Pesto, made with pine nuts and Italian cheese, is not a cheap sauce, mainly because of those pine nuts. It makes sense then, that one should want their pesto to look as good as it tastes, even if it’s been sitting in the fridge for a few days. The problem, though, is oxygen.
Oxygen has the pesky habit of oxidizing things—in this case, the basil, which turns from a bright, vivid green to a dull, murky brown. Luckily, there are three ways you can prevent this from occurring: two that are easy and lazy, and a slightly more involved, less lazy, but still not hard, way.
Top off your pesto with olive oil
The first easy way: You know what makes a great little oxygen-free barrier? Fat! Oil! The very stuff you mixed into your pesto the make it luscious and spreadable in the first place. Once you’ve packed your pesto in a pretty little jar, smooth out the top and pour a layer of olive oil on top. The oil will keep air out of your pesto, allowing it to stay fresh and green for up to a week. Stir before serving.
Add some (more) lemon juice to pesto
The second easy way: You’ve probably heard that vitamin C is an antioxidant, and now you get to see it in action. According to Cook’s Illustrated, adding straight ascorbic acid and/or citric acid can give pesto an unpleasant sour flavor, but lemon juice (which contains both acids) blends in. Aim for a ratio of 2 teaspoons fresh juice for every cup of packed basil.
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Blanch the basil
The slightly more involved way: Oxygen cannot oxidize all on its own. The enzymes in the green leaves also play a part. According to Cook’s Illustrated, blanching “deactivates the enzyme that causes browning when cut basil leaves interact with oxygen.” Just submerge the leaves in boiling water for 30 seconds, then plunge into ice water to keep them from “cooking.” Dry thoroughly before adding to your pesto.
