Never eat sad, army green pesto again.
There’s no getting around it: Pesto looks the best right after you make it.
With time, the fresh herb paste can transform from a lovely jade hue to drab army green. There are as many suggested techniques for keeping pesto bright green as there are cooks who make it—but when we tested some of the most popular ones, we found that they aren’t all reliable.
Fortunately, we found two methods that work well: Blanching the basil and adding lemon juice.
To understand how to keep pesto green, it’s important to know what makes it turn brown. The problem is the basil: Cutting, processing, or bruising activates the enzyme polyphenol oxidase in the leaves. This enzyme catalyzes reactions in which the leaves’ colorless monophenol compounds oxidize to form black and brown polyphenols.
One way of preventing oxidation is to inactivate the enzymes that cause it. This can be done by blanching.
To test this method, we made two batches of pesto: one with fresh basil and one with blanched leaves. Both tasted the same, but the unblanched batch started to darken as soon as we scooped it out of the food processor, while the blanched pesto stayed bright green—even after sitting at room temperature for a few hours, in the refrigerator for a week, and in the freezer for several more weeks.
Another way to prevent basil oxidation? Lower its pH with lemon juice.
Browning happens most readily at neutral pH, so making the pesto’s pH more acidic can help slow down this process.
We found that lemon juice, which contains both citric and ascorbic acid, preserved the pesto’s green color without compromising its flavor, adding just a hint of pleasant acidity.
Add 4 teaspoons of lemon juice per 2 cups of packed basil.
If made with blanched basil or lemon juice, pesto will keep in the fridge for up to a week. Stash it in an airtight container, covered with 1-2 tablespoons of oil. (Without oil, the surface of the pesto is exposed to more air and can turn black. The methods above can prevent oxidation from the air that’s already in the pesto, but continued air contact will lead to browning).
Pesto freezes very well. Store it in an airtight container topped with oil and consume within one month.