Ingredient

Basil

Basil is the herb most damaged by being treated like any other. Its flavour is almost entirely volatile, its leaves bruise on contact with a knife, and it turns black in the fridge — all three of which point at the same handling.

Why it goes black

Two separate things blacken basil and both are worth distinguishing.

Cold damage. Basil is a tropical plant and it suffers below about 10°C — the fridge is well under that. The cold ruptures cell membranes, enzymes escape and oxidise phenolic compounds in the leaf, and within a day or two the leaves are covered in black patches. This is why basil is the one herb that should not go in the fridge. See black basil leaves.

Bruising. Chopping with a blunt knife, or chopping at all in some views, crushes cells and starts the same oxidation locally, which is why chopped basil darkens at the cut edges within minutes. Tearing the leaves by hand does less damage than a poor knife does, and a genuinely sharp knife does less than tearing.

Handling it

• Add it at the end, off the heat. Basil's aroma is volatile and disappears within a minute or two of cooking. Basil simmered into a sauce contributes almost nothing.
• Tear rather than chop for salads and finishing, unless your knife is genuinely sharp.
• Keep it out of the fridge. Stand the stems in a glass of water on the counter, loosely covered, like cut flowers. It will last several days.
• Do not wash it until you use it. Wet basil deteriorates fast.
• Use the stems. They carry a lot of flavour and can go into a sauce early, to be fished out, while the leaves go in at the end.
• Blanch it for pesto — five seconds in boiling water and straight into ice — if you want the pesto to stay green. It costs a little aroma and buys a lot of colour.

The basils

Genovese / sweet basil

Large, soft, bright green leaves. Sweet, peppery, faintly aniseed. Pesto, tomatoes, Italian cooking.

Thai basil

Narrower leaves, purple stems, much sturdier. Strongly aniseed and clove-like. Holds up to heat far better, which is why it goes into stir-fries.

Holy basil (kraphao)

Peppery and slightly medicinal. The basil in pad kraphao, and not interchangeable with Thai basil despite the confusion.

Lemon basil

Citrus-scented. Good with fish and in Southeast Asian salads.

Dried basil

A different and much weaker thing. Usable in a long-cooked tomato sauce; useless where fresh basil is the point.

Pesto, briefly

• Basil, garlic, pine nuts, Parmesan, olive oil, salt. The proportions are a matter of taste and the method matters more.
• A mortar gives a better texture than a processor, because it crushes rather than shreds and heats the leaves less. A processor works if you pulse it and keep it brief.
• Salt the garlic first — it acts as an abrasive and stops the garlic sliding around.
• Add the oil last and mostly by hand. Blitzing oil hard can make pesto taste bitter.
• Cover the surface with a film of oil to store it, which keeps the air off and slows the blackening.

Pesto, and why the method changes the result

The most useful thing to do with more basil than you can eat.

• Pound it, do not blend it. A blade slices and bruises the leaf, which speeds the enzymatic browning that turns basil black and bitter, and it heats the herb through friction.
• Order matters in a mortar: garlic with coarse salt first, then pine nuts, then basil a handful at a time, then cheese, and the oil stirred in last rather than pounded.
• Blanch for five seconds and refresh in iced water if the colour matters more than the last of the aroma. It stays green for days.
• Cover the surface with oil in the jar. Air is what browns it.
• Toss it with pasta off the heat, loosened with cooking water. Heating pesto in a pan dulls it.
• Freeze it without the cheese, in ice cube trays, and stir the cheese in when you use it.

Common questions