Why the method changes the result
Basil browns through the same enzymatic process that darkens a cut apple, and the more cell damage there is, the faster it happens. A high-speed blade damages a great deal of tissue very quickly, and it also heats the basil through friction.
A mortar crushes rather than slices. The cells rupture, the oils release, but the tearing is coarser and slower, and the basil stays green far longer.
The texture is different too. Blitzed pesto is a uniform emulsified paste; pounded pesto is rougher, with the ingredients still distinguishable, and it clings to pasta differently.
In a mortar
Order matters as much as method.
- Pound the garlic with a pinch of coarse salt first, until it is a smooth paste. The salt is an abrasive as much as a seasoning.
- Add the pine nuts and pound them into the garlic until broken down.
- Add the basil leaves a handful at a time, with a little more salt, and grind them against the side of the mortar with a circular motion rather than hammering.
- Keep adding basil until it is all incorporated and you have a coarse green paste.
- Work in the grated cheese — traditionally a mixture of Parmesan and Pecorino.
- Stir in good olive oil last, gradually, until it loosens to the consistency you want. Do not pound the oil in.
- Taste for salt. Pesto usually needs more than expected because the cheese varies.
Keeping it green, and variations
- Blanch the basil for five seconds and refresh in iced water before pounding. It costs a little aroma and keeps the colour for days.
- Cover the surface with oil in the jar. Air is what browns it.
- Add a squeeze of lemon, which is not traditional and slows the browning.
- Wild garlic pesto, in spring, using the leaves in place of basil and garlic both.
- Rocket, parsley or basil-and-mint, all of which work and none of which are pesto genovese.
- Walnuts, almonds or hazelnuts in place of pine nuts, which are expensive and often stale.
- Pesto alla trapanese, the Sicilian version with almonds and tomato.
- Toss it with pasta off the heat, loosened with a little cooking water. Heating pesto in a pan dulls it.
The ingredients
- Basil
- Genovese basil, small-leaved and sweet, is the traditional one. Large-leaved supermarket basil is more aniseed-like.
- Pine nuts
- Expensive and prone to rancidity. Smell them before use. Toasting is not traditional in Genoa and many people do it anyway.
- Cheese
- Parmesan and Pecorino together is traditional. Parmesan alone is milder.
- Garlic
- One clove goes a long way. Raw garlic dominates quickly.
- Oil
- A mild, fruity olive oil. A very peppery one fights the basil.
Common questions
Does a mortar really make a difference?
Yes. A blade bruises basil and turns it black and bitter faster, and gives a smoother, more uniform texture.
Why does my pesto go black?
Enzymatic browning from cell damage and air. Blanch the basil briefly, or cover the surface with oil.
Can I use a food processor?
Yes, in short pulses and with cold ingredients. It is not the same, and it is much faster.
Do I toast the pine nuts?
Not traditionally in Genoa. Many people do and it gives a nuttier, less fresh result.
Should I heat pesto?
No. Toss it with hot pasta off the heat, loosened with cooking water.
Try this next
- Pesto the ingredient itself
- Basil the herb, and why it blackens
- Pestle and mortar the tool that changes the result
- Black basil leaves the browning problem