Podcast: Episode 8, stability and compatibility

Audio episode, stability and compatibility tests


Stability and compatibility tests under accelerated aging conditions:

  • Choice of containers: glass pillboxes for the stability test; final packaging for compatibility test
  • Test temperatures: cold: -4*C, ambient temperature: ~25*C, hot: from +40*C to +50*C, alternating cold / hot
  • Test duration: from 3 to 6 months
  • Benefits : anticipate and evaluate the state of the product during its life

Episode Notes


Hello, you are listening to episode 8 of Les Sous de la Cosmétique, the Podcast which reveals and explains to you with clarity and sincerity the world of cosmetics. I am Julie Magand Castel, chemical biologist and cosmetologist specialized in natural cosmetics and today we are going to talk about stability and compatibility tests.

Before being put on the market, our cosmetics had to be carefully examined to ensure their safety, their robustness, and their effectiveness. We listed these tests in episode 4 on cosmetic development, in which we mentioned the stability and compatibility tests. The stability test certifies the robustness of the sensoriality of the product over time, and the compatibility test certifies that there is no involuntary reaction between the packaging and the formula. So how is this determined knowing that it is not viable for a brand to check the condition of its finished products for 3 years before placing them on the market?

Indeed, cosmetics is a very changing sector, which implies a certain dynamism and a certain reactivity, so it is not customary to wait for the end of a real-time stability test to launch your product. To Instead, brands perform stability tests under accelerated conditions. That is to say that the product is put in glass pillboxes, therefore in glass containers and kept in ovens at different temperatures.

The product is put in glass pillboxes for two reasons: first, glass is an inert material and therefore reacts little or not at all with the container, so we make sure that if there is instability it is only to the formula; and secondly because the glass is transparent and therefore it is very easy to judge if the product is out of phase. Then the product is kept in the refrigerator to check that it is resistant to cold, it is placed in warm ovens, between 40100 and 50C for several months, between 3 and 6 months to mimic the aging it could have endured over several years between 2 and 3 years.

In addition, it is possible to make the product undergo alternating cold and hot cycles to imitate extreme transport conditions, for example. Thus the product endures temperatures of -10C at +40C in the same day and this for several days or even tens of days. The exact procedures depend on the laboratories.

These accelerated aging conditions make it possible to predict the state of a product if it had lived for several years under normal conditions. So in a few months we have a very good idea if the product is suitable for use a few years after its production, from the point of view of its sensoriality only. In order to realize any instability in time, the product is checked, which is kept in the ovens frequently and at precise time intervals so that it can be compared from one formula to another. The sooner we realize that the formula is unstable, the sooner we can reformulate and follow the new stability test.

It is also possible to put the product in the centrifuge to test its robustness, this test is extremely practical because in about ten minutes it is possible to have a first evaluation of the stability of the product. A centrifuge is a machine that allows the product to undergo very rapid rotations, which can destabilize an emulsion if it is not robust enough.

So, since the beginning of this episode, we've been talking about instability, so how it translates into instability. Take the example of an emulsion, which is a difficult system to stabilize. As a reminder, an oil-in-water emulsion is a system composed of very small droplets of oil dispersed in an aqueous system.

I invite you to listen or re-listen to theepisode 2, mayonnaise and vinaigrette which explains in more detail what an emulsion is.

The emulsion is stable thanks to the active surfactant also called emulsifier, small molecules which are placed at the interphase between oil and water. The emulsifier separates the droplets from each other and increases the repulsion that the droplets have with each other. And thanks to this force of repulsions, the emulsion is stable. However, the emulsifier is too weak and this repulsion force is not strong enough, so the droplets come together, this is called coalescence to form increasingly larger droplets. Eventually, all the droplets regroup and form a distinct phase, this is the phase shift. The system is then composed of two distinct phases.

Generally the oily phase is found above because its density is less than 1, so its density is lower than that of water. So you can imagine that the customer who buys his cream will not be happy to open the jar and discover an oil phase on the surface. It is therefore imperative to check that the stability of the cream is optimal before placing it on the market. We will talk about other instability in a future episode devoted to this subject.

Then, you have to check the compatibility of the formula with the packaging, we say container/content compatibility. Indeed some ingredients can interact with some materials.

It is possible for example that certain alkane deforms certain plastics, this does not mean that these ingredients are dangerous but it is necessary to ensure that the tube of cream does not deform over time, that it does not leak. etc And for that the test is very similar to the stability test except that the formula is placed in the final packaging and not in glass pillboxes in the oven. And we follow the evolution of the finished product over several months under accelerated aging conditions.

I hope you enjoyed this episode and know more about animal testing in the cosmetics industry. A new episode will be released next Saturday and you can find the ratings for these episodes on mastelcosmetics.com in the underside of cosmetics section. Mastel is written MASTEL and is the contraction of my two surnames Magand Castel.

Do not hesitate to react in the comments, to ask your questions or to indicate what you would like to hear next. You can subscribe to the Podcast Les sous de la beauté today on various Podcast platforms including Spotify and follow its news on Instagram and Twitter with the username mastelcosmetics. Also and of course feel free to share this podcast with your friends if you think they might enjoy it - thank you. I wish you a good week and see you next Saturday for an episode on stability and compatibility tests.

Leave a comment

Please note, comments must be approved before they are published

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.


A chemist in the cosmetics industry for more than 5 years and a graduate of the Natural Raw Materials in Cosmetics Master's degree from ISIPCA, Julie is an expert in the development of natural cosmetic products.