Actually very easy

Pesto

The beliefPesto is assumed to need a mortar, a particular basil and pine nuts, and a processor is said to be wrong.

The truthA green leaf, oil, a hard cheese and an oily seed. A processor does it too — you only have to keep the heat down.

15 minutes

How to make it

What you need

60 g basil leaves
40 g pine nuts or walnuts
2 garlic cloves
120 ml extra virgin olive oil
50 g grated hard cheese
Half a teaspoon of salt

How the engine read this list (6 lines)

60 g	Basil leaves	basil
40 g	Pine nuts	pine-nut
10 g	Garlic	garlic
110 g	Extra virgin olive oil	extra-virgin-olive-oil
50 g	Grated hard cheese	kasar
3 g	Salt	salt

An empty weight means that line could not be turned into grams — either the amount is open ("to taste") or the ingredient has no record here. Neither enters the sum as zero; they do not enter it at all.

Toast the nuts in a dry pan for three minutes until they begin to colour, then cool them. Untoasted nuts leave pesto tasting floury.
Chill the processor bowl and blade for ten minutes. The whole point is to keep the friction heat down.
Process the garlic, nuts and salt first. Add the basil last with half the oil and run it for ten to fifteen seconds, then stop.
Stir the rest of the oil in by hand, and keep the cheese out of the machine — fold it in.
Transfer it to a jar and cover the surface with a finger of olive oil.

Why it works

Whether pesto stays green or goes brown comes down to one enzyme: polyphenol oxidase. Tear a basil leaf and the enzyme leaves the cell, meets oxygen and turns the phenolics into brown pigments — the same mechanism as a cut apple. A mortar beats a processor for a physical reason rather than a romantic one: a blade generates friction heat as it cuts, and the enzyme speeds up above 30 °C. If you use a processor, chill the bowl and blade, add the basil last and run it for no more than fifteen seconds. That is the olive oil’s second job too: a film of fat keeps oxygen off the leaf. The cheese should be stirred in by hand rather than processed, because friction splits its fat and leaves the pesto oily and grainy.

The rough sum

273 grams going in

1450 kcal in total

100% of the mass has figures behind it. This is the sum of what GOES IN rather than of what comes out. Wherever something evaporates, drains or ferments the two differ: a kilo of milk is 150 g of cheese.

Diets it qualifies for

Gluten-free Vegetarian · with care Lactose-free

Storage & shelf life

How long it keeps

Fridge, in a jar sealed under a layer of olive oil: 1 week
Without the cheese, the same way: 2 weeks
Freezer, in an ice tray: 6 months
Room temperature: A few hours; basil darkens fast

Pesto darkens sooner than it spoils. A finger of olive oil over the surface cuts off the oxygen and holds the colour for a week. Pesto with cheese in it keeps less well, because milk protein is food for bacteria — if you mean to freeze it, add the cheese after thawing for both a longer life and a better texture.

When to bin it A soured-milk smell, any swelling, or mould on the surface. Garlic and olive oil together can create an oxygen-free pocket, so do not leave home-made pesto at room temperature for days.

History

Where it comes from

Pesto is Genoa’s sauce and its name comes from the verb to pound — the same root as pestle. The modern version was written down in the nineteenth century, but its ancestor is Rome’s *moretum*: garlic, cheese, herbs and oil, all crushed in a mortar. Every corner of the Mediterranean has a relative — Provence’s *pistou* has no cheese and no nuts, Sicily’s *pesto alla trapanese* is made with almonds and tomato. The nearest logic in Turkish cooking is tarator with walnuts and garlic: the same three things — an oily seed, garlic and fat — with a different green.

The knack

Dry the basil thoroughly if you washed it. Drops of water break the emulsion in the oil and the pesto separates.
Parsley, rocket, nettle or carrot tops all work: the rule is the ratio, not the leaf.
Turn the heat off before you stir it through pasta and add a ladle of the cooking water. In a hot pan pesto cooks and turns bitter.

Pastes and sauces