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Ask Jenna is a monthly beauty advice column from Bazaar’s beauty director, Jenna Rosenstein. Do you have a burning beauty question that you want answered in next month’s edition? Don’t be shy: You can always slide into Jenna’s DMs on Instagram.
Dear Jenna,I use a vitamin C serum every morning, and it’s turned brown—it’s not even old! It also smells weird, though it always smelled weird. Is this serum even doing anything? How do I know if it’s gone bad?
If there’s one thing I know about vitamin C serums, it’s that they’re notoriously unstable—and kind of stinky. Some of the most popular serums out there have a scent best described as “hot dog water,” and “old pennies,” and come in shades ranging from pale yellow to deep amber. So it’s a great question: how can you tell if your vitamin C serum is still effective at protecting and brightening your skin?
I’m no scientist, so I reached out to Michelle Wong, a science educator and cosmetic chemist, who is known online to her 650,000 followers as @LabMuffinBeautyScience. Wong conducted independent experiments on this exact question using a basic L-ascorbic acid formula, one of the most potent and widely used forms of vitamin C in skincare. “That’s the ‘true’ type of vitamin C, and it’s the most unstable,” she says. “With other vitamin C derivatives, they tend to go bad a lot slower, and it’s harder to tell.”
Your first sign that your serum may be off is the smell. “Vitamin C is weird. It breaks down into a bunch of different reactions, and which one dominates depends on the conditions—temperature, air exposure, light exposure,” she says. “With one of the reactions, the smell does change.” So if you notice a significant change in the scent of your serum, it’s time to toss it.
Now for the color. In her experiment, Wong put one bottle of L-ascorbic acid serum in the fridge, another at room temperature, and a third in direct sunlight. The colors of each serum changed dramatically. The one in direct sunlight darkened the most, turning to a deep brown. The one at room temperature darkened as well, but not as much as the first. And the one in the fridge? It stayed pale. But here’s what’s interesting: the difference in oxidation among the three formulas wasn’t huge—they all showed reduced efficacy to some degree. “The color changing means it’s going to be somewhat less effective. But it’s probably not entirely ineffective,” she says. “I also tested some years-old, super dark-orange products from a few brands, and they still had some activity.”
Wong advises checking the expiration date on your serum bottle and sticking to it, as most reputable beauty brands will have conducted tests to determine the efficacy. And when it comes to the packaging—darkened glass for light protection, airless pumps to prevent oxidation—the delivery system doesn’t matter all that much. “Airless pumps aren’t as airless as they might seem since oxygen molecules are tiny, and can get through any gaps and even through a lot of plastics,” the chemist says.
Her best advice for daily vitamin C users? “Get the most out of your serum by using it consistently, and not buying more than you can use up,” she adds. “Don’t get swept up in the rush of buying new products, which is easy to do on social media.”
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