Preservative Efficacy (Challenge Testing)

This topic is part of the SG Systems Global cosmetic safety & manufacturing glossary.

Updated November 2025 • EU 1223/2009, UK Cosmetics Regulation, USP <51>, ISO 11930, ISO 29621 • Regulatory, Microbiology, R&D, Quality, Manufacturing

Preservative efficacy testing—often called challenge testing—is the experimental proof that a cosmetic product’s preservation system can prevent or control microbial growth during its shelf life and normal use. In practice, a defined cocktail of bacteria, yeast and mould is deliberately added to the product; counts are measured at multiple time points; and log‑reductions are compared to acceptance criteria (ISO 11930, USP <51> or in‑house standards). Challenge testing translates a theoretical preservation design (water activity, preservative blend, packaging, pH, chelators) into hard evidence that the product will not become a microbial soup once it leaves the factory. For Responsible Persons, it is one of the non‑negotiable pillars under the CPSR and PIF.

“You do not have a preservative system because you added a preservative. You have a preservative system when challenge data show that real microbes cannot take hold.”

1) Purpose & Regulatory Context

The purpose of preservative efficacy testing is to demonstrate that the product, not just the individual preservative ingredient, is adequately resistant to microbial growth across its realistic contamination scenarios. Regulatory frameworks (EU 1223/2009, UK Cosmetics Regulation, MoCRA‑aligned expectations) require that cosmetic products under normal and reasonably foreseeable conditions of use do not pose a microbiological risk to human health. For water‑containing or high‑aw creams, lotions, wipes and scrubs, that requirement is impossible to defend without challenge data or a scientifically justified exemption.

Authorities, notified bodies and sophisticated retailers expect that any high‑risk formula has been tested using a recognised method (ISO 11930, USP <51>, EP 5.1.3 or equivalent) and that the results are documented in the PIF and referenced in the CPSR. If your answer to “how do you know the preservative works?” is anything other than a clear test report and rationale, you are running on luck, not compliance.

2) What Challenge Testing Actually Involves

Challenge testing is conceptually simple but operationally exacting. A typical study includes:

• Selecting representative test organisms: usually S. aureus, P. aeruginosa, E. coli, C. albicans and A. brasiliensis, plus additional strains for special risks (e.g. B. cepacia complex, environmental flora).
• Inoculating the product with a known microbial load at T0.
• Storing the inoculated product under defined conditions (usually room temperature, sometimes elevated).
• Sampling at multiple time points (e.g. 7, 14, 28 days) and enumerating surviving organisms.
• Comparing the observed log‑reductions at each time point against targeted acceptance criteria.

The result is not simply “pass/fail”; it is a log‑reduction profile over time that reveals how quickly and completely microbes are suppressed. If a bug persists or regrows after an initial drop, that is a flashing warning light. Challenge data inform whether the formula is robust enough as‑is, needs optimisation, or needs restricted use conditions (e.g. airless packaging, short shelf life after opening, or more stringent GMP on filling lines).

3) Standards & Acceptance Criteria (ISO 11930, USP <51>)

Several harmonised methods are widely used to define how challenge tests are run and interpreted:

• ISO 11930 – method and criteria specifically tailored to cosmetics, including risk‑based approaches and packaging considerations.
• USP <51> / EP 5.1.3 – antimicrobial effectiveness tests commonly used for personal care and some OTC products.
• Company or retailer in‑house standards – often stricter than pharmacopeial baselines for high‑risk categories (wipes, baby care, eye products).

Acceptance criteria are expressed as minimum log‑reductions for bacteria and fungi at defined time points, and sometimes as “no increase” or “no growth” conditions. For borderline cases, ISO 11930 allows a risk assessment that considers packaging, usage profile and GMP controls. The key point: you cannot invent criteria after seeing the data. They must be defined up‑front in the protocol and justified relative to product risk and regulatory expectations.

4) Formulation Factors that Drive Preservative Performance

Challenge testing is where formulation theory meets biology. Several factors determine whether the preservative system works on paper or in the real formulation:

• Water activity and phase structure – oil‑in‑water, water‑in‑oil, gels, emulsions; how much free water is available to microbes.
• pH – many preservatives have narrow effective pH windows; micro shifts after manufacturing or over shelf life can cripple them.
• Solubility and partitioning – preservatives stuck in the oil phase cannot protect the aqueous phase where microbes live.
• Interactions with other ingredients – surfactants, polymers, proteins, clays and fragrance components can bind or inactivate preservatives.
• Preservative synergy or antagonism – certain blends enhance each other; others interfere or lead to instability.

Challenge outcomes often expose uncomfortable truths: the preservative dose looked fine in the lab, but in the actual emulsion, at scale, with your specific raw‑material variability, it under‑performs. This is not a lab failure; it is the system doing its job by forcing you to adjust formula, process or packaging before release—not after complaints and recalls.

