The Correct Duration for Stability Tests

Dear Valerie: I’ve heard some people say an accelerated stability of three months yields a shelf life of two years, while others say three. Which is it?  

— Unstable on the Bench

Dear Unstable:

Stability testing is interesting because no universal protocol sets rules for calculating a product’s stability and its corresponding shelf life. There are generally accepted methods, and several published guidelines are available online or for purchase. While companies follow similar testing protocols, I’ve found that there are subtle differences between organizations, such as temperatures and time points in which the product is evaluated.

The reason there is no globally accepted protocol for determining shelf life is there is very little published research or established protocols that accurately determine shelf life for every product. The type of products we produce are vastly different—a facial cleanser and hair color have completely different physical and chemical properties. They go in different types of packaging. They are also evaluated for different aesthetics, physical and chemical parameters, light stability and micro, to name a few. (Permanent hair color has the luxury of no micro-testing!)

Therefore, it’s pertinent that a protocol be developed based on the product type, using know-how in how products change over time, and determining for what attributes products should be evaluated. By gathering historical data over various time points and temperature conditions, plus the use of mathematical equations like the Arrhenius equation, you can develop a stability protocol that uses scientific reasoning to justify a 1-, 2- or 3-year shelf life for a product.

As you can see, this variability makes it completely plausible to develop a 2- or 3-year shelf life off a 3-month accelerated stability test, depending on the product and temperature conditions used. It’s not meant to be a blanket statement, but one that should be backed in scientific reasoning. I also think it should be coupled with real-time monitoring of products, as some products tend to perform not so great under certain temperature conditions in stability, but it’s not reflective of how they perform in real life. It’s rarely the other way around. I also tend to follow the OTC-drug line of thinking and conduct six-month stabilities for a three-year shelf life. This gives me a little extra assurance in how the product performs over time. I don’t like surprises in the market!

Valerie George

askvalerie@icloud.com

Valerie George is a cosmetic chemist, science communicator, educator, leader, and avid proponent of transparency in the beauty industry. She works on the latest research in hair color and hair care at her company, Simply Formulas, and is the co-host of The Beauty Brains podcast. You can find her on Instagram at @cosmetic_chemist or showcasing her favorite ingredients to small brands and home formulators at simply-ingredients.com