Ingredient

Kombu

Kombu is a thick, dark, dried kelp, and it is the foundation of Japanese stock. Steeped in water it releases a great deal of glutamate — the compound behind savouriness — which is why a broth made from nothing but water and a piece of seaweed tastes of something substantial.

What it does

Kombu is dried at length and cured, and during that process free glutamate accumulates in and on the leaf. A white powdery bloom on the surface of a good piece is largely mannitol and salts, and it is a sign of quality rather than something to wash off — wiping a piece of kombu with a damp cloth is right, rinsing it under a tap is not.

Dropped into cold water and warmed gently, it gives up that glutamate into the liquid. The result is kombu dashi, and it is the base layer of an enormous share of Japanese cooking.

The classic dashi adds katsuobushi — dried, smoked, fermented skipjack tuna shaved into flakes — which contributes inosinate. Glutamate and inosinate together produce a savoury effect much greater than either alone, and that combination is what makes dashi taste the way it does. A vegetarian version pairs kombu with dried shiitake, which contributes guanylate and works on the same principle.

Making kombu dashi

The one rule that matters: do not boil it.

1. Wipe the surface of the kombu with a damp cloth. Do not scrub off the white bloom — that is flavour.
2. Put it in cold water and leave it, ideally for half an hour or several hours in the fridge. Cold steeping alone makes perfectly good dashi.
3. Warm the pan slowly. Bring it towards a simmer over a low heat and take the kombu out just before the water boils, when small bubbles are rising at the edges.
4. If it boils with the kombu still in it, the stock turns cloudy, slippery and faintly bitter. This is the single most common mistake and it is not recoverable.
5. For a full dashi, add katsuobushi flakes off the heat, let them steep for a minute or two, and strain.
6. Keep the used kombu. It can be simmered in soy, mirin and sugar into tsukudani, or sliced into a salad or a stew.

Other things to do with it

• Put a strip in with dried beans or pulses as they cook. It seasons them savoury, and it is a long-standing Japanese habit.
• In a vegetarian stock with dried shiitake, for anything that would otherwise want chicken stock.
• Simmered into a braise — it dissolves partially and thickens slightly while adding depth.
• Tsukudani — used kombu simmered down with soy, mirin and sugar until sticky and intensely savoury. Eaten with rice.
• In a pickling liquid, where it rounds out the sharpness.
• In water for cooking rice, a small piece, which is subtle and good.
• Kombu-cured fish. Pressed between sheets of kombu for a few hours, white fish takes on savouriness and firms up. A restaurant technique that works at home.

The seaweeds, told apart

They are sold near each other and are not interchangeable.

Kombu

Thick, dark, leathery sheets of kelp. For stock. Not usually eaten as it comes.

Wakame

Thin, dark green, sold dried in pieces. Rehydrates into a soft, silky green seaweed. For miso soup and salads, not for stock.

Nori

Pressed into thin sheets, toasted. For sushi, onigiri and as a garnish. Falls apart in liquid.

Dulse

Reddish-purple, Atlantic. Chewy, salty, faintly bacony when fried. Irish, Scottish and Icelandic rather than Japanese.

Hijiki

Fine black strands. Cooked in soy and mirin as a side dish. Worth noting that food safety authorities in several countries advise limiting hijiki because of inorganic arsenic — check current guidance from the Food Standards Agency.

Common questions