Key takeaways
- ISO 11930 defines the organisms, the inoculation and the log reduction criteria.
- Test at release and at end of shelf life, not only at release.
- A single preservative almost never passes without a booster.
A challenge test, also called a preservative efficacy test, inoculates a product with defined microorganisms and measures how quickly the preservative system reduces them. It is the only evidence that a formula is adequately preserved, and it is a protocol rather than a single measurement.
1. The organisms
The standard set is Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Candida albicans and Aspergillus brasiliensis, representing Gram-positive bacteria, Gram-negative bacteria, yeast and mould. Some protocols add a product-specific isolate from the factory environment.
2. The inoculation
Each organism is inoculated at 10 to the fifth or sixth colony forming units per gram. The product is sampled at day 2, 7, 14 and 28 and the surviving count is measured. The reduction over time is the result.
3. Pass criteria
ISO 11930 sets criteria by product category: for a leave-on product, a defined log reduction of bacteria by day 7 and of yeast and mould by day 14 or 28, with no re-growth at the end. The criteria differ for rinse-off products and for products with a high alcohol content.
4. Why it fails
Most failures come from a dose that is too low, a pH outside the preservative’s active range, a high oil load that binds the preservative, or a pack that exposes the product to contamination. A failure is informative; a re-test without a change wastes a month.
5. Timing
The test takes 4 to 6 weeks and should start as soon as the formula is fixed. Running it in parallel with stability saves a month. Running it after stability delays the launch by a month for no reason.
6. End of shelf life
A product that passes at release can fail at month twelve because the preservative has degraded or the pH has drifted. The test should be repeated on an aged sample, typically at the twelve-month stability pull. This is the step most brands skip.
7. The pack
Test in the commercial pack. A jar and a pump present different contamination challenges, and a test in a beaker answers a question nobody asked. For a jar, consider testing after repeated simulated use.
8. Natural systems
A preservative-free or natural system must pass the same criteria. If it does not, the product needs a different system, a lower pH, a lower water activity or a different pack. There is no exemption for a natural positioning.
9. Factory isolates
Adding an isolate from the factory’s own environment makes the test more relevant. A product that passes the standard panel and fails on a factory isolate has a real problem that the standard test would have missed.
10. Records
Keep the protocol, the organism list, the raw counts, the pass criteria and the conclusion. The record should be traceable to the batch tested, because a test on a lab sample does not cover a production batch made with a different raw material lot.
11. Who signs for the result
The test is performed by a microbiologist and approved by a second person. A result signed by the analyst alone is a result with no review. In a market enquiry the reviewer’s name is the evidence that the pass criteria were applied by someone other than the person who produced the number.
12. Where it fits
Preservation testing runs across the cleanser range; the system design continues in the preservation entry.
Reference: Challenge test protocols and laboratory reports, 2025
