Preservative challenge testing

Since preservative efficacy may vary according to the product, analysis of preservative content often has to be supported by a preservative challenge test. This may also be relevant with certain emulsion systems where the preservative may partition between the different oil and water phases. For simple distribution phenomena, the partition or distribution law normally applies. 

Many users of such easy-care coolants are of the opinion that additional use of a chemical preservative is not necessary. We have investigated this question in extensive laboratory tests by subjecting such water-mixed coolants to a preservation challenge test (S M Boko test Figure 5). 

The preservation challenge test is discontinued when + + + microorganism growth has been found consecutively on at least three streak preparations. After at most twelve inoculation cycles with negative results (-) the test is also discontinued. 

As with evaluation of preservative challenge tests, manufacturers are at liberty to set their own evaluation criteria for such tests. When evaluating user-tests, it is reasonable to expect the presence of micro-organisms in the product immediately after use. Analogous to the preservation challenge test one would expect a reduction in microbe number over a pre-specified time. Returned product samples with > 10 cfu/g suggest product preservation is insufficient. Lower microbe numbers should be reduced over an acceptable time period to < 100 cfu/g. 

Heinzel, M., 1999a. Hygiene of Refillable handwash dispenser systems and validation of preservation challenge tests. Seifen ole Fette Wachse, 125. Jhrg., 6/99, pp. 25-29... 

To evaluate the effectiveness of a preservative, it is essential to test finished formulas. There is no standard for preservative challenge testing, but several different test designs can be used to demonstrate the microbiological stability of a product s preservation system. 

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