Why Pesto Turns Brown (And How To Stop It)

The main cause of pesto turning brown is that the herbs have been exposed to oxygen for too long and have started to oxidise. The best way to slow that process is to reduce the amount of time the sauce is exposed to air. This is most simply achieved by ensuring there is always a thin layer of oil on top of your sauce when you store it in your fridge.

Brown pesto doesn't mean it has gone bad

Let's start with a quick word about safety. Just because your pesto has started to go brown doesn't necessarily mean you need to immediately discard it. Far more important is that you don't leave an opened jar of pesto lying around at room temperature for longer than two hours. If you do, the safest thing is to throw it away without a second thought.

What causes pesto to turn brown?

If your jar of pesto has been opened and left in the fridge for a day or two, it's most likely that the change in colour is simply a result of oxidation. This is a chain reaction that happens when delicate herbs are exposed to oxygen and is what causes the deterioration of food, making it a dull colour and sometimes resulting in off-smells or off-flavours too.

This oxidation is largely unavoidable and isn’t dangerous in itself, but even though it's unlikely to make you sick, we recommend throwing away the jar. People eat with their eyes as much as their mouths, and life is too short to eat pesto that isn't in anything other than peak condition.

How to slow the browning of homemade pesto

There's nothing you can do to completely stop pesto's colour change, but there are ways to slow the process down. The first is to briefly blanch the basil leaves before incorporating them into the sauce. This destroys many of the decomposing enzymes that cause the browning in the first place.

Anyone who has ever tried to prevent an avocado from going grey will intuitively know that acids are helpful in slowing down the process too. We like our pesto to have a little hint of citrus, so adding a little lime or lemon juice is a win-win for us. The addition of a little vitamin E powder can slow down oxidation too.

How to stop your shop-bought pesto from turning brown

The only way consumers can slow down the browning of pesto is by limiting its exposure to air once it is opened. Vacuum sealing is one option, but few people have that luxury in their homes.

The other way is to drizzle a thin layer of oil (ideally the same kind of oil used in the pesto) on top of your sauce before transferring it to your fridge. That oil creates a barrier between the sauce and the air and should at least halve the speed of browning and keep your pesto green for longer.