Ingredient

Pesto

Pesto is a raw sauce. Nothing in it is cooked, and that single fact governs everything — why it must never be heated, why it goes grey, why the jarred version tastes so different from the fresh, and why a spoonful of pasta water is the difference between a sauce that coats and one that sits in a clump.

What is actually in it

Classic pesto alla genovese has seven ingredients and no more: basil, pine nuts, garlic, Parmigiano Reggiano, Pecorino Sardo, olive oil and salt. The name comes from pestare, to pound, and traditionally it is made in a mortar rather than a machine.

The pounding is not nostalgia. A blade chops and bruises, generating heat and forcing air into the basil, which accelerates the browning and gives a slightly bitter, metallic edge. A mortar crushes, releasing oils without heating them. A processor makes perfectly good pesto and a mortar makes better, and if you use a machine, pulse it briefly and keep everything cold.

Most jarred pesto departs from that list considerably: cheaper oils in place of some of the olive oil, cashews or sunflower seeds instead of pine nuts, potato flake or fibre as a bulking agent, and hard cheese rather than the two named ones. It is a different product rather than a bad one — but it explains why the jar and the fresh sauce taste so unalike, and why the jar is usually much saltier.

Using it on pasta, properly

The commonest mistake is treating it as a sauce to heat. It is a dressing to fold in.

1. Cook the pasta in well-salted water. Trenette, linguine or trofie are traditional; anything works.
2. Put the pesto in a wide bowl — the serving bowl, not a pan. It never goes near direct heat.
3. Before draining, take out a mugful of the starchy cooking water. This is the step people skip and it is the one that matters.
4. Loosen the pesto with two or three tablespoons of that hot water, stirring until it goes from a thick paste to a pourable cream. The starch in the water emulsifies with the oil and turns the pesto into something that clings.
5. Drain the pasta, still dripping slightly, and tip it into the bowl.
6. Toss hard and fast, adding more water as needed, until every strand is coated. The residual heat of the pasta warms the pesto through, which is all the heat it should ever get.
7. Serve immediately, with more cheese. Traditionally boiled potato and green beans go in with the pasta, and it is worth trying once.

Why it goes grey, and how to keep it green

Basil browns for two separate reasons and both are manageable.

• Enzymatic browning. Damaged basil cells release enzymes that oxidise phenolic compounds and turn the leaf dark, in the same way a cut apple browns. This starts within minutes.
• Blanch the basil to prevent it. Ten seconds in boiling water, straight into iced water, then squeezed very dry. It deactivates the enzyme and locks the colour in, at a small cost in freshness of flavour. This is the single most effective measure.
• Keep everything cold — chill the leaves, the bowl and the blade — which slows the enzyme considerably.
• Work fast and do not over-process. Heat from a spinning blade speeds the reaction.
• Cover the surface with a film of olive oil in the jar. Oxygen is a reactant, and excluding it stops the browning at the top.
• Add a little lemon juice, which lowers the pH and slows the enzyme, at the cost of a slightly sharper sauce.
• Grey pesto is not spoiled. It has oxidised, which affects colour and dulls the flavour slightly. It is a cosmetic problem, not a safety one.

Beyond pasta, and beyond basil

• Stirred into soup at the table — minestrone with a spoonful of pesto is the Ligurian original, and it works in almost any vegetable soup.
• On roasted vegetables or potatoes, after they come out of the oven.
• Thinned into a dressing with more oil and a little lemon or vinegar.
• On grilled fish, chicken or lamb, as a finishing sauce.
• In a sandwich or on bruschetta, with tomatoes and mozzarella.
• Folded through rice, orzo or a grain salad.
• Other herbs. Parsley, rocket, wild garlic, coriander and mint all make good pestos. Wild garlic in spring is exceptional and costs nothing.
• Other nuts. Walnuts, almonds, hazelnuts, pistachios and toasted sunflower seeds all work, and pine nuts have become expensive enough that the substitutes are worth taking seriously — see sunflower seeds.
• Pesto rosso, the Sicilian version with sun-dried tomatoes and almonds, which is a genuinely different sauce.

Common questions