Preservative Challenge Testing: ISO 11930 Explained

Why Challenge Testing Matters

A 2024 study by Alshehrei found something disturbing: 81% of tested cosmetic products showed microbial contamination despite being unused. Even more concerning, 2 of 6 non-contaminated products failed challenge testing against Staphylococcus aureus.

This is why challenge testing exists - to prove a preservative system actually works before products reach consumers.

The ISO 11930 Protocol

ISO 11930:2019 is the international standard for cosmetic preservative efficacy testing. Here's how it works:

Challenge Test Procedure

Inoculation: Product is deliberately contaminated with standardized microorganisms
Incubation: Stored at 20-25°C for 28 days
Sampling: Microbial counts taken at days 7, 14, and 28
Evaluation: Log reduction calculated against Criterion A or B

The Test Organisms

Products are challenged with five representative microorganisms:

Organism	Type	Why It's Tested
Pseudomonas aeruginosa	Gram-negative bacteria	Common water contaminant, opportunistic pathogen
Staphylococcus aureus	Gram-positive bacteria	Skin pathogen, causes infections
Escherichia coli	Gram-negative bacteria	Indicator of fecal contamination
Candida albicans	Yeast	Common fungal contaminant
Aspergillus brasiliensis	Mold	Environmental mold, spoilage indicator

Criterion A vs. Criterion B

ISO 11930 defines two acceptance criteria:

Criterion A (Preferred)

Timepoint	Bacteria	Fungi
Day 7	≥3 log reduction	-
Day 14	No increase from Day 7	≥1 log reduction
Day 28	No increase from Day 14	No increase from Day 14

Criterion B (Minimum Acceptable)

Timepoint	Bacteria	Fungi
Day 14	≥3 log reduction	≥1 log reduction
Day 28	No increase from Day 14	No increase from Day 14

3 log reduction means killing 99.9% of the bacteria. 1 log reduction means killing 90% of the fungi.

Real-World Correlation

Does lab testing actually predict consumer safety? A landmark 1987 study by Brannan et al. answered this definitively:

Challenge Test vs. Consumer Use (Brannan 1987)

Lab Classification	Consumer Contamination Rate
Poorly preserved	46-90% contaminated after use
Marginally preserved	0-21% contaminated
Well preserved	0% contaminated

Products classified as "well preserved" in challenge testing showed zero contamination after consumer use.

This study established challenge testing as a reliable predictor of real-world safety - and it's still cited today as foundational validation of the methodology.

Product Categories and Limits

The EU sets different microbial limits based on product risk:

Category	Products	Limit (CFU/g)
Category 1	Children <3 years, eye area, mucous membranes	≤10² (100)
Category 2	Other cosmetic products	≤10³ (1,000)

Baby products and eye products have 10× stricter limits because contamination risks are higher in these sensitive applications.

Natural Preservatives: Do They Work?

A 2018 study by Kočevar Glavač and Lunder tested 13 antimicrobials (11 natural, 2 conventional) against ISO 11930 criteria:

Natural vs. Conventional Preservatives

Most natural antimicrobials met Criterion A requirements
Exceptions: Levulinic acid, lactobacillus ferment, grapefruit seed extract failed in specific challenge scenarios
Surprise finding: Phenoxyethanol and paraben combinations were inefficient at minimum studied concentrations
This suggests some products may be over-preserved with synthetic preservatives

The takeaway: natural preservatives can work, but require careful formulation and testing. "Preservative-free" claims should be viewed skeptically - either the product has extremely short shelf life, or something else is providing antimicrobial protection.

Rapid Testing Methods

Traditional challenge testing takes 28 days. A 2020 study by Almoughrabie et al. developed a rapid alternative:

HCS-CLSM Rapid Method

Uses High Content Screening and Confocal Laser Scanning Microscopy
Results in 4 hours vs. 28 days for traditional testing
Predictive modeling accurately forecasts 7-day log reductions
Tested chlorphenesin vs. benzyl alcohol against S. aureus
Chlorphenesin showed superior efficacy (Dc values 0.45-17.89 h vs. 1.07-28.09 h)

This accelerates product development while maintaining ISO 11930 compliance standards.

Common Preservative Systems

Based on the research, here are preservative systems that consistently pass challenge testing:

System	Typical Concentration	Notes
Phenoxyethanol + Ethylhexylglycerin	0.8-1.0% + 0.3-0.5%	Most common modern system
Phenoxyethanol + Parabens	0.5% + 0.2-0.4%	Broad spectrum, well-studied
Benzyl alcohol + Dehydroacetic acid	0.5% + 0.3%	COSMOS-approved option
Sodium benzoate + Potassium sorbate	0.2% + 0.2%	pH-dependent (<5.5), food-grade

When Products Fail

Challenge test failures typically occur due to:

Insufficient preservative concentration - trying to use the minimum amount
pH incompatibility - some preservatives only work at specific pH ranges
Ingredient interactions - surfactants can deactivate certain preservatives
Formulation matrix - emulsion type, water activity affect efficacy
Single-preservative reliance - broad spectrum requires multiple actives

Practical Implications

What This Means For Consumers

EU products are tested - the Cosmetic Product Safety Report (CPSR) requires preservation efficacy data.
"Preservative-free" is a red flag - unless the product has very low water activity or very short shelf life.
Expensive doesn't mean better preserved - the 2024 Alshehrei study found premium products failing tests.
Natural preservatives can work - but require higher concentrations and careful formulation.
Follow storage instructions - heat and contamination from fingers compromise preservation.

References

Halla N, et al. (2018). Cosmetics Preservation: A Review on Present Strategies. Molecules. DOI: 10.3390/molecules23071571
Kočevar Glavač N, Lunder M (2018). Preservative efficacy of selected antimicrobials of natural origin in a cosmetic emulsion. International Journal of Cosmetic Science. DOI: 10.1111/ics.12461
Brannan DK, Dille JC, Kaufman DJ (1987). Correlation of in vitro challenge testing with consumer use testing for cosmetic products. Applied and Environmental Microbiology. DOI: 10.1128/aem.53.8.1827-1832.1987
Almoughrabie S, et al. (2020). Rapid assessment and prediction of preservative efficiency using HCS-CLSM. PLoS ONE. DOI: 10.1371/journal.pone.0236059
Alshehrei FM (2024). Microbiological Quality Assessment of Skin and Body care Cosmetics by using Challenge test. Saudi Journal of Biological Sciences. DOI: 10.1016/j.sjbs.2024.103965
ISO 11930:2019. Cosmetics - Microbiology - Evaluation of the antimicrobial protection of a cosmetic product.