Why does my pesto go brown or taste bitter?

Pesto has two separate faults with two separate causes, and both are usually blamed on the basil. Browning is oxidation of the leaves. Bitterness is nearly always the oil or the garlic reacting to a fast blade. Once you know which is which, both are easy to design out.

What you'll need

• Basil
• Garlic
• Hard cheese
• Nuts or seeds
• Mild olive oil and extra virgin olive oil

Step by step

1. Blanch the basil if you want it to stay green.Five seconds in boiling water, then straight into iced water, then squeezed dry. This deactivates the enzymes that cause browning. The pesto stays bright green for days rather than hours, at the cost of a slight loss in aroma.
2. Use a mild oil in the machine and finish with the good oil by hand.This is the key to avoiding bitterness. Blending extra virgin olive oil at speed ruptures its cells and releases bitter polyphenols. Blend with a mild or light olive oil, then stir the extra virgin in with a spoon at the end for flavour.
3. Toast the nuts and check they are fresh.Pine nuts go rancid readily and rancid nuts are bitter and slightly metallic. Smell them first. A light toasting in a dry pan improves the flavour considerably — watch them, as they burn in seconds.
4. Go easy on the garlic, and add it whole rather than pre-chopped.One small clove per large bunch of basil. Garlic shredded at high speed becomes harsh and acrid. Remove the green germ from older cloves, which is markedly more bitter.
5. Pulse rather than run the machine continuously.Short bursts, scraping down, keeps the temperature low and the texture coarse. Heat from a running blade both browns the basil and brings out bitterness. A mortar and pestle produces the best pesto of all for exactly this reason.
6. Add the cheese at the end, off the blade.Grated Parmesan or Pecorino stirred in by hand. Blending cheese makes the pesto pasty and the salt in it is easier to judge when you add it last.
7. Store it under a film of oil.In a jar, pressed down flat with a centimetre of oil over the surface, it keeps a week in the fridge without browning. Freeze it in an ice-cube tray, without the cheese, for months.

Tips

• Never heat pesto. Stir it into hot pasta off the heat, loosened with a spoonful of the pasta cooking water. Cooking it dulls the basil and separates the oil.
• Salt at the very end. Parmesan and Pecorino are both salty and the amount varies.
• A squeeze of lemon brightens a pesto and slows browning slightly, though it also softens the basil flavour.
• Freeze pesto without cheese and add the cheese when you use it. It keeps better and tastes fresher.
• The mortar and pestle is not romanticism. Crushing rather than cutting gives a better texture and less bitterness, and it takes about ten minutes.

Alternatives

• Walnut, cashew or almond pesto: Walnuts give a richer, slightly tannic pesto and are far cheaper than pine nuts. Cashews give a creamy, mild version. Almonds are the traditional nut in Sicilian pesto with tomatoes. All work directly in place of pine nuts.
• Sunflower or pumpkin seed pesto: Cheap, nut-free and perfectly good. Toast them first. Pumpkin seeds give a striking green colour as well.
• Rocket, parsley, wild garlic or kale pesto: All work in place of basil. Rocket is peppery, wild garlic is seasonal and superb, kale needs blanching first. Basil is the mildest, so the others need more cheese and more oil to balance them.
• Pesto rosso: Sun-dried tomatoes, almonds, garlic, Parmesan and oil. Keeps far longer than green pesto, does not brown at all, and suits robust dishes.

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