“Does vitamin C serum have any potency after it turns yellow?”

“Should I throw out my vitamin C serum after it turns yellow?”

“Is L-ascorbic acid pro-oxidant after it turns yellow?”

I have gotten this question many, many times in the past decade. Vitamin C serums tend to turn yellow or crystallize after repeated exposure to light, heat, and air. The change in hue has lead to widespread anxiety over whether or not vitamin C serum that is no longer clear still has benefit for the skin.

L-ascorbic acid is notoriously difficult to stabilize in skin care because it is such an excellent antioxidant. What this means is vitamin C is a very enthusiastic electron donor, cheerfully donating electrons to the oxygen content in air, forming dehydro-L-ascorbic acid (DHAA). If further oxidized, the ring will open and the molecule (now called diketogulonic acid) will become completely useless. All of this happens within hours after initial exposure.

But does vitamin C serum actually do anything to help your skin after it turns yellow? I decided to run an experiment to test to see how much L-ascorbic acid is actually present in vitamin C serums with the following characteristics:

15% L-ascorbic acid in water 15% L-ascorbic acid, 1% vitamin E, and 0.5% ferulic acid 15% L-ascorbic acid, 1% vitamin E, and 0.5% ferulic acid exposed to light, heat, and air over a 48-hour period 5% L-ascorbic acid in water

How the Experiment Works

Dichlorophenol-indophenol (DCPIP), a form of indophenol, is often used to determine the presence of vitamin C, or ascorbic acid (Journal of Pharmacological Science).

The darker the color, the LESS vitamin C is present.

The lighter the color, the MORE vitamin C is present. If there is a high concentration of vitamin C in the solution, it “neutralizes” the indophenol, preventing the formation of the purple color. So the amount of vitamin C present in a solution may be measured by adding a small amount of each vitamin C solution, and then adding concentrated indophenol solution dropwise. The solutions with the most vitamin C will appear the lightest purple; the solutions with the least vitamin C will appear the darkest purple.

Our Results Indicate Store-Bought Serums with Stabilizers Like Ferulic Acid are More Stable than 5% or 15% L-Ascorbic Acid-Only Serums

Going into this experiment, I thought the 15% L-ascorbic acid, 1% vitamin E, and 0.5% ferulic acid serum would contain more vitamin C than the 15% L-ascorbic acid in water, followed by the 15% L-ascorbic acid, 1% vitamin E, and 0.5% ferulic acid exposed to light, heat, and air over a 48-hour period, and then the 5% vitamin C solution.

But I was wrong.

It turns out this is the relative amount of vitamin C left in each solution, from highest to lowest:

15% L-ascorbic acid, 1% vitamin E, and 0.5% ferulic acid 15% L-ascorbic acid, 1% vitamin E, and 0.5% ferulic acid exposed to light, heat, and air 15% L-ascorbic acid in water 5% L-ascorbic acid in water

It appears that ferulic acid has enough of a stabilizing effect to best even the 15% L-ascorbic acid solution I made fresh today. Studies show that ferulic acid is an effective stabilizer, but I must admit that I was surprised it was effective enough to preserve more vitamin C than plain water even after exposure to significant light, heat, and air. (For the record, I sat the vitamin C serum on the windowsill for 48 hours, and microwaved the solution for 8 seconds). The fact that ferulic acid creates that much of a stabilizing effect is astounding.

Each Shade of Yellow Appears to Make Vitamin C 10-20% Less Potent

As for the effect of light, heat, and air on vitamin C serums, I will say that it appears a shade or two difference in color means 10-20% more DHAA is present, which makes vitamin C content about 10-20% lower. If your vitamin C serum is significantly yellow or orange, I would lower my estimate of vitamin C by about 40%-50%.

What FutureDerm Labs Determined About Vitamin C Serums

The most important aspect of vitamin C serums is that they include a stabilizer. Our results show that 15% L-ascorbic acid in water loses potency faster than 15% ascorbic acid + 0.5% ferulic acid exposed to 48 hours of light, heat, and air exposure. Heck, I even microwaved the tube, and it still contained more vitamin C than a 15% solution without a stabilizer exposed to air for minutes! I was shocked by how fast vitamin C decays without a stabilizer. You absolutely need a stabilizing ingredient like ferulic acid.

A far second in determining vitamin C serum potency is its color. Curiously, a shade different in color only amounted to 10-20% difference in vitamin C content. Even the darkest vitamin C serums are about half as potent as the clearest vitamin C serums. So while a Vitamin C product needs to be properly packaged in tube packaging or an airless pump bottle, it does not appear to make much of a difference. What makes the most significant difference by far is that the ingredients also include a stabilizer like ferulic acid.

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