Stability testing timeframes

I was wondering what experience people had with accelerated testing of products (natural) over different time frames. If 45C for 8 weeks is approx equivalent to 1 year at RT for stability purposes, would 2 weeks at 45C be an indicator your product is stable for 6 months or do more products tend to fail between 2-4 weeks at the higher temp? I’m making small batches of products so 6 months stability would be an acceptable start point for me at this stage. Thanks for any advice!


  • There is no such equivalence. Lots of semisolids are not stable at higher temperatures, so putting such a product in accelerated conditions will just tend to fail. I only do this test just because it has to be done (45 C / 80% RH 4 weeks), the results are not reliable.
  • PerryPerry Administrator, Professional Chemist
    @Biochemist - No. There is not a linear relationship. In fact, the 45C for 8 weeks predicting RT for a year is just a guideline for many formulas. It only gives you a probability of having a stable formula. It doesn't guarantee it.  For any new formula, you still have to have a sample that goes out to 1 year at RT to see if the 45C sample at 8 weeks is predictive.

    There is no shortcut to stability testing.

  • Thanks both for the advice, makes perfect sense.
    when do you suggest running preservative efficacy testing in relation to stability testing- concurrently or make sure the product is stable then test preservatives or vice versa?
    I need the most cost effective way as I don’t have access to facilities or resources to keep paying for multiple tests. 
  • I'd make the sure product is stable and than verify preservatives, in any case I can modify the preservative without creating problems for the stability. 
  • PerryPerry Administrator, Professional Chemist
    We always did a PET at the start of a stability test and on 45C, 8wk samples.
  • Thanks, appreciate the tips!
  • Bill_TogeBill_Toge Member, Professional Chemist
    as challenge testing requires the services of a third-party microbiology lab, we tend to start the challenge test at 8 weeks; that way, we can be reasonably confident the product is stable before committing to (and paying for) the test

    if the product type is genuinely unprecedented, and we have no stability records for anything similar, we hold off until 12 or 16 weeks
    UK based cosmetic chemist with 13 years' experience at the bench. I've worked with pretty much everything apart from pressed powders, soap, solid lipstick and aerosols.
  • @em88 for the products that you mention you know probably won’t be stable at higher temperatures, do you only do room temp testing and specify on storage conditions not to exceed that temp? 
  • With our new hair conditioner recipe, at room temperature (around 30 C here in the tropics) it appears stable after 8 weeks. However, put it into our 45 C rapid stability test oven and it seperatesinto two layers fairly quickly.

    Would I be correct in assuming this is normal for some hair conditioner recipes?

    Does present us with a dilema. Dop we only then do room temperature testing? And for how long?
    Dr. Mike Thair
    Cofounder & Chief Formulator
    Indochine Natural
  • Thanks @Bill_Toge- in the case where you have a stable product but the preservative efficacy fails, do you change the preservative and repeat stability testing again, or are you fairly confident you have a stable product so only repeat PET? (Assuming your new preservative doesn’t thin your emulsion etc)
  • PerryPerry Administrator, Professional Chemist
    @mikethair - The problem with only testing at RT is that you then don't know whether your product is stable during shipping and storage conditions. If it separates at 45C that quickly, I wouldn't consider the formula stable.

  • Biochemist, If I know that one of the ingredients will not be stable at 45C I only do the stability test for 4 weeks, and try to improve the product to not separate. For ex. urea degradation is triggered by water presence, acid medium and temperature. If you have an emulsion o/w and an acid active ingredient, doing the stability test at 45 C will only lead to urea degradation. You will see the pH will increase a lot (8-9), the cream surface will have some hoes due to CO2 formation. 
    I mention on the package to keep at room temperature, or if it is the case at 8-15 C. It is also mentioned on the label for bigger packages the same thing, to avoid problems in transport or storage facilities. 
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