How to Stop Ingredient Oxidation 

Dear Valerie: I know it’s common for skin brightening ingredients like retinol, kojic acid and hexyl resorcinol to oxidize over time. Why does it happen and how do I prevent it? I’d be grateful for any formulation tricks! 

—Rusty Onabench

Dear Rusty:

These pigment-busting molecules certainly are delicate. Whether it’s air, light or the presence of water, it can be very challenging to keep them stable. While I don’t personally formulate with these ingredients, I do have a bit of experience in preventing precious ingredients from oxidizing because this is the basis of hair color chemistry.

If you look closely at the chemical structure of these molecules, you’ll see a common feature—they’re all electron-rich, ring-based systems. This subatomic opulence gives them a magnanimity highly desired on skin because it’s the basis of their efficacy!

Essentially, they function as antioxidants; meaning, they easily donate electrons when they counter reactive oxygen species (free radicals). These things want to react. It’s most ideal if you can save this electron-sharing generosity for when and where you want it. By that, I mean, keep them non-reactive while they bide their time on the shelf; have them react on the consumer’s skin when they need that efficacious punch. If only it were that simple.

I always find it funny—the strong, tough-guy anthropomorphic language we use to characterize the efficacy of antioxidants. Put reckless-free electrons out of their misery, saving skin! While this can be the case, an antioxidant’s ability to lose electrons is a little too easy. They’re very delicate! They want to react, and they’re non-discriminatory about it. The irony of it is, once an antioxidant has neutralized a free radical, it itself has been oxidized and will not be restored. And the more efficacious an antioxidant is, the more reactive it is. The less efficacious antioxidants don’t really do much for us and are less desired.

Fortunately, there are some formulation tactics to help reduce the sensitivity. One—add more antioxidants (we use the term reducing agents in my world). By bolstering your formulation with even more protective ingredients like ascorbic acid, tocopherol, cinnamic acids and acetyl zingerone, you can help reduce the impact on your sensitive ingredients. There is also a handful of evidence that tocopherol and vitamin C (specifically) help regenerate each other, like a game of hot potato, so consider combinations to reduce some of the burden.

Think about using more stable forms of these ingredients, such as the ester versions of retinol and kojic acid, retinyl palmitate and kojic acid dipalmitate, respectively. They’re more stable because hydroxy groups have been replaced with ester moieties. This also makes them less antioxidant-y, so formulate wisely.

Encapsulation is another tool to reduce oxidation. Contact your raw material suppliers to learn how encapsulation can essentially hide the sensitive molecule by loading it into a lipophilic core. The molecule is released once it’s applied to skin. These can be pricier than the molecule itself, and you must consider the “active level” of the molecule inside the liposome. For example, an encapsulated retinol may only have 1% retinol in the encapsulation. You would need to use 10% of the encapsulate to get 0.1% active retinol. This can be costlier, but also save you a formulating headache.

Avoid oxidizers. Fortunately, we do not add many intentional oxidants to cosmetics, but there are unintentional sources, like plant oils. As a plant oil is exposed to air, heat or light, it generates minute quantities of peroxides. These peroxides create more peroxides, which makes sense, considering they themselves are oxidizers. If you are using plant oils in a product with a sensitive antioxidant, make sure the oil has a very low peroxide value specification, and don’t use oils easily prone to rancidity – like oils high in omegas 3 and 6. There is one supplier with an internal stabilization process to remove peroxides generated during expelling, ICSC, so that also may be a consideration.

Other ways to avoid oxidation include manufacturing in a vacuum, choosing smart packaging (airless versus jar, choosing a plastic with an oxygen barrier), and selecting smaller packaging sizes, so the consumer has an opportunity to use everything quickly.

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