How do you make pesto that doesn't go brown?

Pesto is a sauce that looks its best for about ten minutes and then starts quietly ageing in front of you. The colour is not a moral question, and a slightly darker pesto still tastes of basil. But if you want that green that looks lit from inside, there are a few habits that make the difference.

Start with leaves that want to be pesto

Dry, perky leaves pulled off the stems just before you start. Wet leaves dilute the sauce and make it slide around instead of breaking down, so wash early and let them dry properly on a towel. Tired, blackened or floppy basil is already halfway to brown before it meets the mortar; that bunch belongs in a soup.

Bigger, older leaves can taste a little minty and sharp. Smaller leaves from the top of the plant are sweeter. Neither is wrong, it just changes the pesto.

Keep it cool and quick

Friction and warmth are the enemies. In a blender, the blades spin, the jug warms, and the leaves get beaten rather than cut. What we do:

• Chill the blender jug or food processor bowl for a while before starting.
• Add the oil early, so the leaves are coated as they break up, rather than pouring it in at the end.
• Pulse in short bursts and scrape down. Stop as soon as it looks like sauce, not purée.
• Use cold oil and cold pine nuts if the kitchen is warm.

Some cooks plunge the leaves into boiling water for a few seconds and then into iced water before blending. It holds the colour remarkably well, and the trade is a flatter, slightly cooked basil flavour. Worth trying once so you know which camp you are in.

Mortar or blender?

This is a taste choice, not a test of character. A mortar crushes the leaves rather than slicing them, which gives a looser, more aromatic sauce with visible texture and a rougher, more rustic finish. It takes longer and your forearm will have opinions. A blender is fast and smooth and, handled coolly, perfectly good. We use the mortar on a slow Sunday and the blender on a Wednesday, and nobody at the table has ever filed a complaint.

The oil cap

Once it is made, scrape the pesto into the smallest jar it fits, press it down to remove air pockets, and pour a thin layer of olive oil over the top so no green is exposed. Lid on, into the fridge. Each time you take some, smooth the surface and top up the oil. This is the single most useful trick on the page.

A mild, fruity oil works best here; a very peppery one can turn the whole jar bitter over a few days.

Freezing pesto

Pesto freezes well in small portions: ice-cube trays, or small jars with an oil cap. We freeze it without the cheese and stir the grated cheese in after thawing, because the cheese is better fresh and the texture stays cleaner. Garlic can get a bit sharper in the freezer, so go easy on it if you are making a batch to store.

At the table

Pesto never goes into a hot pan. Toss it with the drained pasta off the heat, loosened with a splash of the cooking water, so it coats rather than fries. Heat is what turns the sauce dull in seconds, which is a shame after all that care.