Basil Pesto, and Why It Turns Bitter

There is a moment about twenty seconds into making basil pesto when the garlic and the leaves hit each other and the whole kitchen changes. That smell is the reason to make it rather than buy it. Nothing in a jar has ever come close.

It is also easy to get wrong, in two particular ways. Homemade pesto has a habit of coming out bitter, or turning brown in the fridge overnight, and often both at once. Neither has much to do with the recipe, since it is usually the same few ingredients. Both come down to how long the machine runs and how the oil goes in.

Start With Dry Basil

Wash the leaves and dry them properly, which a salad spinner does in about ten seconds. Water clinging to basil thins the sauce and dulls everything in it, and pesto should be thick enough to sit on a spoon and stay there.

Ordinary sweet basil is what you want, the big-leaved kind sold everywhere and sometimes labelled Genovese. Two cups packed is the amount you want, and packed means wildly different things to different people, so if you have a scale it is two ounces.

Why the Oil Goes In Last, and Slowly

Here is the part that decides whether your pesto turns bitter.

Basil bruises easily. A spinning blade generates heat, then the cut leaves get introduced to the air, and the longer the machine runs the more of both you get. It is what happens to torn basil left sitting in a bowl, only faster and hotter.

The trick is keeping the basil in there as briefly as possible. Chop the nuts and garlic first, on their own. Add the basil and pulse just until it breaks down, not a second longer. Then, with the machine on low, pour the oil through the feed tube in a thin steady stream.

That slow pour is not fussiness. It emulsifies, which is why good basil pesto is thick and glossy and pale green rather than a loose oily slurry with green bits floating in it. Tip the oil in all at once and it will not come together, and you will find yourself running the machine another minute to fix it, which is the exact thing that made it bitter in the first place.

The cheese goes in by hand at the end. A processor turns Parmigiano pasty, and pasty cheese in basil pesto is its own kind of disappointment.

The Third Thing the Oil Does

Pesto browns because of air, not because of time. The surface oxidizes and goes dark while everything underneath is still bright green, which is why a jar can look dull and be perfectly good a spoonful down.

The fix is a thin layer of olive oil poured over the top before the lid goes on. It sits there as a physical barrier and keeps the air off. It is a step that gets passed along without much explanation, which is a shame, because once you know the reason you also know to top the layer up again each time you dig in.

Worth saying too that the oil here is not a background ingredient. Half a cup against two ounces of basil makes it one of the two things you are actually tasting. A flat supermarket oil gives you a pesto that is fine and a little dull. A fresh peppery one gives that catch at the back of the throat people notice in good pesto and usually falsely credit to the garlic.

Making Pesto Without Pine Nuts

Pine nuts are traditional and they have become genuinely expensive. There is no reason to feel bad about reaching for something else.

Walnuts are the usual answer and they are a good one. Cheaper, easier to find, and once everything is blended most people could not tell you what is in there. Almonds and pistachios both work. Pumpkin or sunflower seeds will do it if you are avoiding nuts altogether, though the result reads a little more like a basil dressing than a pesto.

Toast whichever you use for a few minutes in a dry pan first. Thirty seconds of work and it deepens everything.

Two Things We Leave Out

The first is blanching, dipping the leaves in boiling water and then into ice. It does work for color, since the heat kills the enzyme that browns the leaves. It also costs you aroma. Given that the whole point of making pesto at home is that it smells better than the supermarket jar, that is the wrong trade, and the oil layer holds the color well enough on its own.

The other is lemon. Classic pesto alla Genovese has none, and there is a reason for that. A small squeeze does brighten the sauce and help hold the green, but it is very easy to tip past that and end up with something that tastes mostly of lemon. A teaspoon, if you want it at all.

Basil Pesto

Makes about 1 cup  ·  10 minutes

Ingredients

• 2 cups packed fresh basil leaves, about 2 oz, washed and dried well

• ⅓ cup walnuts, or pine nuts

• 2 garlic cloves, roughly chopped

• ½ cup extra virgin olive oil, plus more for storing  

• ½ cup finely grated Parmigiano-Reggiano

• 1 teaspoon lemon juice, optional

• Fine sea salt and freshly ground black pepper

Instructions

1. Process the nuts and garlic on their own for about ten seconds, until coarsely chopped.

2. Add the basil and pulse until it breaks down into a rough paste. Do not let the machine run continuously.

3. With the processor on low, pour the olive oil through the feed tube in a thin steady stream. Stop when it turns thick and glossy.

4. Tip it into a bowl and stir the Parmigiano in by hand, with the lemon juice if you are using it.

5. Taste, then season with salt and pepper. The cheese is already salty, so taste before you add any.

How to Store Pesto

Spoon it into a jar, smooth the surface flat, and pour a thin layer of olive oil over before sealing. About a week in the fridge. Use a clean spoon each time and top the oil layer back up afterwards. For longer, freeze it in an ice cube tray and move the cubes to a bag once they are solid. A single cube stirred into hot pasta on a grim Tuesday in February is one of the better arguments for making a big batch in August.

Serving It

Stirred through hot pasta with a splash of the cooking water is the obvious one and still the best. Beyond that, spoon it over roasted potatoes, spread it in a sandwich with mozzarella and tomato, stir it into scrambled eggs at the last second, or drop a spoonful into a bowl of soup. A jar in the fridge changes what is possible on a weeknight.

Six ingredients, and the oil is half the volume of the finished sauce. Di Marco Olio is grown and pressed on one estate in Cefalà Diana, Sicily, picked by hand in the fall and cold pressed within forty-eight hours.