Stability testing for cosmetics: What you need to know (2024)

Testing the minimum durability of cosmetics is important and obligatory

It is crucial for cosmetics to be safe for use. Not only directly after opening a new product but also after a week, month or even a year. Just like in food, this is called the minimum durability, or expiration.

It is required to test the durability of cosmetics before the product is allowed for sale, since the minimum durability must be indicated on the product packaging.

The durability of a product is tested using various testing, such as stability testing, compatibility testing (= stability test in final packaging) and microbial challenge testing.

Why are stability tests important for the minimum durability of cosmetics?

Cosmetic products are substances or mixtures of substances that consumers can use for extended periods. During this entire time period, these products must remain stable. The last thing you want is that the product starts discolouring or deteriorating when it is still being consumed.

Not just because such unstable products look weird and less attractive for users, but also because this can cause safety risks for the consumer. A sunscreen product that is unstable may have as a consequence that you apply less UV filter than you’d think, causing you to get sunburned much more quickly.

Various types of stability tests

Stability tests are done to test the product’s shelf life. The most ideal test would be to test the product at least as long as the expiration date indicates on the product labelling.

This can be for example 2 years, and this type of testing is what we call real-time testing. With a 2-year test, you know that the product is indeed durable for 2 years.

Sounds great and all, but that also means that the product is only allowed to be sold after the test is finished – in the example, this would be after 2 years (because: first the product must pass all testing and only then it may be sold).

When you realize that the average product life cycle is 3 years (and then reformulated or replaced by a new variant), this would mean that you’d often be waiting almost for the same duration as you’d actually be selling the product.

A better way to test cosmetics’ shelf-life

To fix this problem, the cosmetics industry has adopted a much more efficient testing method. This is based on the work of the Swedish chemist Svante Arrhenius.

He discovered that molecules (including the substances in cosmetic products) vibrate with a certain frequency. When he raised the temperature, the molecules would increase in vibration frequency.

This insight showed that a higher temperature would result in more molecule vibrations in the same time period. With that, you can predict how often these molecules would vibrate at room temperature compared with a higher testing temperature.

That sounds quite technical, but it’s a fantastic fix for the testing time of cosmetic products. By increasing the testing temperature, testing time can be reduced from years to months.

A test done at a higher temperature is also called an accelerated stability test because the shelf-life of a product is simulated in an accelerated manner due to the elevated temperatures

Stability testing at various temperatures

Often various temperatures are used when doing stability testing. Because: the higher the temperature, the faster you have results, but the temperature can also get too high.

Just like baking an egg, a cosmetic product can be irreversibly boiled due to the elevated temperature. That doesn’t say anything about the durability of the product, since this is only caused by the high temperature.

Another example would be a deodorant stick, where a too-high temperature causes the product to melt. This also doesn’t provide a realistic view of the true product’s durability. So products are tested at various temperatures which ensures that you get fast, but also realistic results from the stability test.

Naturally, it is also advisable to do a real-time test on the product so you can also get information from this scenario.

Other methods for even faster testing

Using various temperatures for an accelerated stability test is a great way to speed up the check of whether a product is stable.

Besides this method, there are also other ways to pretty quickly get a sense of whether a product formula is unstable.

These include tests that can be done even faster compared with accelerated stability testing. These tests often give a less accurate view of how long a product can be stable. But these tests do give a good answer to whether a product can withstand the most extreme stress.

So you can see it more as a first check, if a product fails this step it doesn’t need to go through the 3-6 months of accelerated stability testing.

Examples of these super-fast methods are

• Centrifuge testing for creams
• Temperatuur stress testing
• Mechanical stress testing

Centrifuge testing for cosmetics, half an hour

With centrifuge testing, creams are heated and centrifuged at 3000 rounds per minute. If cream is unstable, it will show this in the form of oil droplets on the surface of the cream.

This is actually called ‘creaming’, and a stable cream shouldn’t be creaming. This test only takes half an hour to do which makes it a perfect first test for an emulsion formulation like a cream.

Temperature stress testing or ‘cycle’ testing

Temperature-based stress tests are also called ‘cycle’ tests because the temperature is changed every 24 hours between warm (25 degrees Celsius) and cold (-10 degrees Celsius).

If you’ve ever visited a sauna you know that changing temperatures can be quite spicy, and that’s exactly the same for cosmetics. If a product passes this test with flying colours, then it is almost certain that the product is super stable.

The added bonus of this test is that it also mimics product transport and shipping, during which products are also exposed to varying temperatures.

Mechanical stress testing on roller or shaker machines

Mechanical stress tests are tests during which products are shaken vigorously, to check whether they can withstand the physical aspects of shipping.

No, that doesn’t mean that lab people are secretly throwing a cocktail-shaking competition in the lab (shaken, not stirred), this process is done on specialized equipment.

Not recommended: do-it-yourself stability testing

A lot of these methods are aimed at the first check for product stability.

Especially in the past, many old-school product developers working at manufacturers also used some out-of-the-box tricks. Just by throwing a couple of product samples in the back of their car, driving them around and leaving them out in the burning sun, this was a perfect combination for testing new formulations.

Nowadays testing is of course much more modern since the quality requirements have become much stricter. And the stakes have become higher, needing to produce results as soon as possible.

What does a professional stability test for cosmetics look like?

Often stability tests are done in glass containers. This is because glass is inert, it reacts to (basically) nothing. If the product is unstable, then you know it’s really due to the formulation itself.

In parallel, the product is also tested in its final packaging. Is the product stable in glass, but not in its final packaging, then you know the problem lies with the packaging (or the combination of product + packaging).

It’s not a strict requirement to test in glass, but it can add a lot of information to your results, especially if you don’t have a lot of experience with the stability of a particular formulation or packaging.

Testing in the final packaging: the compatibility test

A stability test in its final packaging is also called a compatibility test because you look at how compatible the formulation and the packaging are. Because a product can leak out of its packaging, substances can migrate from the packaging into the product and vice versa, the product may discolour if the packaging isn’t airtight.

You want to know that the product has a great shelf-life in its final packaging, before the product is sold on the market.

Cosmetics with a long shelf-life are safe cosmetics

Cosmetic products must have long minimum durabilities. Two of the most important tests to check product shelf-life are the stability of the product formulation and the compatibility between the formulation and its packaging.

These can be studied using various methods, with the most common test being the accelerated 2-4 months stability test.