per week for 6 weeks) and the requirement for acceptable preservation is essentially zero survivors after all six challenges.
We feel that products evaluated by the Surety procedure, using microorganisms isolated from real situations, and subjected to six challenges will result in a formulation that can be considered satisfactorily preserved against the organisms tested. Since it is impossible to test products against the millions of strains and types of microorganisms present in the world, we feel that satisfactory results in our test are indicative of the fact that this product can be deemed “adequately preserved” during manufacture, use and storage.