Challenge test in cosmetics: What you need to know (2024)

Why do you need to test cosmetics’ shelf-life?

Cosmetics must be safe for use. Not just for single use, but for a longer period of time. You wouldn’t want your body lotion to expire after using it twice, right? That is why it is key that a product has a long durability, and is protected against the external environment.

This durability consists of 2 parts: chemical durability and microbial durability. The chemical durability is tested by stability and compatibility tests. The microbial durability is tested using a challenge test.

What is a challenge test for cosmetics?

A challenge test is meant to check the microbial durability of a product. But what does this mean exactly?

Many products are ideal environments for bacteria and molds to grow. This causes the product to rot (also called a “microbial attack”). Not very pleasant, which is why the product must be adequately preserved to prevent this from happening. Cosmetic products too can grow mold or rot.

Various preservatives are allowed for use in cosmetics. They are no silver bullets, often acting against either bacteria, molds, or even just against a specific subgroup. That’s why often a mixture of preservatives is used.

A product developer or a manufacturer wants to test whether their preservative mix indeed protects well enough against yeasts and bacteria alike.

This test is called a challenge test because the preservation system is “challenged” by adding a ton of bacteria and molds on purpose to the product. When the product successfully passes a challenge test, it is highly likely that the product is preserved sufficiently against virtually all bacteria and molds and therefore has a long shelf-life.

When is a challenge test needed?

It’s super important to ensure that a product cannot rot or grow mold. Nevertheless, not all products require a challenge test. First and foremost: bacteria and mold need water to live and grow. Products that do not contain water are therefore less susceptible to microbial growth. Take make-up powders or dry shampoo for example.

To get very technical, this is due to low water activity, where the cosmetic formulation does not have enough free water molecules available to prosper. Low water activity is also the reason why dried fruit, peanut butter, and honey doesn’t spoil easily.

Products that are intrinsically protected against bacteria and mold, often also do not need a challenge test. An example is a soap bar with a very high pH, causing bacteria and mold to die (that’s why soap acts as soap), or a facial peeling with a very low pH.

Also, products that do not contain water but are oil-based products, don’t require a challenge test. Think of massage oils or after-shower oils.

Furthermore, certain ingredients can also prevent microbial growth. Take for example high alcoholic (>20% alcohol) products such as fragrances, ammonia or monoethanolamine-containing hair dyes, polar organic solvents (ethyl acetate, butyl acetate in nail polish), and so on.

Besides the product and its ingredients, also the manufacturing process and the packaging material may affect whether a microbial challenge test is needed.

If a mixture is heated for more than 10 minutes at 65 degrees Celsius during the manufacturing/filling of the product in its packaging, virtually all bacteria and moulds are killed. This is similar to pasteurizing milk, the heating process ensures that the milk has a longer shelf life.

If packaging is used that remains closed-off to the external environment, such as a pressurized spray can, then it has a very low chance that microbes can get into the product during use. Unlike a jar of cream where you can just stick your fingers in, which has a high probability of microbial contamination and thus requires a challenge test.

Checklist to determine whether a product needs a challenge test

You can use this checklist to see if your cosmetic product needs a challenge test:

1. Check the amount of water/water activity in your product
2. Measure the pH of the product
3. Does the product contain specific ingredients which create an anti-microbial environment?
4. What is the production method, and does it contain heating steps?
5. How well does the packaging protect the product?

How is a challenge test done?

There are various protocols to conduct a challenge test. Some protocols are created by international communities (ISO standards, Pharmacopoeia,…), while other protocols are developed by companies (often by suppliers of preservative ingredients).

These protocols contain differences, which is why not all protocols are readily accepted for cosmetics on the EU market. For example, the EP (European Pharmacopoeia) is a method widely accepted in Europe, while this isn’t automatically true for an in-house method developed by a company outside of the EU.

Be sure to check this with a microbial laboratory or a safety assessor so you are absolutely certain that you have an acceptable method for challenge testing.

Microbes that are normally used for challenge testing often include 3 bacteria (Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa) en 2 yeasts/mold (Candida albicans and Aspergillus brasiliensis).

What is the duration of a challenge test for cosmetics?

Despite the multitude in challenge testing protocols, most methods have similar timelines.

At day 0 a product is prepared for challenge testing, where the product is divided among a couple of test plates, and per plate a different bacteria or mold is added.

Every so often (e.g. after 6 hours, 1 day, and 1,2,4 weeks) the samples are measured to determine the amount of microbes on each plate. Most methods have a 28-day duration, but this doesn’t mean that you have the final result at exactly day 28.

This is because, for analysis, bacteria and molds need some extra days after harvesting. Bacteria grows quite fast and mold takes some more time to grow.

And of course, the analysis and conclusion must be written in a report and signed off by a microbiologist. That’s why it is better to assume 40 days instead.

How much does a challenge test cost on average?

There are various challenge test protocols. Some methods are more strict or more extensive with a lot more data points or using more microbes. This causes the pricing between microbial laboratories to vary.

Besides that, sometimes it may be also needed to include some other bacteria and molds, which also affects pricing. The usual costs that we see in the field are somewhere between 150 and 300 euro per challenge test.

Occasionally we encounter higher prices, this often is a laboratory specializing in medical devices or medicinal products – product types that, of course, require even stricter approvals.

The need for cosmetic product safety tests

Cosmetic products need to be safe for the duration of their entire shelf-life. An important aspect of this is that the product doesn’t mold or rot.

Some products have product characteristics that make them not susceptible to microbes, like lack of water, extreme pH, specific ingredients, manufacturing methods, and packaging.

Products that can spoil, can be protected by adding preservatives. A challenge test is done to test whether adding these preservatives offers sufficient protection to microbes.