What Is a Cosmetic Challenge Test?

A cosmetic challenge test, also known as a preservative efficacy test (PET), is a microbiological test used to evaluate the effectiveness of the preservation system in a cosmetic product.

During the test, a known quantity of selected microorganisms is deliberately introduced into the cosmetic formulation. The product is then monitored at defined time points to determine whether the microorganisms are reduced, remain controlled or continue to grow.

The purpose is to demonstrate that the formulation has an adequate level of antimicrobial protection and is capable of resisting microbial contamination during its intended use.

A challenge test is particularly important for cosmetic products containing water or other ingredients that can support microbial growth. However, the need for testing should ultimately be considered as part of the product's overall microbiological risk assessment.

Why Is a Challenge Test Important?

The presence of a preservative in a cosmetic formulation does not automatically mean that the finished product is adequately preserved.

The effectiveness of a preservation system depends on the complete formulation, including factors such as pH, water availability, raw materials, emulsifiers, surfactants, packaging and the interaction between the preservative and other ingredients.

A preservative system that performs well in one formulation may therefore perform differently in another.

The challenge test provides practical evidence of how the preservation system performs in the finished formulation.

Evaluating Preservative Efficacy

The primary purpose of the test is to determine whether the selected preservation system can control the microorganisms introduced into the product.

Supporting Microbiological Safety

Microbial contamination can cause deterioration of a cosmetic product and may present a safety concern. Effective antimicrobial protection helps minimise this risk.

Supporting Product Stability

Microbial growth can change the appearance, odour, texture and other characteristics of a formulation. Effective preservation helps maintain product quality during its intended shelf life.

Supporting Formulation Development

Challenge testing is not only a final quality-control exercise. It can also be an important formulation-development tool, allowing the formulator to identify preservation problems while there is still an opportunity to modify the formula.

How Does a Cosmetic Challenge Test Work?

The basic principle is relatively straightforward.

A defined quantity of selected microorganisms is introduced into separate samples of the cosmetic product. Samples are then examined at predetermined time points to determine the number of microorganisms that remain viable.

The results are expressed as changes in microbial counts, commonly as logarithmic reductions.

The greater the reduction in microbial numbers, and the longer that reduction is maintained without subsequent growth, the stronger the demonstrated antimicrobial protection.

ISO 11930 describes a reference approach for evaluating the antimicrobial protection of cosmetic products and includes a preservation efficacy test as well as an overall assessment of antimicrobial protection.

Which Microorganisms Are Tested?

The challenge test uses microorganisms representative of the main groups of microorganisms that may contaminate cosmetic products.

The original A&T formulation describes four representative groups:

Gram-Negative Bacteria

Pseudomonas aeruginosa is used as a representative Gram-negative bacterium and is particularly relevant to the microbiological evaluation of water-containing cosmetic products.

Gram-Positive Bacteria

Staphylococcus aureus represents Gram-positive bacteria and is included to evaluate the preservation system against this group of microorganisms.

Yeasts

Candida albicans represents yeast contamination.

Moulds

Aspergillus brasiliensis represents mould contamination.

Current ISO 11930 methodology also identifies these microorganisms as reference strains for preservation efficacy testing. Some testing programmes may include additional microorganisms when justified by the characteristics or risk profile of a particular product.

Microbial Inoculation and Monitoring

The microorganisms are introduced into the product as calibrated inocula. Each microorganism is generally evaluated separately so that the response of the preservation system against different microbial groups can be assessed.

The samples are then stored under controlled conditions and examined at defined intervals.

Microbial counts are determined at these time points and compared with the initial inoculum.

The resulting data show whether the microorganism population:

• decreases,
• remains controlled, or
• increases during the test period.

This is important because a preservation system should not merely produce an initial reduction in microbial numbers. It must also prevent subsequent recovery and growth during the evaluation period.

Log Reduction in Challenge Testing

Challenge-test results are commonly expressed as log reductions.

A log reduction describes the decrease in the number of viable microorganisms compared with the initial microbial count.

For example, a:

• 1-log reduction corresponds to a 90% reduction,
• 2-log reduction corresponds to a 99% reduction,
• 3-log reduction corresponds to a 99.9% reduction.

This provides a quantitative way of assessing the performance of the preservation system.

The required reduction depends on the microorganism and the test criteria being applied.

Challenge Test Acceptance Criteria

Different standards and methods can use different acceptance criteria.

Bacteria must show at least a 2-log reduction within 2 days and a 3-log reduction within 7 days, with no subsequent increase in microbial numbers through the later observation points.

For fungi, the original criteria require at least a 2-log reduction within 7 days, again with no subsequent increase through the end of the evaluation period.

The exact acceptance criteria should always be interpreted according to the specific standard and test method used for the product. ISO 11930 provides its own evaluation framework for preservation efficacy and overall antimicrobial protection.

