Why Pesto Turns Brown (And How To Stop It)
The cause of pesto turning brown, or grey is usually because the herbs have started oxidising. The best way to slow that process is to reduce the amount of time the sauce is exposed to air.
First things first, if you accidentally leave your homemade or store-bought pesto out overnight and wake up to find it has changed colour, then we strongly advise you to discard the whole batch.
If, on the other hand, the jar has been opened and left in a 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 in the presence of oxygen and is what causes the deterioration of food - making them a dull colour and sometimes resulting in off-smells or off-flavours too.
This oxidisation is largely unavoidable and isn’t dangerous per se. 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 producers slow down the browning of pesto
There's nothing anyone can do to completely stop pesto's colour change, but there are various things that producers can do to slow it down. The first is to briefly blanche the herbs 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 going grey will intuitively know that acids are helpful in slowing down the process too. We like a little citrus kick in our sauces, so the use of lime or lemon juice is win-win for us. Our sauces also contain a tiny amount of vitamin E that can slow down oxidation.
How you can help stop your pesto turning brown or grey
The only way consumers can slow down the browning of pesto is by limiting its exposure to air once opened. Vacuum sealing is one option, but few people have that luxury in their home.
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 little bit of oil basically 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.