5) Packaging, Use Pattern & Real-World Contamination

A product’s contamination risk is driven as much by packaging and use as by formula. Challenge testing, especially under ISO 11930, must account for:

• Pack type – jars routinely dipped into by fingers, open‑neck bottles, pumps, droppers, sprays, tubes, airless containers.
• Intended use – rinse‑off vs leave‑on, facial vs body, baby vs adult, eye‑area vs heel balm.
• Usage environment – bathroom shelves, wet shower, gym bag, beach, handbag; frequency of opening and closing.
• Consumer behaviour – common misuse such as adding water to thick products, decanting, sharing jars.

High‑risk use patterns may justify stricter acceptance criteria, higher preservative levels (within safety limits), or packaging that minimises headspace exposure and finger contact. It is delusional to design preservative systems as if every consumer has clean, dry hands and follows instructions perfectly. Challenge testing is where that delusion is forced to collide with reality.

6) Low-Preservative & “Preservative-Free” Strategies

Market pressure for “low preservative” or “preservative‑free” products does not change microbial physics. If you reduce or remove classic preservatives, you must compensate elsewhere and prove it via challenge tests or justified exemptions. Strategies include:

• Designing low water activity systems (anhydrous oils, powders, hot‑pours, sticks) that genuinely do not support growth.
• Using hurdle technology: pH control, chelators, multifunctional ingredients with secondary antimicrobial effects, controlled packaging.
• Implementing airless or unit‑dose packaging to reduce in‑use contamination.
• Short post‑opening periods and clear usage instructions.

“Preservative‑free” claims for high‑aw creams in open jars are, bluntly, reckless unless backed by a credible alternative system and rock‑solid challenge data or a scientifically justified position. Regulators are increasingly hostile to fear‑selling around legally permitted preservatives; if you choose to play that game, your microbiology has to be bulletproof.

7) When & How Often to Perform Challenge Testing

Challenge testing is not “once per product forever”. At minimum it should be performed:

• During development, once a near‑final formula and pack are selected.
• For major changes: preservative swap or level change, pH shift, base change, new fragrance or key excipient, new packaging.
• For site transfers or significant process changes that could impact micro load or product structure.
• Periodically for long‑running hero products, especially if complaint trends or environmental monitoring data change.

A risk‑based strategy may allow reduced frequency for low‑aw, low‑risk formats and higher frequency or confirmatory testing for high‑risk categories (wipes, baby products, eye‑area products, spa‑use bulk formats). The important part is that the rules are written down, understood, and actually followed—not reinvented ad hoc when launch dates loom.

8) Laboratory Execution, Controls & Data Integrity

Challenge tests are only as good as the microbiology lab behind them. Key execution factors include:

• Use of qualified strains with documented provenance and storage.
• Validated neutralisation and recovery methods to avoid under‑counting survivors.
• Appropriate controls: growth controls, neutraliser controls, method suitability checks.
• Correct sample handling to avoid cross‑contamination or preservative evaporation artefacts.
• Robust data‑integrity practices: contemporaneous records, audit trails, review and approval workflows.

Outsourcing to a CRO does not absolve you of responsibility. You still need to review methods, verify competence and sanity‑check results against expectations. If a challenge test comes back with suspiciously perfect log‑reductions for every organism at every time point, either your product is miraculous—or the method is not probing reality hard enough.

9) Integration with CPSR, PIF & Stability

Challenge results feed directly into the Cosmetic Product Safety Report (CPSR) and PIF. The safety assessor evaluates whether micro risk is acceptable given product type, target population, usage, packaging, GMP status and preservative toxicity margins. The PIF must hold full challenge reports, not just one‑line “pass” statements.

Challenge data must also be viewed alongside stability data. Preservative efficacy can drift over shelf life as pH, viscosity, phase distribution or raw‑material characteristics change. If you rely on borderline challenge results at initial release, you are betting they will not degrade over two or three years on shelf and in consumers’ bathrooms. Sensible organisations build a margin of safety into preservative performance and periodically re‑confirm it, rather than running at the edge of failure for the sake of marginal “cleaner label” optics.

10) Link to Manufacturing, Water Systems & Environmental Monitoring

A preservative system is not a magic shield against dirty manufacturing. Challenge testing assumes baseline control of raw‑material contamination, distribution practices, equipment hygiene and water quality. If your water system and environmental monitoring are weak, even a strong preservative system can be overwhelmed by massive inocula at fill.

Outputs from EM, water testing, and in‑process bioburden checks should feed back into the preservative risk assessment. Rising background counts are a signal to investigate and potentially tighten challenge criteria or adjust formulations. Conversely, strong GMP and consistently low counts may justify leaner preservative levels, provided challenge data support performance under realistic contamination loads. Pretending these systems are independent is a convenient fiction that regulators will not endorse.

