Here’s the biggest difference between what you experienced in your college organic chemistry lab versus a cosmetic formulation lab. In an organic lab, you mix chemicals together and hope something 
happens. Ideally, you get a chemical reaction you expect. As a formulating chemist, you mix chemicals together and hope nothing happens. Cosmetics are mixtures of chemicals that mostly aren’t supposed to react with each other.
Unfortunately, they often do react (or otherwise change) so you need to test your formulas to see how long they will last. This is called Stability Testing and is something a cosmetic scientist spend much of her time doing. In this post, we’ll give a brief description of the test and suggest when, why and how it should be done.
What is stability testing?
Stability testing is simply an experiment in which you create a batch of your formula and put samples of it at different environmental conditions for a set period of time. These conditions vary in temperature and light levels and are meant to simulate what will happen to the product during its life cycle.
At select intervals you evaluate your samples for various physical, chemical and performance characteristics to see how they have changed. If the changes are minimal according to your company standards, then your formula is said to have “passed” stability testing. This means you can have confident that when the formula is shipped to stores and ultimately customers, it will still be as good as when it was first manufactured.
The underlying assumption in stability testing is that increasing storage temperature speeds up any aging reactions that will occur. A handy rule of thumb is that a sample stored at 45C for 8 weeks is equivalent to one that is stored at room temperature for one year. This isn’t an exact predictor, but is good enough for the purposes of cosmetic products.
A sample stored at 45C for 8 weeks is equivalent to one stored at room temperature for a year
When do you perform stability testing?
Since you’ll be making hundreds or thousands of prototypes during your career, it won’t be practical to run a stability test on all of them. You’ll also find that changes happen so rapidly at your company, you won’t have time to properly test many of your formulas. But there are times when you need to do stability testing. Here is a short list of some of the most important times to conduct a stability test.
1. New prototypes – Whenever you make a new formula and are satisfied with the way it performs, you’ll want to do a stability test to ensure that it will stay together. Don’t bother testing all your prototypes, just the ones that work the way you want.
2. New raw materials – Whenever you have to change the fragrance, color, or other raw material in a formula, you’ll have to do a stability test to make sure there aren’t unacceptable changes. Also, when you have a new raw material source (or supplier) you’ll want to run a test.
3. New manufacturing procedure – Manufacturing is always trying to find faster ways to make formulas. This often means they change some order of addition or shorten mixing time. Whenever changes like these happen, it could affect your formula. Run a stability test to see if the change is acceptable.
4. New packaging – Cosmetic products change their look almost yearly so packaging is constantly being modified. Whenever you get a new package, you’ll have to determine if the formula continues to be compatible. Stability testing helps ensure that it is.
How do you stability test a cosmetic?
There are no set rules on how you must conduct a stability test for cosmetic products. Of course, for cosmetic OTC products like sunscreens, AP/DO, or dandruff shampoos the FDA has specific stability test requirements that you have to follow. See the FDA website for more information.
Here is a basic format you can follow for conducting a cosmetic formula stability test.
Step 1 – Make your batch. Calculate how much to make based on the number of samples you’ll be using for the test. It’s a good idea to make 30-40% more than you think you’ll need.
Step 2 – Fill your samples. Ideally, you’ll have the correct packaging but don’t count on it. When appropriate, fill glass jars with the product along with the finished package. In stability testing, you want to do both glass and packaging if possible. The number of samples depends on how much testing your doing but at minimum you should have 2 samples for each storage condition.
Step 3 – Take initial readings. Once you have a sample filled test it for all the characteristics you’re going to evaluate later. The exact tests depend on the product but minimally you’ll want to record notes about the appearance, color and fragrance. You’ll also want to take pH and viscosity readings. For aerosol products you will test spray patterns.
Step 4 – Put samples at different conditions. Stability testing requires different temperature and light conditions. Some standard temperatures include 50C, 45C, 37C, 25C (RT), and 4C. You’ll also want to conduct a freeze/thaw stability test which involves cycling your product through 24 hours of freezing then 24 hours of thawing. Different lighting conditions involve a fluorescent light box and a natural light box (to simulate sunlight).
Step 5 – Evaluate the product. Samples should be evaluated at the following intervals. 2 weeks, 4 weeks, 8 weeks, 12 weeks, and 52 weeks. Only the RT, 37C and 4C samples will be evaluated after one year. The highest temperature samples and the light exposed samples only need to be evaluated for the first three test intervals. The evaluation tests should be the same ones you conducted when taking your initial readings.
Step 6 – Determine stability. After 8 weeks you can confidently decide whether your formula is stable or not. Nearly all products will exhibit some change so it will be up to you (and your boss) to decide whether the product passed or not.
Early in your career, stability testing will be one of the most common activities you’ll do. If you can create a system that you consistently follow, you’ll avoid burn-out and be able to confidently communicate when a product is appropriate to launch.
How does this compare to your company’s stability procedure? Leave a comment and let the rest of the cosmetic chemists here know.
{ 2 trackbacks }
{ 27 comments… read them below or add one }
One of the more common questions when you are talking about stabilities are: How do I run a an accelerated stability test?And how much shelf life this stability will represent? How do i do a correlation between the time I have on stability and real days.
This is very informative info for anyone who is just getting their feet wet in our industry. This is a pretty through break down. Thank You Perry!
Accelerated stability tests just involve storing samples at higher temperatures.
A rough estimate accepted in the cosmetic industry is this.
