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Tradition says to use a mortar and pestle when making pesto, but tradition says a lot of things. Are they all true? [Photographs: Vicky Wasik]

Pesto is, on the surface, a very simple sauce. It requires no cooking, and has few ingredients. And yet read just any authoritative source on pesto and you're confronted with a laundry list of Dos and Don'ts.

I wanted to explore these rules of pesto-making, so I've been running recipe tests all week long, and in the process I've realized an important thing: it's really easy to get sick of pesto. Now, don't get me wrong, because I love pesto and I get excited for it when summer rolls around and basil crops up at the farmers market. But man, if I'm to be completely honest, I tire of it very quickly.

The reason I can only eat so much is because pesto is, for all its simplicity, a pungent, in-your-face sauce. Don't believe me? Just think of what's packed into it: heaps of fresh, aromatic basil (an assertive herb all by itself), raw garlic, aged cheeses, and rich nuts, plus a good glug of olive oil to hold it all together. There's nothing mild about it. I make this point for a reason, because it helps in understanding what the secrets are to a good pesto. Because what I've come to realize is that the best pesto leans towards milder, sweeter, rounder flavors, but even if you don't achieve that goal, it's still really hard to mess it up, because all those flavors, even at their mildest, are just so dang pungent. This is good news because it means you actually have a wider margin of error than most experts will admit.

Still, we want the best pesto, right? So how do we get there? Let's take a closer look.

The Pesto Pantheon

Despite its simplicity, there are several variables to explore when making pesto. Read those who wax poetic on proper pesto, and you'll usually encounter the following rules:

  • First, there's the basil itself; do it right, and that basil should be freshly picked from the Ligurian hillsides when the leaves are small and the basil plants are flowering. Obviously, that's out of the question for most of us.
  • Next, there's the olive oil, which should be buttery and mild, freshly pressed from Ligurian taggiasca olives. Most of us can find this oil, but it tends to be expensive.
  • Then there's the cheese: Parmigiano-Reggiano and Pecorino Fiore Sardo, and preferably not the sharper, saltier, tangier (and much more widely available) Pecorino Romano that many recipes in the Unites States call for.
  • The nuts are typically pine nuts, though some recipes will substitute walnuts. And most recipes call for them untoasted.
  • Lastly, there's the method: should you use a mortar and pestle, as tradition dictates, or go the easy route and whip it up in a food processor?

I also found, on an Italian website, some necessary "attitudes" when making pesto:

  • Enthusiasm
  • A little common sense
  • Some elbow grease

I decided to run some taste tests in the SE test kitchen to see which of these rules really matter (and whether that website was right about the attitudes). Care to hazard a guess as to which made the most difference?

Probing Pesto

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Before getting into each part, I want to address the glaring omission to my tests: the basil. As essential as basil is to Genovese-style pesto, I concluded that most of us have a limited set of options for our basil. Unless you're reading this from Liguria itself, you probably don't have access to the prized basil of that region. Otherwise, if you grow your own, that's great. If you have access to a good farmers market with beautiful tufts of fragrant basil, go for it. If you have to settle for supermarket clam-shells of hydroponic herb, well, then that's what it's going to be. And frankly, some of that stuff isn't half bad. Basically, get the best fresh basil you can, and that's all there is to it.

The Ratio

Without a doubt, in a sauce as simple as pesto, the ratio of ingredients is of paramount importance. Because I've made a lot of pesto in my life, and because I consulted dozens of recipes in books and online, I started out with a pretty good general sense of how much of each ingredient to use. To arrive at what I think is a pretty great ratio of ingredients in my recipe here, I dialed in the quantities during the process of testing the other components, gradually adjusting and tweaking through each successive batch until my colleagues and I agreed that I'd nailed the sweet spot. Still, it's a matter of personal taste, so if you want it with more garlic, or less cheese, just go ahead and adjust it to your own tastes.

The Olive Oil

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The first test I wanted to do was the olive oil. I'll admit, I went into this one thinking that it would be one of the most important factors in the final sauce. Because there are so many thousands of brands of olive oil on the market, there was no way to try all of them, so I kept it simple here to test a basic premise: does good Ligurian olive oil matter?

To find out, I pitted a bottle of pricey Ligurian oil against a months-old, jumbo tin of cheapo, all-purpose olive oil that we use in the test kitchen for everyday tasks, making two equal batches of pesto with the only difference being the oil.

Nearly every taster in the office (there were about 4 or 5 in most cases, who, I should note, never knew which variable I was testing), preferred the pesto made with the Ligurian oil—just one gravitated to the pesto with the cheaper oil. But while the Ligurian oil came out on top, being more buttery and rounded in flavor, all agreed that the differences were incredibly subtle. In fact, few realized the oil was different, assuming I had changed the garlic or some other ingredient.

Going back to the pungency of pesto, this starts to make sense: When loaded with basil, garlic, aged cheeses, and nuts, the nuances of a good oil become much harder to taste. That doesn't mean it makes no difference, but the differences aren't as stark as one might think.

I'd imagine that if you used an absolutely awful, rancid oil, or a very, very spicy, aggressive olive oil, those differences might be more apparent. But as long as you're using a decent, somewhat mild olive oil, your pesto is going to be just fine; if you feel like using an even higher quality, not-too-spicy oil, whether Ligurian or not, it may be subtly better.

The Cheeses

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From left, Pecorino Fiore Sardo, Pecorino Romano, and Parmigiano-Reggiano.

