Key takeaways
- Challenge testing at 30 °C is not the same as challenge testing at 40 °C.
- Humidity affects the water activity at the product surface, not the bulk.
- A system that passes ISO 11930 in one lab may fail in another — ask for the report.
Preservative systems are validated by challenge testing: the product is inoculated with specified organisms and the log reduction is measured over 28 days. The standard is ISO 11930, and the pass criteria are fixed. What is not fixed is the temperature and humidity of the warehouse your product will sit in.
A system that passes comfortably at 20 °C can fail at 35 °C, because microbial growth kinetics roughly double for every 10 °C. If your market is the Gulf, Southeast Asia or southern China, test at the upper end of the storage range, not the middle.
The second variable is packaging. A jar invites finger contamination; a pump or tube does not. Challenge testing a formula in a jar and then packing it in a pump is a false comfort — the test should use the real pack, or at least the real worst case.
The third variable is pH. Many systems depend on a narrow pH window, and a formula that drifts during storage will lose protection. Build a pH drift check into the stability protocol, not just a single time-zero measurement.
Practically, ask for the challenge test report, not a claim that the product is preserved. The report should state the organisms, the inoculum level, the temperature, the sampling days and the pack format. If any of those are missing, the test does not tell you what you need to know.
Reference: ISO 11930:2019; internal challenge test protocols
