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Cosmetic challenge testing (ISO 11930 preservative efficacy testing)
Cosmetic challenge testing proves that your preservative system actually works. A finished product is deliberately inoculated with bacteria, yeast and mould, then monitored for 28 days to see whether the formulation suppresses them. CIGREG runs the test to ISO 11930 and hands you a challenge test report your safety assessor can use in a Cosmetic Product Safety Report (CPSR) for the EU, UK, Swiss and US markets.
Reviewed by Cassandra Maddocks, chemist & biochemist · last reviewed 22 September 2026
Key takeaways
- Challenge testing is the standard way to demonstrate preservative efficacy in any product that can support microbial growth.
- CIGREG runs the ISO 11930 reference method: five organisms, sampling on days 2, 7, 14 and 28, graded against Criteria A or B.
- Testing is quoted per product at a fixed price. Formulation, packaging, sample count and target markets drive the figure.
- Your report arrives as a signed document with raw counts, log reductions and an explicit pass or fail.
What is a challenge test?
The 28-day run is why a challenge test cannot be rushed: the time is the data. A formulation can look clean on day 2 and collapse by day 28. Only the later counts show whether the preservative holds up under repeated consumer use.
That gap has commercial weight. It is the difference between a product that stays safe in a bathroom for two years and one that grows mould by week three - after the pallet has shipped.
If your product contains water, a challenge test is usually how you prove the preservative system works, and how an assessor justifies it in the safety report. The principles behind the method are long settled in the literature (Russell, Challenge testing: principles and practice).
What is a preservative efficacy test (PET)?
A PET test for cosmetics is always run on the finished formulation, in its final packaging, from a representative production batch. Test a bench sample and you test the wrong thing. Change the fragrance, the preservative blend or the packaging material afterwards, and the result no longer describes the product you sell.
Retailers and distributors increasingly want PET testing in cosmetics to be evidenced rather than asserted, especially for products sold in jars or used around the eyes. A written study answers that question once, cleanly.
Lock the formulation before you book the run. A late preservative or packaging change means paying for the test twice.
ISO 11930: the method we run
ISO 11930:2019 is the current published edition, and ISO lists the standard as under revision. CIGREG runs the published method and states the edition in your report so your assessor can reference it without ambiguity.
Reference organisms
The panel uses five reference strains: Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Candida albicans and Aspergillus brasiliensis. Two Gram-positive and Gram-negative bacteria, one yeast, one mould - a spread that exposes preservative systems with narrow gaps.
Every strain is traceable to a recognised culture collection, and passage numbers appear in the report. Where a product type warrants it - wipes, aqueous gels, formulas for vulnerable users - the panel can be extended. Agree that before the run, not after.
Sampling days: 2, 7, 14, 28
Day 0 is inoculation. Counting follows on days 2, 7, 14 and 28. Each interval answers a different question: how fast the kill starts, whether it holds, and whether the population recovers. A formulation that creeps back up between day 14 and day 28 has a preservative problem, not a test problem.
Criteria A and B
Results are graded against two acceptance criteria. Criteria A is the stricter of the two and applies to products for children under three, the eye area and mucous membranes. Criteria B applies to most other product categories.
Both use the same organisms and the same sampling days. They differ in how much reduction is required, and how quickly it must be achieved. Both also require the inoculum not to increase between day 14 and day 28. Fail either grade and the preservative system is inadequate for the intended use - the fix is formulation work, not a second run of the same recipe.
ISO 11930 log reduction requirements
| Organism group | Criterion | Day 2 | Day 7 | Day 14 | Day 28 |
|---|---|---|---|---|---|
| Bacteria | Criterion A | ≥ 3 log | ≥ 3 log | ≥ 3 log | ≥ 3 log |
| Bacteria | Criterion B | ≥ 2 log | ≥ 3 log | ≥ 3 log | ≥ 3 log |
| Yeast and mould | Criterion A | ≥ 2 log | ≥ 2 log | ≥ 2 log | ≥ 2 log |
| Yeast and mould | Criterion B | — | ≥ 1 log | ≥ 1 log | ≥ 1 log |
Log reduction is expressed against the initial inoculum. A 3 log reduction means the surviving population is 1,000 times smaller than the amount introduced. Criterion A requires no increase on the initial inoculum at any sampling point; Criterion B permits the population to remain at or below the initial inoculum at day 2 for yeast and mould.
The five ISO 11930 reference strains
ISO 11930 specifies five reference strains: three bacteria and two fungi. Each is named by species and culture collection number.
- Staphylococcus aureus — Gram-positive bacterium, representative of skin-borne contamination.
- Pseudomonas aeruginosa — Gram-negative bacterium, the most common cause of failed challenge tests in water-based formulations.
- Escherichia coli — Gram-negative bacterium, representative of faecal and water-borne contamination.
- Candida albicans — yeast, representative of fungal contamination from skin and the environment.
- Aspergillus brasiliensis — mould, representative of spore contamination from air and raw materials.
Culture collection numbers for each strain are stated in the test report. A challenge test that uses fewer than these five strains does not demonstrate ISO 11930 compliance.
