Why vitamin C serums turn brown, and what stops it

You buy a vitamin C serum, and two months later it has gone yellow. A month after that it is orange-brown. At that point it is not just doing less, it can irritate the skin.

That is oxidation, and it is the reason ferulic acid keeps turning up on ingredient lists next to vitamin C.

What ferulic acid does

It is an antioxidant that occurs naturally in plants, in bran, oats, rice and the seeds of apples and oranges. In the plant it is part of the defence system against sun and stress. On skin it does two things.

First, it catches free radicals itself, the unstable molecules that UV and pollution generate.

Second, and this is the useful part, it keeps vitamin C and vitamin E stable for longer. Vitamin C on its own oxidises quickly. With ferulic acid alongside it, the whole formula stays active longer, both in the bottle and on your skin.

What the research shows

A 2005 paper in the Journal of Investigative Dermatology tested vitamin C, vitamin E and ferulic acid together. Its title says the result plainly: ferulic acid stabilises that combination and doubles its sun-damage protection.

Online you will find claims of eight times more protection. That is not in the paper. Doubling is already a solid result.

What it is not

It is not sun protection. It does not block UV and it does not replace your SPF. It cleans up the damage that happens despite your SPF, which is different work.

And despite the name, it is not an exfoliant. The word acid here describes the chemistry, not an effect on your skin. It does not sting and it does not peel.

Why you rarely buy it separately

Standalone ferulic acid serums exist, but there is not much point. It earns its keep in company, and if it is already in your vitamin C serum, it is doing its job there.

That is also why you never see it on the front of a bottle. Its work is invisible: it makes sure the ingredient you actually paid for still works by the time you get to the end.

Where ours sits

Protective Power combines ferulic acid with vitamin E and ascorbic acid stabilised using gold. The same three components the research is about.

Why the percentage on a vitamin C serum tells you less than you would think is covered in the article on vitamin C.

Frequently asked questions

How do I know my serum has gone off?

By the colour. Clear to pale yellow is fine. Clearly orange or brown means the vitamin C has oxidised, and the ferulic acid can no longer help.

Morning or evening?

Morning. Antioxidants are most useful at the moment UV and pollution reach your skin.

Do I need it if my serum already contains it?

No. Adding a second product alongside adds very little.

Sources

• Lin FH et al., Ferulic acid stabilizes a solution of vitamins C and E and doubles its photoprotection of skin, Journal of Investigative Dermatology, 2005.