Preservation challenge test for cosmetics: when it is needed and what it involves

In a challenge test the product is deliberately contaminated with microorganisms in the laboratory. Over a defined period, the development of their numbers is measured. This shows whether the preservation protects the product adequately during normal storage and use.

The test is therefore different from microbiological quality testing, which shows whether the finished product stayed clean during manufacture, and different from stability testing, which looks at physical and chemical changes.

Annex I, Part A, point 3 requires the safety report to contain the microbiological specification of the substances or mixtures and of the cosmetic product, and the results of the preservation challenge test. Particular attention is paid to products used around the eyes, on mucous membranes and damaged skin, on children under three, on elderly people and on people with compromised immune responses.

The Commission’s guidelines add what the data is for: it justifies the effectiveness of the preservation system, the indicated minimum durability and the period after opening in terms of safety.

The guidelines distinguish by microbiological susceptibility. First, low microbiological risk products, for example with more than 20% alcohol, based on organic solvents or with a high or low pH: neither a challenge test nor microbiological quality tests on the finished product are needed.

Second, single-use products and products that cannot be opened, for example because the packaging doses the product without it coming into contact with air: only microbiological quality tests are needed. Third, all other products: they need both. In the first two groups the exception must be justified scientifically, and the safety assessor confirms the justification in Part B.

According to the guidelines, the main parameters of microbiological quality are the original level of contamination and the possibility of microbial growth. Water-based mixtures, protein-rich materials and plant or animal raw materials are particularly susceptible; organic solvents, by contrast, do not support growth.

In practice this means that microbiological specifications of raw materials belong in the supplier data, and a new raw material or supplier can change the assessment.

For the test itself, laboratories often work with ISO 11930:2019 (amended by Amendment 1:2022), which describes the evaluation of the antimicrobial protection of a cosmetic product. ISO 29621:2017 contains guidelines for risk assessment and for identifying microbiologically low-risk products. Products classed as low-risk under it are excluded from the challenge test in ISO 11930.

The standards are paid documents, and which criteria apply in a given case is determined by the laboratory and the safety assessor from the text of the standard. Important for documentation: the method and edition of the standard belong with the result, not just the test type.

A passed test applies to the sample tested. If the preservative, the water phase, the pH, the packaging or the manufacturing process changes, a professional judgement is needed on whether the existing data still applies. The same goes for a change of supplier for susceptible raw materials.

This judgement should be documented even when it concludes that no new test is needed. Otherwise it cannot be shown later why the old data still holds.

A challenge test runs for several weeks and is often outsourced. Samples, dates and results therefore affect milestones such as customer approval and market launch. Ordering the test only after the formula has been approved costs time if it fails and the preservation has to be adjusted.

In VYVA every test protocol has a section for microbiology tests. It belongs to a formula and a batch number, and the laboratory report is attached to the test as a file. Alongside the result, formula, batch and date are therefore fixed as well.

This makes it possible to answer later which sample was tested and whether it matches the product going to market. Evaluating the result remains the job of the laboratory and the safety assessor.