Dried kelp
Dried kelp in stiff, dark green sheets, used mainly to steep in stock, which it gives a deep, rounded savouriness.
Caution: Kombu is very high in iodine, so a small piece is enough for a stock, and the cooked kelp itself should be eaten only occasionally and in small amounts, especially by people with thyroid conditions.
Kombu, called dasima in Korean, is dried kelp, a brown seaweed sold in stiff, leathery sheets or strips. It tastes little of the sea and more mild and savoury, because it is rich in natural glutamic acid, which is why it serves as the base of stocks and broths. The white, powdery coating on the surface is not mould but part of the flavour, so the sheets should at most be wiped briefly with a damp cloth rather than washed. Kombu is steeped in cold water or heated slowly and removed before the liquid boils, since long, hard boiling makes the stock slimy and slightly bitter. It is sold in Asian grocery shops, often also in organic and health food shops, and by mail order; kept dry and sealed, it lasts a very long time.
Substitute:
There is no equivalent substitute. A few dried shiitake also give a stock savoury depth, but taste noticeably earthier and colour it dark; instant dashi granules are a stopgap, but usually contain fish and taste stronger and saltier.