Can pesto really be prevented from darkening by blanching the basil before adding it to the food processor?
To find out, we made two batches of pesto: one with fresh basil and one with blanched leaves. The pesto made with fresh basil started to darken as soon as we scooped it out of the food processor, but the blanched batch stayed bright green even after sitting for a few hours on the counter. When we sampled the sauces, tasters found them virtually identical in flavor. The good news continued: After an entire week in the refrigerator, the blanched-basil pesto was still a brilliant green, as was a sample that we froze for three weeks and then thawed. The sample that had been frozen tasted great, too.
Here’s why blanching works: Cutting, processing, or bruising activates enzymes within the basil leaf that promote rapid oxidation, darkening its bright green color. Blanching (dunking the leaves in boiling water for 20 to 30 seconds, then plunging them into ice water) inactivates those enzymes, so the color holds fast.
If you’re making a limited quantity of pesto to use right away, blanching is hardly worth the trouble. But if you’re transforming a bumper crop into a year’s worth of pesto, the process will ensure vivid color that lasts.