Particularly popular in Japanese cookery, kombu is one of the two basic ingredients used for dashi (soup stock). It's a long dark brown to grayish-black seaweed, which, after harvesting, is sun-dried and folded into sheets. Kombu is sold in Japanese and health-food markets and when stored unopened in a dry place it will keep indefinitely. After opening, store in a cool, dry place for up to 6 months. Kombu has a natural white-powder covering that delivers considerable flavor. For that reason, the surface should be lightly wiped off, not washed. Kombu is used to flavor cooked foods as well as for sushi. It's sometimes pickled and used as a condiment. Kombu is also called simply kelp.