11) Complaints, Micro Failures & Post-Market Review

Real‑world performance is the ultimate test. Preservative systems that look fine in small‑scale challenge tests can fail under consumer abuse, hot climates, distribution delays or packaging damage. Signals include:

• Microbiological OOS results on finished‑product release or stability testing.
• Visible mould, off‑odours, gas formation or texture changes reported by consumers.
• Clusters of adverse events linked to infections or irritation potentially tied to contamination.
• Recurring micro deviations during manufacture or filling.

Every confirmed micro issue should trigger a structured investigation, including review of challenge data, formulation, packaging, EM, water and filling practices. Weak or outdated challenge reports often surface as part of root‑cause analysis. If you repeatedly solve micro issues by “re‑cleaning the line” without revisiting preservative efficacy, you are treating symptoms, not the disease.

12) “Micro-Minimalist” Brand Positions & Reality Checks

Brands positioning themselves as “clean”, “minimalist” or “preservative cautious” face extra pressure to get challenge testing right. Removing traditional preservatives may please some consumers, but it forces careful trade‑offs: lower aw systems, fewer botanical extracts with inconsistent micro loads, more controlled packaging, tighter GMP and shorter shelf lives. All of that costs money and operational discipline.

There is no regulatory sympathy for mouldy “natural” products. If your marketing team wants aggressive messaging about preservative avoidance, your technical and quality teams must be equally aggressive in designing, executing and documenting preservative efficacy strategies and justifications. Otherwise, you are selling a story that will collapse the moment regulators or retailers start opening random jars from the field and sending them to a lab.

13) Working with External Labs & Interpreting Borderline Results

Most brands use external microbiology labs or CROs for challenge testing. Effective control means:

• Written, technical agreements covering methods, criteria, reporting and data retention.
• Review of protocols before studies start, including risk‑based organism selection and justification of criteria.
• Ability to obtain raw data and method details for regulatory inspections and investigations.
• Clear decision rules for handling borderline results: additional repeats, formulation adjustments, packaging restrictions or product‑type reclassification.

“Pass with reservations” is not a comfortable place to live, especially for high‑risk categories. If your preservative performance is consistently scraping over the line, consider the message that sends: you are one raw‑material deviation, one summer heatwave or one water‑system upset away from systemic failures. It is cheaper to build margin into preservative design than to manage chronic borderline performance forever.

14) Governance, Documentation & Integration with QMS

Preservative efficacy is a quality‑system topic, not an isolated lab activity. Governance should include:

• Formal SOPs for when challenge testing is required and which standards apply.
• Integration into formulation and recipe‑management workflows, so new and changed products cannot progress without a micro assessment.
• Storage of protocols, reports and rationales in a controlled DMS, referenced in the PIF.
• Links to deviation, CAPA and Management of Change (MoC) systems.

KPIs might track the percentage of at‑risk SKUs with current challenge data, frequency of preservative‑related deviations, and cycle time between formulation freeze and availability of micro results. If preservative issues repeatedly appear in management reviews and nothing structural changes, do not be surprised when regulators conclude that your QMS is decorative rather than effective.

15) FAQ

Q1. Do all cosmetic products need preservative efficacy (challenge) testing?
No—but most water‑containing or high‑aw products do. Truly anhydrous, low‑aw or single‑use formats can sometimes be exempt based on a documented risk assessment and supporting data. Declaring yourself exempt because “we don’t add water” to a clearly water‑containing emulsion will not survive regulatory scrutiny.

Q2. Can we rely on preservative supplier data instead of product‑specific challenge tests?
Not realistically. Supplier data prove that the preservative can work under certain conditions; they do not prove it works in your formulation, with your pH, packaging, raw‑material variability and manufacturing environment. At best, supplier data complement, not replace, product‑specific challenge testing for at‑risk categories.

Q3. How often should we repeat challenge testing?
At least whenever there is a significant change in formula, preservative system, pH, key excipients, water source, packaging, manufacturing site or micro risk profile—and periodically for high‑risk, long‑running SKUs. A written, risk‑based policy beats improvised decisions every time.

Q4. What if our challenge results just barely meet the criteria?
Borderline passes should trigger hard questions: can we improve formulation or packaging, refine manufacturing hygiene, or adjust acceptance criteria with a defensible risk assessment? Treating “just barely” as “good enough forever” is a strategic choice to live at the edge of failure.

Q5. What is the first practical step to strengthen preservative efficacy control?
Build an inventory of all water‑containing SKUs, map which ones have valid, current challenge data and which don’t, and risk‑rank them by pack type, target population and complaint history. Close the biggest gaps first, then update your SOPs and MoC processes so the holes do not re‑open with every reformulation or new launch.

Related Reading
• Safety & Regulatory: CPSR | PIF | ISO 22716 | Allergen Control
• Microbiology & Operations: Environmental Monitoring (EM) | Water System Microbiological Control | Cleaning Validation | Weighing & Dispensing
• Quality & Governance: QMS | DMS | Deviation / NCR | CAPA | MoC