The Preservative Efficacy (Challenge) Test plays a critical role in verifying the effectiveness of a cosmetic product’s preservative system. Its main purpose is to confirm that a product remains microbiologically safe throughout its declared shelf life and during the Period After Opening (PAO). For most water-containing cosmetic products, this test is mandatory and forms a core element of cosmetic safety assessment.

The Principle of the Challenge Test

The Challenge Test earns its name because laboratories deliberately expose the cosmetic formulation to high levels of microorganisms. This approach simulates real-life contamination scenarios, such as repeated consumer contact with fingers or applicators during normal use.

ISO 11930 serves as the primary international standard for performing this test. It specifically defines how laboratories must evaluate preservative efficacy. ISO 29621, by contrast, focuses on microbiological risk assessment and does not govern test execution. In some markets, authorities may also accept regional standards such as USP or the European Pharmacopoeia.

Key Steps of the Challenge Test Procedure

The Challenge Test follows a structured and controlled process designed to measure preservative performance accurately.

During inoculation, the laboratory introduces known quantities of standardized microorganisms into the product. These typically include bacteria such as Pseudomonas aeruginosa and Staphylococcus aureus, yeast such as Candida albicans, and mold such as Aspergillus brasiliensis.

After inoculation, the product remains under controlled storage conditions, usually at room temperature. Analysts then measure microbial levels at specific time points, most commonly on days 7, 14, and 28.

The evaluation phase focuses on how quickly the preservative system reduces microbial populations and whether it prevents regrowth throughout the test period.

Interpretation of Results: Pass or Fail

ISO 11930 defines clear acceptance criteria to determine whether a product passes or fails the Challenge Test. The standard categorizes these limits as Criterion A or Criterion B.

For example, bacterial populations must typically show at least a 3-log reduction within the first seven days. Microbial counts must then remain low and stable until day 28. When a product meets these requirements, it demonstrates that its preservative system can protect against contamination during normal consumer use.

The Role of ISO 29621 in Preservative Assessment

ISO 29621 supports preservative assessment by guiding manufacturers through microbiological risk evaluation. Unlike ISO 11930, it does not test preservative efficacy directly.

This standard allows manufacturers to identify products that qualify as low risk and may not require a Challenge Test. Products with very low or very high pH, high alcohol content, or anhydrous formulations often fall into this category because their composition naturally inhibits microbial growth.

Conclusion

The Preservative Efficacy (Challenge) Test remains a fundamental requirement for cosmetic products that contain water. By actively confirming preservative performance, the test ensures that products stay safe, stable, and compliant throughout their intended use by consumers.

Ensure your Preservative Efficacy Test meets ISO 11930 requirements and supports product safety with reliable data. Contact us!