ISO 11930 and Cosmetic Challenge Testing

ISO 11930 – Cosmetics — Microbiology — Evaluation of the antimicrobial protection of a cosmetic product is one of the key international standards used when evaluating the preservation of cosmetic products.

The standard describes a preservation efficacy test and a procedure for evaluating the overall antimicrobial protection of a cosmetic product.

Importantly, antimicrobial protection does not necessarily come only from the chemical preservative. ISO 11930 recognises that protection can also be influenced by:

• The formulation itself
• The preservation system
• Packaging design
• Manufacturing conditions
• The overall microbiological risk of the product

This means that preservation should be considered as a complete system rather than simply as the presence or absence of a particular preservative.

Which Cosmetic Products May Require Challenge Testing?

Challenge testing is particularly relevant to cosmetic formulations that contain water and are potentially susceptible to microbial growth.

Examples include:

Creams and Lotions

Emulsions containing an aqueous phase generally require careful consideration of their preservation system.

Serums and Gels

Water-based serums and gels can provide favourable conditions for microbial growth if they are not adequately protected.

Shampoos and Shower Gels

These products often contain substantial amounts of water and therefore require appropriate microbiological protection.

Liquid Cleansers

Water-based cleansing products can also require effective preservation depending on their composition and packaging.

Other Water-Containing Products

Many other cosmetic formulations may require challenge testing depending on their microbiological risk profile.

Not every cosmetic product necessarily requires a conventional challenge test. ISO 11930 specifically provides for products that have been determined to have a sufficiently low microbiological risk according to the relevant risk-assessment approach.

Challenge Testing During Cosmetic Product Development

One of the most important reasons to perform a challenge test is to identify preservation problems before the formulation is finalised.

A formulation may appear completely satisfactory during development while still having inadequate microbiological protection.

Challenge testing can therefore be incorporated into the development process alongside other formulation and stability evaluations.

If the formulation fails the test, the formulator may need to investigate factors such as:

Preservative Selection

The selected preservative or preservative combination may not provide sufficient broad-spectrum protection for the formulation.

Preservative Concentration

The concentration may be insufficient under the specific formulation conditions.

pH

The activity of certain preservatives can depend strongly on formulation pH.

Ingredient Interactions

Other ingredients can affect the availability or effectiveness of a preservative.

Packaging

The packaging system can influence the level of microbial exposure during consumer use.

A challenge-test failure therefore provides valuable information for formulation optimisation rather than simply indicating that the product cannot be developed.

Challenge Test and Packaging

Packaging is an important part of the overall microbiological protection of a cosmetic product.

A product used repeatedly by consumers may be exposed to microorganisms during normal handling. The design of the package can influence how much opportunity there is for contamination.

Examples of packaging considerations include:

• Open jars
• Pumps
• Airless systems
• Single-use packaging
• Applicator systems

The formulation and packaging should therefore be considered together when assessing the microbiological protection of the final cosmetic product.

ISO 11930 includes consideration of packaging characteristics as part of the overall evaluation of antimicrobial protection.

Challenge Test vs. Microbiological Quality Testing

A challenge test should not be confused with routine microbiological quality testing.

Microbiological Quality Testing

Routine microbiological testing evaluates whether microorganisms are present in a production sample and whether the product meets defined microbiological quality requirements.

Challenge Testing

A challenge test deliberately introduces selected microorganisms into the formulation to evaluate the effectiveness of the preservation system.

The two tests therefore answer different questions and can form complementary parts of a cosmetic product's microbiological quality and safety strategy.

What Happens If a Cosmetic Fails the Challenge Test?

A failed challenge test does not necessarily mean that the entire product concept must be abandoned.

Instead, the result can help identify weaknesses in the preservation system.

Depending on the formulation, possible development actions may include:

• Changing the preservative system
• Optimising preservative concentration
• Adjusting the formulation pH
• Modifying the formulation composition
• Introducing synergistic antimicrobial ingredients where appropriate
• Reviewing manufacturing conditions
• Changing the packaging system
• Repeating the challenge test after reformulation

The reformulated product should then be appropriately re-evaluated to demonstrate that the revised preservation system provides sufficient protection.

Challenge Testing and Cosmetic Product Safety

Preservation is an important part of cosmetic microbiological safety, but it should not be considered in isolation.

A complete microbiological risk assessment can consider:

• Raw material microbiological quality
• Manufacturing hygiene
• Water quality
• Product formulation
• pH and water availability
• Preservation system
• Packaging
• Intended consumer use
• Product application area
• Expected shelf life

ISO 11930 explicitly places preservation efficacy testing within the broader evaluation of the overall antimicrobial protection of a cosmetic product.

When Is a Challenge Test Not Necessary?

A conventional challenge test is not necessarily required for every cosmetic product. The need for preservative efficacy testing should be determined as part of the product's microbiological risk assessment, taking into account the formulation, packaging, manufacturing process and intended use.

