Just as your green beans will get greener when you blanch them in a pot of boiling water, so will basil. Following that logic, for greener basil send the leaves into boiling water (then shock them in an ice bath to stymie the cooking process).
It's a proven technique—that much we can agree on.
But what the pesto people cannot reach a consensus on is how much the blanching affects the overall product: Does it give a less flavorful sauce? The answers are mixed. While most agreed that blanching the basil dulls its flavor to some degree, the extent is contested.
Here's how it shakes out:
I was amazed at how much flavor the blanched basil retained, and how green my pesto was even after days in the fridge! I was a convert. I also found that, just like with fruits and vegetables, blanching my basil before freezing it made a world of difference.
In a departure from almost every other pasta sauce out there, all the charm of pesto is dependent on its fresh, raw flavor. Heat, and in particular prolonged exposure to high heat, is just about the worst thing for it. That's why most store-bought pesto is so disappointing: The high-heat sterilization necessary for canning and bottling cooks the basil, turning its volatile anise-mint scent dull.
So which is it? Does blanching really make a negligible difference?
I doubled Martha Rose Shulman's Bright Green Pesto, blanching (just for 5 seconds, not 15) and shocking the basil in one batch, using raw leaves for the other.
The difference in color was remarkable and most of us thought the taste was, too. The pesto made with raw leaves was herbier, with an immediate and earthy basil flavor; the basil in the blanched pesto was more mellow, leaving more opportunity for the garlic, cheese, and salt to come to the forefront: It was flavorful, but not particularly from the basil.
Ultimately, most of us preferred the raw. (Sorry, Tom Colicchio.) But our photographer Bobbi could barely taste the difference, and said she would definitely blanch for the greener color alone.
My personal conclusion: I'd skip the blanching if planning to serve (or consume) the pesto that same day. But if I'm going to store it in the refrigerator (or the freezer) for some time, where it will continue to oxidize, the flavor degrading anyway, that's when I'd consider blanching first.
Does oxidized pesto bother you—and why? Tell us in the comments!