What is challenge testing?

Whether your products contain traditional preservatives, or avoid contamination via alternative means, antimicrobial fastness is a legal must. Barbara Derczo and Marco Piacentini explain the process of assessing cosmetic antimicrobial efficacy

Cosmetics Business asks, what is challenge testing, and how can we use this knowledge to avoid contamination?


Over to the expert…

One of the key components to take into consideration during product development is ensuring product microbiological stability and, in turn, safety.

Antimicrobial preservatives are ingredients added to products to protect them from micro-organisms that will inevitably contaminate the product during its use by the consumer and once the packaging has been opened.

The load of antimicrobial preservatives depends on the product’s microbiological risk, which can be assessed on a case-by-case basis by toxicologists and/or safety assessors during the product development phase.

Preservatives in a formulation help to protect the consumer from exposure to contaminated products; the following test methods are used to determine the preservative robustness of a product formulation:

SO 11930: This is the referral method recommended by EU cosmetic law 1223/2009, one of the most severe and structured regulatory systems in the cosmetics industry.

As with all ISO norms, it is a standard and harmonised protocol accepted globally. It refers to two different levels of acceptability criteria: criteria A, which is the preferred and the most rigid and criteria B, which is suitable only if accompanied by safety assessor approval.

USP <51> Antimicrobial Effectiveness Testing (EP and JP): This is the most widely used method to evaluate preservative effectiveness in aqueous-based products. Differences occur in the acceptability criteria showing EP as the most stringent method while the JP and USP allow for more flexibility.

CPC Microbiology Guidelines M-3 Method For Preservation Efficacy Testing Of Water-Miscible Personal Care Products: The Microbiology Committee of the Personal Care Products Council (formerly known as the Cosmetic, Toiletry and Fragrance Association) felt a need for an alternative test method to 51. Through industry-wide collaborative testing, M-3 was designed with a more stringent pass/fail criteria as compared with USP <51>.

PCPC Microbiology Guidelines M-4 Method For Preservation Efficacy Testing Of Eye Area Personal Care Products: In the case of eye area cosmetics, adequate preservation of the product is crucial. These products are used frequently and in the case of a product like mascara the same brush is used repeatedly to apply the product, so in effect the brush is exposed to micro-organisms and placed back in the product container until the next use.


One of the key components to take into consideration during product development is ensuring product microbiological stability and, in turn, safety


PCPC Microbiology Guidelines M-5 Method For Preservation Efficacy Testing Of Nonwoven Substrate Personal Care Products: This method refers to products known as ‘wipes’.

PCPC Microbiology Guidelines M-6 Method For Preservation Efficacy Testing Of Atypical Personal Care Products: Some personal care products are anhydrous and cannot be tested by conventional methods. These include oils, powders and wax-like products. Anhydrous products having a low water activity may be considered ‘self-preserved’, since micro-organisms require water to grow.

This method, M-6, addresses the issues in testing these types of products.

PCPC Microbiology Guidelines M-7 A Screening Method For Preservation Testing Of Water-Miscible Personal Care Products: The time from lab bench to the market is crucial in the very competitive personal care product industry. This method is used to screen numerous preservative systems to help choose the most effective one. This method does not replace the M-3 Method or USP <51>, but does identify those systems that have a high probability of meeting the passing criteria for these methods.


All above methods involve the introduction of a controlled inoculum to the test product.

Test organisms include (but are not limited to) Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Candida albicans and Aspergillus brasilensis.

Testing is then done to determine the number of surviving organisms at different time points. Based on the chosen test method, a specified log reduction in the number of micro-organisms recovered must be obtained at certain time points in order to meet the criteria for passing the test.

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