What Is a Challenge Test? How to Interpret a Challenge Test Report?

Microbial contamination in cosmetic products can cause physical and chemical changes to the product. The microbial load of a product must remain within limits throughout its shelf life. Various preservatives can be used to ensure the microbial safety of cosmetic products. The test conducted to determine the effectiveness of a preservative in any product or to investigate various substances as preservatives is called a challenge test or preservative efficacy test. Preservative Efficacy Tests hold an important place in the reliability of cosmetic formulations and consumer acceptance. For this purpose, the antimicrobial activity of preservative substances used must be maintained throughout the product's use and shelf life.

When Is Challenge Test Applied?

Challenge Test is used to evaluate whether a cosmetic formulation is protected against microbial effects. It is applied to all cosmetic products on the market.

When Is It Not Required?

It is not required for cosmetic products determined to have low microbiological risk. According to ISO 29621 standard, product groups with low microbiological risk are listed below:

Physicochemical Factor Limits

pH ≤3.0 pH ≥10.0 Ethanol or other alcohols ≥20% Filling temperature ≥65.0°C Water activity ≤0.75 Solvent-based products ≥25% Oxidizing products ≥25% Aluminum chlorohydrate ≥25% Which Microorganisms Are Used?

For Challenge Test, the microorganism strains used are Pseudomonas aeruginosa, Staphylococcus aureus, Escherichia coli, Candida albicans and Aspergillus brasiliensis.

What Are the Limit Values for Challenge Test?

The limit values specified in the ISO 11930 standard for Challenge Test are as shown in the table below: T7 T14 and T28: These represent the status of the sample on days 7, 14 and 28. N value: The number of bacteria detected when the product is analyzed on days 7, 14 and 28 after the relevant bacteria have been inoculated into the product. N0 value: It represents the amount of microorganism inoculated into the product. R value: The difference between the logarithm of the N0 value and the logarithmic value of the number of microorganisms detected when the product is analyzed on days 7, 14 and 28. (Logarithmic calculation is performed in microbiological efficacy tests.) Example Calculation: Calculation for R value on day 7: It is calculated by subtracting the logarithmic value of the bacterial count obtained from the inoculation performed after 7 days from the logarithm of each bacterium's N0 value. If a bacterial growth of 100 CFU is observed for Staphylococcus aureus, the logarithm of 100 CFU is first taken for the result (R=2 for 100). Then this value (R=2) is subtracted from the logarithm of the N0 value (R=6.50), resulting in R=4.5. After all these calculations are completed, results are evaluated according to Criterion A and Criterion B.

How Is It Interpreted?

For ISO 11930 Challenge Test, cosmetic products are expected to meet one of the following 3 criteria. a) If the formulation meets criterion A, it is accepted that the microbiological risk is tolerable and the cosmetic product meets the standard requirements without any additional justification. b) If the formulation meets criterion B, the microbiological risk analysis must demonstrate the presence of control factors independent of the formulation. For example, a protective packaging such as a pump provides a higher level of microbial protection than a jar. This pump should be considered as a protective device to reduce microbiological risk. c) If the formulation does not meet the requirements of criterion A or B, then the condition of the product must be evaluated independently according to microbiological risk assessment. For example; single-use products can be considered in a compensable microbiological risk group if the microbiological analysis results of the finished product are satisfactory at the time of market release. Therefore, if the risk analysis demonstrates the presence of enhanced control factors (risk reduction), then the cosmetic product is accepted to meet the standard requirements. If the cosmetic product does not comply with any of the three situations above, it is accepted that it does not meet the standard requirements. Çağlayan Çatal / Technical Director / Saniter