Historically, the cheeses of true Ligurian pesto are Parmigiano-Reggiano and Pecorino Sardo (or Fiore Sardo), a Sardinian sheep milk cheese. When pesto first became popular in the United States, however, Fiore Sardo was pretty much unavailable here, so recipe writers substituted the next best available thing: Pecorino Romano. But Pecorino Romano is saltier, sharper, and tangier than Fiore Sardo.

So my question was: Does it really matter? To test this, I made two batches of pesto. The first had equal parts Parmigiano-Reggiano and Pecorino Fiore Sardo, which tends to be the ratio of those two cheeses given by most recipes I looked at. In the other, I did Parmigiano-Reggiano and Pecorino Romano, but here I used 50% more of the parmesan and 50% less of the Pecorino Romano—using more of the parm and less of the romano tends to be the solution most recipes use to account for romano's more assertive flavor.

Tasters preferred the batch with Fiore Sardo, with its ever so slightly sweeter, fruitier, less harsh flavor. And yet here too the differences were incredibly minor. So, once again, if you can find the Fiore Sardo, it will make a marginally better pesto sauce, but Pecorino Romano makes a darned good one too (frankly, Parmigiano-Reggiano alone makes a great one as well).

The Pine Nuts

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In all the recipes I looked at, the pine nuts are tossed into the sauce as-is, but I wondered whether there was any benefit to toasting them first, which would enhance their nutty flavor.

Side-by-side, though, the two batches I made were nearly indistinguishable from each other: not one taster could tell the difference. Because I had made them and knew what flavor I was looking for, I could faintly detect the roasted nut flavor in the lingering aftertaste of the sauce, and frankly, I didn't think it did the sauce any favors—it detracted from the sweet roundness that makes a good pesto taste good. But if they're not looking for it, I'm not sure most people would even notice.

The verdict: toasting is not worth it.

Do You Butter?

In a couple recipes, including Marcella Hazan's in The Essentials of Classic Italian Cooking, a small amount of butter is worked into the pesto in addition to the olive oil. I made a batch with butter, but no one could figure out how it was different. It may marginally help bind and emulsify the sauce when you toss it with pasta, but it turned out to not be an essential addition.

The Method: Mortar or Processor

If there's one shortcut most people make with pesto sauce, it's using a food processor, which turns a somewhat laborious sauce into one of the quickest and easiest things to make. But just look at the name, pesto. It comes from the Italian verb pestare, which means to crush or mash. A mortar and pestle can crush and mash (just look at that word pestle too!), but a food processor chops and minces. Is this an acceptable shortcut? Two side-by-side batches would reveal all.

Food Processor

For my food-processor batch, I used a mini-processor, because I was working with a half batch of pesto, and made sure to stir the olive oil in at the end, since blending olive oil at high speeds can give it bitter flavors, which seemed like an unfair thing to do in this test.

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I started by pulverizing the garlic and pine nuts together.

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Then I added the basil and pulsed it until finely minced.

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I added the olive oil in a thin stream, stirring it in.

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Looks good!

Mortar and Pestle

I made the same recipe, using the exact same ratio in the mortar and pestle as well.

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I started by combining the garlic and some sea salt, and working the garlic into a paste.

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Then I added the basil in handfuls, crushing the leaves against the side of the mortar until they were reduced to little bits before adding the next handful. Bashing the basil with the pestle doesn't work well here and will quickly give you a headache from the loud noise. Instead, work the pestle in a circular motion, grinding the leaves down.

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Once all the basil has been crushed into little bits and the green juice has stained the side of the mortar, it's time to add the pine nuts (some recipes have you do the garlic and pine nuts at the same time, followed by the basil, but I figured it would be easier to break down the basil if there wasn't too much "padding" already in the mortar).

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For the pine nuts, I found that I had to target them, smashing individual ones under the pestle, before they yielded into a paste.

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The cheese is the last addition.

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I worked it until I had a pretty good, uniform-ish paste going.

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Next, I drizzled in the olive oil, working it in with a wooden spoon.

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The final mortar and pestle pesto was lookin' pretty good too.

The Method Results:

The proof, though, was in tasting them.

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Side-by-side, it can be hard at first to see the differences, but the mortar and pestle pesto (left) had pieces of basil of different sizes, and their texture was soft, silken, and tender, totally yielding. The food processor pesto, meanwhile, had lots of uniform little bits, but in the mouth you could feel them all, almost like grit compared to the mortar one.

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The mortar and pestle version, top, was silkier and more luxurious in texture.

The flavor was different too. The mortar and pestle pesto was infused with the flavors of all the ingredients.

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On pasta, the differences really become clear. The mortar and pestle pesto (left) coated the noodles beautifully, bathing them in a gorgeous, green-tinted sauce. The food processor pesto just stuck to the pasta in little bits, like glitter, and the oil had absorbed little of the basil's color. The best pesto, as its name has insisted all along, should be made under the pressure of a pestle.

Conclusion

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 includes selecting a good, mild olive oil, using Pecorino Sardo cheese instead of romano, and not toasting the pine nuts. Not following the rules above won't ruin your pesto, though.

Staying true to tradition and using a mortar and pestle makes a much more noticeable difference. If there isn't a chance in hell that you'd ever use a mortar and pestle, then yes, you can still make pesto in a food processor and it will be fine, but it will be just fine. It won't be special. It won't be magical and beautiful. Because that requires some special attitudes: enthusiasm, common sense, and definitely some elbow grease.

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