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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:
- Fine Cooking calls basil-blanching "The Trick to Smooth, Creamy Pesto." Blanching is not solely for cosmetic purposes, but also for textural reasons: Cooked basil will emulsify more easily, resulting in a smoother sauce. "Blanching will slightly reduce the potency of the fresh basil flavor, but because a good bunch of basil starts out so incredibly fragrant, the reduction is minimal."
- "Doing this leaches out a wee bit of the basil’s vivid flavor, but not enough to change that of the pesto significantly," writes Martha Rose Shulman in her recipe for Bright Green Pesto on the The New York Times.
- Saveur includes blanching and shocking as part of a "classic version" of Genovese pesto to "give the sauce a brilliant green hue and to reduce any bitterness." (But the recipe for Emiko Davies's Pesto alla Genovese here on Food52 starts with raw leaves.)
- Red Fern Farm laments discoloration as "the eternal problem of pesto-makers everywhere":
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.
- The blog "Food and Style" lauds blanching as a method for "all of the flavor, none of the funky brown color" and is adamant on the flavor retention: "You may think that blanching the basil will take away some of its spunk, but be assured that it doesn’t. Indeed, the technique won’t take an ounce of flavor out of the aromatic leaves."
- And different sources recommend different blanching times: Shulman warns against going over 10 seconds, while Michael Chiarello on the Food Network instructs 15.
- And so does Tom Colicchio. Yes, even Tom Colicchio blanches! (Cue gasps.) "But if you make pesto my way—blanching the basil quickly and shocking it in ice water—you'll find that it stays bright green and that it tastes every bit as fresh as when it's made with raw basil. It even keeps the same, big, fresh aroma."
- The Kitchn tested the tip and declared it "mind-blowing... but with a caveat": "The winner here? Unblanched pesto by a mile. It tasted herbaceous, fresher, and well, like fresh basil. The blanched pesto's flavors were muted, and to be honest, didn't really taste like basil anymore." But, they conducted their taste with a 15-second blanch, much longer than Martha Rose Shulman (and others) suggests.
- Daniel Gritzer, writing about pesto pasta for Serious Eats, is firm about the most important rule of cooking with pesto: "Don't cook it." We must assume that this applies to the leaves themselves, too.
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.
- But in another article, this one called "The Best Pesto," he concludes that "Because pesto is such a strong sauce, anything you do to reduce its pungency just a little and steer it towards a sweeter, rounder-flavored sauce, will help." That leaves the question, not addressed in that post, up in the air: Does blanching tone down the sauce, or make it less herby and more garlic- and cheese-forward and therefore sharper and more biting?
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!