ISO 11930 recognises that some cosmetic products may have a sufficiently low microbiological risk that a conventional preservation efficacy test is not required. However, this conclusion should be supported by a documented and scientifically justified assessment rather than simply assuming that a product does not need testing.

Anhydrous Products

Products containing no water, or only negligible amounts of water that are not available to support microbial growth, may have a low microbiological risk.

Examples can include certain:

• Anhydrous oils
• Oil-based balms
• Some lip products
• Certain wax-based products

However, the absence of added water alone does not automatically mean that microbiological risk is zero. The complete formulation and manufacturing conditions should still be evaluated.

Products with Very Low Water Availability

Some formulations may contain water but have conditions that make microbial growth highly unlikely. Low water activity (aw) can significantly limit the ability of microorganisms to grow.

In these cases, the microbiological risk assessment may provide sufficient justification for not performing a conventional challenge test.

Products with Extreme pH

Formulations with a sufficiently low or high pH may create an environment that is unfavourable for the growth of many microorganisms.

However, pH alone should not automatically be used as justification for omitting a challenge test. The resistance of different microorganisms to the specific formulation conditions and the complete product composition should be considered.

Products with High Alcohol Content

Certain products containing a sufficiently high concentration of alcohol can have a low microbiological risk because the formulation itself provides antimicrobial protection.

Examples may include some alcohol-based products and specific hydroalcoholic formulations.

The actual alcohol concentration and the complete formulation need to be assessed before concluding that conventional challenge testing is unnecessary.

Products in Protective or Single-Use Packaging

Packaging can significantly reduce the possibility of microbial contamination during consumer use.

Examples include:

• Single-dose products
• Individually packaged products
• Certain airless dispensing systems
• Packaging systems that minimise repeated contact with the formulation

Nevertheless, packaging should be considered as one element of the microbiological risk assessment, rather than automatically replacing preservation efficacy testing.

Products with an Inherently Low Microbiological Risk

Some products may have a combination of characteristics that makes microbial growth unlikely. This can include a combination of low water availability, an unfavourable pH, antimicrobial formulation characteristics and protective packaging.

For such products, a documented risk assessment may conclude that a conventional challenge test is not necessary.

When Can a Challenge Test Be Omitted?

The important principle is that a challenge test should not be omitted simply because a product appears unlikely to support microbial growth.

Instead, the decision should be based on a documented microbiological risk assessment demonstrating why the product has sufficiently low microbiological risk.

Factors that may be considered include:

• Water content and water activity
• Formulation pH
• Alcohol concentration
• Presence of ingredients with antimicrobial properties
• Product composition
• Packaging design
• Manufacturing conditions
• Intended method of application
• Frequency of consumer use
• Risk of contamination during use
• Product's intended area of application

If the overall assessment demonstrates that the product has a sufficiently low microbiological risk, a conventional preservative efficacy test may not be necessary.

Low-Risk Does Not Mean No Microbiological Testing

It is important to distinguish between not requiring a challenge test and not requiring microbiological evaluation at all.

Even when a product is considered low risk, other aspects of microbiological quality may still need to be assessed as part of the product safety and quality-control process.

The decision to omit challenge testing should therefore be supported by appropriate technical justification and documented within the product development and safety documentation.

Challenge Test or Risk Assessment?

The decision can essentially be viewed as two different approaches:

Products with a meaningful microbiological risk:
A preservative efficacy/challenge test can be used to demonstrate that the formulation provides adequate antimicrobial protection.

Products with sufficiently low microbiological risk:
A scientifically justified microbiological risk assessment may demonstrate that a conventional challenge test is not necessary.

This distinction is particularly important during professional cosmetic formulation because the objective is not simply to include a preservative or perform a test, but to demonstrate that the finished product has an appropriate level of microbiological protection for its intended use.

A challenge test is therefore an important tool, but it is not a universal requirement for every cosmetic formulation.

Why Challenge Testing Matters in Professional Cosmetic Formulation

A cosmetic formula can have excellent texture, appearance, stability and sensory performance and still be unsuitable for launch if its microbiological protection is inadequate.

Challenge testing provides evidence that the preservation system works within the actual formulation, rather than simply assuming that a particular preservative will be effective because it is commonly used.

For professional cosmetic development, this makes challenge testing an important tool for:

• Preservative selection
• Formula optimisation
• Microbiological risk management
• Product safety
• Shelf-life development
• Packaging decisions
• Regulatory and safety documentation

A well-designed preservation strategy, supported by appropriate microbiological testing, helps ensure that the finished cosmetic product remains adequately protected throughout its intended period of use.

References

Source: Cosmetic Formulation Principles and Practice – Heather A.E. Benson, Michael S. Roberts, Vânia Rodrigues Leite-Silva, Kenneth A. Walters.