How To Keep Pesto Green

Lemon juice, a source of citric and ascorbic acid

Short answer
There's no silver bullet when it comes to keeping pesto green, but the biggest gains can be had by either adding a squeeze of lemon juice or limiting the sauce's exposure to oxygen.

Long answer
We're going to start looking at what you can do to keep pesto green if you're making it yourself, and then what you can do if you're working with a shop-bought sauce. Firstly, let's get our heads around the science of why the sauce's vibrant green colour can quickly fade.

Fruits, vegetables, and herbs are made up of many different molecules, including enzymes, which are responsible for speeding up the rate of chemical reactions. This includes enzymatic browning, a term used to describe what happens when phenolic compounds encounter oxygen. This changes the phenols into the pigment melanin, which has a brown colour. You've probably heard it called oxidation, which doesn't just make foods look unappealing but can affect their taste and nutrient content too.

There's nothing you can do to completely stop pesto from turning brown, but there are things you can do to limit the browning process by preventing the herbs' enzymes from doing what they want to do. We've detailed below some of the methods you can employ.

Bright green pesto

If you're making pesto at home, blanching basil for 15 seconds will deactivate the enzymes' ability to brown. Just make sure to stop the cooking process by plunging the leaves into an ice bath straight after. You will also need to dry them thoroughly before adding them to your pesto.

Some people think that blanching herbs to retain their colour is unnecessary. The three Michelin-starred chef, Massimo Bottura, recommends adding an ice cube when making breadcrumb pesto in a blender. This, he says, is to prevent the heat of the blades from oxidising the basil. It's not for us to question the wisdom of one of the world's best chefs, but we don't like the idea. Not only will it make your pesto watery, but making pesto in a blender is not recommended. The crude action of the blades can turn the sauce bitter, which is probably why he needs to add a bit of sugar to his recipe.

Often confused with preservatives (which slow down the growth of bacteria and microorganisms), antioxidants specifically inhibit oxidation. There are hundreds of natural antioxidants, but anything high in vitamin A, C, or E works well. Another popular antioxidant in the food industry is flavourless rosemary extract, so don’t be perturbed if you see this on the ingredients list of your favourite sauce.

Browning enzymes don't just love oxygen; they perform best within a specific pH range. If the pH is high (alkaline) or low (acidic), then enzymes' effectiveness will be significantly reduced. In the case of pesto, adding a little acidity (normally in the form of lemon or lime juice) doesn't just slow down the browning thanks to its citric and ascorbic acid content, but can make the sauce taste better too.

The presence of oxygen is the main reason why herbs and pesto turn brown, so this is where the biggest gains are to be had. Vacuum-sealing the sauce is ideal, but a much easier option is to add a thin layer of oil on top of the sauce before putting it in the fridge. This protects the sauce from the air, although the browning process will resume as soon as the presence of oxygen is re-established.

Time and refrigeration
Regardless of all the tips and tricks we've outlined, refrigerating your pesto and eating it as soon as possible after opening is the best thing you can do. As soon as the sauce is made, or the jar is opened, it's on a downward spiral towards a less than perfect product.