Recently, a reader wrote to us, asking: How do accelerated and conventional preservative testing compare? What can each measure?
This led us to industry expert John Yablonski, of BioControl, who provided the following answer.
The accelerated preservative efficacy challenge method has been determined to be comparable yet more rigorous than the USP, BP/EP, PCPC (CTFA), AOAC or ASTM methods. It is somewhat unique in that the procedure accurately measures both the rate of kill and the capacity or robustness of a preserved product over a relatively short time period; i.e., 14 days vs. 28 days or more with conventional methodologies.
This is accomplished by determining the rate of kill by plating inoculated samples at 1, 3 and 7 days after the initial inoculation, and including a second inoculation at 7 days to determine robustness at 14 days. The pharmacopeial and standard cosmetic industry methods do not have this capability, although the PCPC methods do allow for the option for re-inoculation.
Conventional preservative challenge protocols generally use a single inoculation and call for a minimum 28 of days incubation. Although the sequence of individual test days varies between the various compendium and standardized methodologies, the key interpretive data points, where specified organism reductions are required, are essentially the same as the accelerated method; i.e., 7 days and 14 days.
One other important point: the compendium pass/fail criteria of acceptability are generally far less stringent than what the industry generally considers acceptable to protect products from potential contamination issues. This protocol has been in use for years by numerous cosmetic, personal care and household product companies as it provides a relatively rapid assessment of preservative system capability and is capable of accurately predicting long-term preservative system stability.
To gain a bit more perspective on the concept and supporting documentation of this methodology, refer to JI Yablonski and SE Mancuso, Preservative efficacy testing: Accelerating the process, Cosmet Toil 122 (10) 51–62 (2007).