Dear Valerie: Are there any stability tests you use that can actually predict real world failure, or are there any tests that give a false sense of security? —Rhea Shore

Dear Ms. Shore,

When I first started in the industry, I could never quite wrap my head around stability protocols. I mostly couldn’t understand why there isn’t a consistent method used by every company, or why no regulatory bodies in various geographies etch in stone the “how” of stability testing. While there are stability guidelines published by various organizations, they’re merely just that—guidelines. Each product has different needs, and the reality is it’s very difficult to predict real world failure or long-term behavior of a product. Walk the floor of any trade show, and you will see one laboratory instrument company touting a novel method that can better predict stability. Another colleague in a much more stained lab coat simply explained that (in the US) stability protocols for OTC and pharmaceutical products were adopted by the cosmetics industry and it varies from company to company.

Fast forward 16 years, I’m still perplexed by all of this, but now operate in a state of acceptance. You see, I have come to terms with the nebulousness of cosmetics stability and have developed a stability mantra: all products change over time, it’s understanding how they shift and whether you can accept that change. My favorite phrase is to note that change is “typical and expected.” Sometimes things just are!

The most common evaluation, as you know, is accelerated stability testing. Exposing products to accelerated temperatures can help calculate a shelf life, but it doesn’t always model how a product will change over time. Particularly in the case of fragrance, quats, and certain colorants, exposure to heat doesn’t necessarily extrapolate to extended storage in moderate temperature conditions. In hair color, elevated heat is nearly worthless. It’s always best to supplement accelerated stability with real time stability to see if they correlate. Sometimes, real time turns out just fine.

It’s also common to toss a product into a 50ºC oven for the weekend and check how the emulsion or surfactant chassis survived. This is great if you’re trying to model how a product transports in the back of a UPS truck on a summer day but doesn’t say much about long term stability (unless the answer is it didn’t survive the weekend). Sometimes, a shift in this temperature range just means it should bear a warning to store the product in a cool place.

I once worked with a company that required their surfactant systems to undergo four weeks of 50ºC exposure to be considered “passing.” I find this to be quite excessive, unless you’re storing products in an uncontrolled temperature facility during a Las Vegas July. 

Another facility I visited required extensive centrifuging of everything at high speeds and long durations of time. I could only think about bending a paperclip back and forth until it snaps. Extensive stress breaks anything and doesn’t mean your emulsion isn’t stable because it couldn’t survive 8,000 rpm for 2 hours. When I pressed about the relevance of this protocol, I was simply met with a shrug that it was their company policy.

 It forces me to ask at this point, what does passing even mean? As you even asked yourself, do any of these tests even translate to real world exposure?

My recommendation is to just use common sense. Where is your product selling? If it will endure a hard winter in a Canadian storage facility, you should be conducting extensive freeze thaw and extended freezing evaluation (something most chemists overlook!). If customers will purchase it from afar, conduct shipping testing. Put it in the light, put it in the dark. Keep the product in a humid shower, keep it in a dry place. Throw it in a purse. Leave it in the trunk of a car. You get the idea. Whatever consumers are doing in the real world is what you want to do, and you want to do it as early in the development process as possible. Of course, continue with the industry-standard protocols as those do have value (they just shouldn’t be the only things that are done).

I will give you one test that I do conduct routinely and find quite good enough: evaluating emulsions under a microscope. How the phase droplets have associated with one another is pretty telling.

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