ISO 17516: microbiological limits
A challenge test measures what your preservative does under attack. ISO 17516 measures what is actually in the finished product. It caps total aerobic mesophilic microorganisms at ≤ 1 × 103 CFU/g or ml for general cosmetics, and at ≤ 1 × 102 CFU/g or ml for products for children under three, the eye area and mucous membranes. Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa and Candida albicans must be absent in 1 g or 1 ml.
The two tests answer different questions, and a CPSR usually needs both. A product can pass a challenge test and still fail a finished-product release test.
What is included in a challenge test report
Every CIGREG report contains:
- product name, batch and packaging as tested
- sample receipt condition and test start date
- the standard and edition applied, with the criterion selected
- organism list with strain references and inoculation levels
- counts and log reductions for days 0, 2, 7, 14 and 28
- interpretation against Criteria A or B, with a stated conclusion
- named microbiologist sign-off and issue date
This is not optional paperwork. Under the EU cosmetics framework, the safety report must include the results of the preservative efficacy study - a challenge test, where applicable (Commission Implementing Decision 2013/674/EU). A regulator, a retailer's technical team or a notified body may read it line by line.
Ask for the raw counts, not just the verdict. Numbers travel; verdicts get questioned.
How much does preservative efficacy testing cost and how long does it take?
Four things drive the number: the formulation and its water content; the organisms and criterion required; the number of samples, variants and replicate runs; and the market evidence your file needs. One emulsion tested to Criteria B is a different job from a sunscreen tested across three shade variants, with an extended organism panel and a matching ISO 17516 release test.
Timeline begins at sample receipt. The 28-day incubation is fixed by the method and cannot be shortened by paying more. Reporting follows the final count, and CIGREG confirms total turnaround in the written quote alongside the price.
Send the formulation, the packaging specification and your target markets with the samples. That is what converts a quote request into a fixed price - and what prevents a repeat run after a late formula change.
Which products need a challenge test?
Products that are microbiologically low risk can be exempt - but only with a documented risk assessment behind them, never a verbal assumption.
ISO 29621 low-risk exemptions
ISO 29621 is the standard used to identify microbiologically low-risk products. A product qualifies when its environment does not support the growth or survival of microorganisms, during manufacture, during use, or both.
Typical candidates are anhydrous formulations, products with very low water activity, high-alcohol systems, extreme pH products, and single-use or airtight packaging that blocks microbial ingress. Adding a preservative does not make a product low risk; the whole formulation and the way it is used decide that.
Where the exemption applies, the assessment is written up and referenced in the safety report. Where it is arguable, test. A failed exemption becomes a recall; a challenge test is a line item.
Across the EU (Regulation (EC) No 1223/2009), the UK, Switzerland and the US, the wording differs but the obligation does not: your safety file has to show the preservative system is adequate. MoCRA, enacted 29 December 2022, requires US brands to hold safety substantiation records - and those records rest on the same evidence (FDA, Modernization of Cosmetics Regulation Act). One ISO 11930 study on the finished formulation generally serves all four markets.
Frequently asked questions about challenge testing
What is a challenge test?
A challenge test is a microbiological test in which a cosmetic product is deliberately contaminated with five specified organisms and monitored over 28 days to measure whether its preservative system controls that contamination. The test is run to ISO 11930, which defines the organisms, the inoculation procedure, the sampling days and the acceptance criteria. CIGREG runs the test and issues a report stating the log reduction achieved for each organism at each sampling point.
Is a challenge test mandatory for EU or UK market access?
A challenge test is not named as a standalone legal requirement in EU Regulation 1223/2009 or UK Regulation 1223/2009 as retained, but it is the accepted evidence that a product is microbiologically safe. The safety assessor signing the Cosmetic Product Safety Report needs that evidence, and ISO 11930 is the method most assessors expect to see. In practice, a water-containing cosmetic product cannot obtain a defensible CPSR without a challenge test, so CIGREG treats it as a required step for EU and UK placement.
How long does a challenge test take?
An ISO 11930 challenge test runs for 28 days from sample receipt, because the standard requires sampling at days 2, 7, 14 and 28. The 28-day run cannot be shortened — the time is the data. CIGREG adds report preparation and assessor review on top of the 28-day run.
What does a challenge test report contain?
An ISO 11930 challenge test report contains the product identification and batch number, the five reference strains used with their culture collection numbers, the inoculation level, the surviving population count for each organism at days 2, 7, 14 and 28, and the calculated log reduction at each point. The report states whether the formulation meets Criterion A or Criterion B. CIGREG issues the report in a form that can be filed directly in the product information file alongside the CPSR.
Does an anhydrous or low-water-activity product need a challenge test?
An anhydrous product, or one with water activity low enough to prevent microbial growth, may not need a full ISO 11930 challenge test. That conclusion must be justified in the safety assessment with the water activity value and the formulation rationale — it cannot be assumed from the product name. Where a product sits close to the water-activity threshold, CIGREG advises running the challenge test rather than relying on an exemption argument that an assessor may reject.
How does a challenge test differ from stability testing?
A challenge test measures whether a product's preservative system controls microbial contamination; stability testing measures whether the product's physical, chemical and organoleptic properties hold over time and under temperature stress. The two tests answer different questions and neither substitutes for the other. A product can pass stability testing and still fail a challenge test, because chemical stability says nothing about antimicrobial performance. CIGREG runs both, and both reports are filed in the product information file.