When you take the time to whip up a batch of traditional basil pesto, watching its bright green color fade into a dull brown can be a major letdown after all that effort. The culprit at large is a natural process called enzymatic oxidation. When you chop or blend basil leaves, enzymes called polyphenols react with oxygen, causing the green pigment to darken rather rapidly. Just how quickly this devastating color change occurs depends on multiple factors, including how the leaves are handled and their temperature. So, while conventional pesto begins browning minutes after it's exposed to air, there's one surprisingly effective trick you can use for a more verdant version: Start your saucy endeavor by heading to the freezer.
Freezing basil not only preserves the herb's flavor but also slows down the enzymes responsible for that dreaded brown hue. While pesto's discoloration is a natural occurrence, oxidation also negatively affects the herb's taste. Frozen basil makes for a vividly green base that doesn't sacrifice flavor. There are also complementary techniques you can use to ensure that your freezer-treated basil pesto keeps its beautiful, vibrant green color in your favorite dishes, from eggplant parmigiana to pesto Caprese paninis.
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While lemon juice and other additives like lactic acid are popular solutions for helping homemade and store-bought pesto retain its green color, such extras aren't necessary if you stick to the freezer method. You can freeze the basil whole, or blanch the leaves, puree or chop them, and freeze them in an ice cube tray. Besides starting with chilled basil, you should ensure that the cheese, garlic, and pine nuts are also extra cold to maintain the lowest temperature possible while blending, which will help slow oxidation.
If you really want to go the extra mile, you can chill your blender bowl and blades before pureeing rather than running the appliance at room temperature. You should also try to avoid blending for too long and stick to the pulse function. Processing pesto quickly and repeatedly is another way to help maintain the sauce's color by reducing the amount of heat generated during blending.
It's also possible to preserve the vibrancy of leftover pesto, and you can even store the sauce in the freezer for up to one month. But whether you're freezing it or keeping it in the fridge, the less oxygen it's exposed to, the greener it will stay. So, top your pesto with a layer of olive oil, which creates a barrier to lock in all that verdancy and keep oxygen out. With cold ingredients, frozen basil, and air-tight storage techniques, your pesto will keep its luster while leaving your taste buds delightfully satisfied.