How To Keep Pesto Green

There's no silver bullet for keeping pesto green, but the biggest gains can be made by either adding a squeeze of lemon juice or limiting the sauce's exposure to oxygen.

A quick science lesson for keeping pesto green

We will start looking at what you can do to keep your homemade pesto green for longer 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 comprise many different molecules, including enzymes, responsible for speeding up the rate of chemical reactions. This includes enzymatic browning, which describes what happens when phenolic compounds encounter oxygen. You've probably heard it called oxidation, which doesn't just make foods look unappealing but can also affect their taste and nutrient content.

You can't keep pesto green forever, but you can keep it greener for longer

Nothing can stop pesto from dulling in colour over time, regardless of what you add to it or how you store it. However, there are things you can do to limit the browning process by preventing the herbs' enzymes from doing what they naturally want to do. Below, we've detailed some of the methods you can employ.

Blanching herbs can keep them green for longer

If you're making pesto at home, blanching basil for 15 seconds will deactivate the enzymes' ability to brown. Immediately after blanching, plunge the leaves into an ice bath to stop the cooking process. You must also dry them thoroughly before adding them to your pesto.

Some people use ice to stop herbs from browning

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. He says this 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.

Antioxidants are the friends of bright green pesto

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 vitamins 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.

Use acidity to slow the rate of pesto losing its green colour

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

Oil prevents oxygen from turning your pesto brown

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

Eating pesto soon after making (or opening) it is the only way to guarantee maximum greenness

Regardless of all the tips and tricks we've outlined, the best thing you can do is refrigerate your pesto and eat it as soon as possible after opening. When the sauce is made, or the jar is opened, it's on a downward spiral toward a less-than-perfect product.