48 PRESERVATIVES
Challenge testing of preservation systems
Malte Sietzen, Nargiza Cakir, Fernando Ibarra - evident ingredients
Preservation efficacy testing (PET), or challenge testing, has for many been a vital tool in judging a personal care product’s safety and is a requirement for EU market access. According to Regulation (EC) No 1223/2009, manufacturers are required to include the results of a PET in the Cosmetic Product Safety Report, though the exact method to be employed is not specified. In practice, today most PETs in Europe are
conducted according to the methodology laid down in the European Pharmacopoeia chapter 5.1.3, but tests according to ISO norm 11930 are becoming increasingly popular as well. For both tests the execution is similar: the cosmetic product is separately inoculated with five specified microorganisms, and the number of remaining colonies is determined at set points in time over the course of 28 days. From the colony counts, logarithmic
reduction factors of the initial microbial inoculation concentrations are calculated. These factors describe how quickly the preservation system was able to reduce the microbial contamination and are the basis for determining if the sample has achieved one of the passing grades (‘A’ or ‘B’) or failed the test. While PETs are currently the best way to
ensure microbiological safety of personal care products, they also carry a major drawback: the colony count measured is subject to a great amount of measurement uncertainty. In fact, when challenge tests of an identical
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S. aureus P. aeruginosa
E.coli Figure 1: Challenge test results with Evicide levulinate B PERSONAL CARE July 2022 C. albicans A. brasiliensis
sample are conducted in different laboratories, they will commonly yield different colony counts, which may result in different assessment results. This has been investigated in 2012
by GÖCH in a round robin test with 20 participating laboratories. Between laboratories, deviations of the logarithmic colony count reduction of 5-20%, in some cases even up to 40% were found. The same study revealed that this phenomenon is not
A ■ B ■ Failed ■
limited to the case of different laboratories, even twofold repetition by the same laboratory will show a disparity – though this intra-laboratory variance is less pronounced than the inter-laboratory one. Despite this lack of reproducibility and
the obvious presence of measurement uncertainties, we do not wish to discredit PETs as a means of ensuring a products safety and eligibility for market entry. Test protocols and result interpretation make allowance for this, and the methodology is generally well accepted. However, at evident ingredients we also
use challenge tests as an important R&D tool to develop new preservation systems – as well as to better understand those systems already in existence. Yet given the test variance we have just described, how can we trust our test results and develop suitable preservation solutions for various demands? After all, a well-working preservative might be overlooked due to a lacklustre performance in the PET, while at the same time exciting test results may turn out to be a dead end.
Bringing order to the chaos of data As with many experimental measurements afflicted by large uncertainties, it is helpful not to examine a single measurement at a time, but to bundle a number of tests conducted under comparable conditions, analyse the
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Number of Challenge Tests
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