8 weeks at 45C is equivalent to one year stored at Room Temperature
This isn’t perfect but typically if your sample stays stable for 8 weeks at 45C, it’ll be good under normal consumer storage for a year.
How about the freeze thaw cycle test? how to predict the stability of product whether stable or not?
Thanks
We generally run an accelerated study at 4C (Control) & 50C for 3day, 1wk & 2 wk review, (Any longer than 2 weeks at this temp, I believe will overcook the product) depending on what type of product (e.g. Oil, Alc, Lotion etc.) For F/T testing we gernerally run 3 cycles (In/out, in/out, in/out) and monitor for any precipation or particles falling out of the product.
I would like to than you for your article above. It’s very informative. Is it possible that the shelf life prediction from accelerated stability study differs for different type of cosmetic products? Unlike Pharmaceutical Products, at the moment there’s no clear-cut guidelines for the prediction of shelf life of Cosmetic Products from the result of the accelerated stability study. I had talked to many formulators, and was told that 3 months of accelerated study is equivalent to 3 years of shelf life at room temperature (unlike what you had mentioned – 6 months). I had also found out that ISO 11609 (1995) (Section 5.4 Determination of Stability) had indicate that: “The toothpaste shall meet the requirements of this International Standard after storage at 40 deg C for 3 months under such conditions of time and temperature as will simulate at room temperature for 30 months.” The above quote was taken from the ISO which is meant especially for the toothpaste. Do you have any idea that is there a separate guideline for a particular type of Cosmetic Products i.e shampoos, body baths, lotions etc.? I’ve been searching for the answer for quite some time, I would really appreciate it if you can enlighten me. Thank you.
@Sean – Yes, the predicted shelf life would differ for different products. In general, products like shampoo will last much longer than emulsion based products. Accelerated stability tests are more predictive for non-emulsion products.
I didn’t mention 3 months = 6 months. We always used 45C for 8 weeks to be equivalent to one year of room temperature storage.
There is no specific guideline that I am familiar with for any standard shampoo, body wash, lotion, etc. The FDA has some guidelines for OTC products like sunscreens and dandruff shampoos but those are more related to the active ingredients.
How prediction of shelf life in powder product(compact powder/two way cake),eye shadow and mascara product?thanks a lot
I am someone who needed cosmetics becuase of fibromylagia attacks and was very sensitive to interior windcasting between the years of 2004 to 2008. Since being diagnosed with a disability this year I have had to use a home concoction that I would like to have tested for final ingredients. Is their any help to warrant a working cosmetic product that is not commercially manufactured?
slm
sara l. marion
cincinnati, ohio
What is the stability testing procedure for a Baby wet wipes. Is there any specific guideline available.
The article given above is quite informative.Thanks for such an article.I wanted to know the accelerated stability testing of perfumes in cosmetic formulations.What procedure can we follow for the same.
This website is very helpful. What about accelerated stability at 50C? How does this translate to real time?
Thanks!
At 50C, you create some chemical/physical changes that wouldn’t happen at room temperature so it’s not so easy to give a direct time relationship between 50C and room temperature. But if you want a number, 50C for 2 weeks would be about 3 months at room temperature.
You can use the same procedure for a fragrance stability test.
Would you expect significant changes in viscosity at 50 C due to the harsh condition or would you say that it is indicative of an unstable formula? For instance, a decrease in viscosity from 23,000 cps to 12,000 cps in a surfactant system.
@Jen – if the surfactant system contains particles like an opacifier / pearling agent, then it wouldn’t be surprising to see a change in viscosity. But a drastic change as you’ve indicated in a simple surfactant solution could be a stability problem. However, you should not make stability decisions based solely on the 50C sample. Wait a little longer and see what happens with the 45C or 37C samples.
Do any of you a sample form showing how to lay out your study?
@Mitch – I don’t have one at the moment but will try to create one for a future post.
For a three year expiration date on an OTC product, say a sunscreen or OTC analgesic in cream form, what would the acceptable accelerated program look like? 45 degrees C for how many months? (as well as RT, FT, 25 and 40 degrees C).
Does 3 months at 45 degrees C. ever equal a three year shelf life for a sunscreen?
David
Is there an article or handbook that this rule of thumb comes from?
If i want to measure shelf life of a hand soap,45C for 8 weeks is equivalent with one year?This means that 16 weeks are equal with two years?
@constantinos – That would be a reasonable assumption. Although, the farther out in time you go, the less reliable the prediction.
@Lisa – not really. It’s just an industry standard.
@David – The only acceptable accelerated stability for the FDA is the following (according to the sunscreen monograph). If you want to make a 3 year expiration date claim, you have to conduct stability testing for 1 year. If you want to make a 1 year shelf life claim, stability testing for 3 months is acceptable.
We are developing a shampoo line. It took about 6 months to come up with a formula that we really like. We have encountered two issues:
1. The formula seems to decrease in thickness about 50% after one month in the bottle
2. Can’t seems to re-create the exact formula again, i.e., color is different, not performing like the formula that thinned out
Can you give us any suggestions to our problems?
Thanks,
Diana
@Diana – it’s difficult to offer suggestions without knowing more about the formula. So, if you want to email me separately the formula (or at least the ingredient list), I can offer a better opinion.
For the color problem, you’ll just have to do some color matching and keep track of the amount of dye added.
Hi Perry,
Thanks for your quick response, I’m so glad I found your website, lots of valuable info. What’s your email address, and I”ll send you the ingredient list, to send the formula would take a few days, not at that location. Also, I’m interested in your training program, when does the next one start?
Diana