Sold in in health food stores and some Asian stores, kombu is typically available in either dried sheets, strips or the shredded form referred to as nalto kombu. The sheets can be thick or thin and delicate. When buying kombu, avoid the sheets that are thin and wrinkled. In addition to sheets, strips and shredded forms, kombu is available as shaved pieces that are seasoned in vinegar (tororo kombu), brine cured and pickeled (kombu-zuke), as flavored kombu boiled in a seasoning such as soy sauce (shio-kombu), and in a powdered form (kombu-ko)for use as a food seasoning or topping to be added to rice and grains.
To prepare the kombu, wipe the surface with a damp cloth to remove the powder. Avoid washing the sheets or strips excessively, as the repeated use of water to cleanse the vegetable will diminish the flavor. Some recipes require soaking the kombu before preparing, in which case it should be soaked only until it softens and opens. Thicker sheets or strips of kombu will require more time to soak than thin sheets or strips. When preparing dried kombu, place the amount desired in the liquid (generally 1 oz of kombu to 6 cups of water) and allow it to boil for 30 minutes before removing the kombu. The broth can then be used either to be served without additional ingredients or as a broth for use to be added with other foods. If the kombu is to be consumed, it is best to cut it into smaller pieces prior to or after boiling. Similarly, when used as a flavor enhancer, allow a strip or two of kombu to simmer in the liquid for 10 to 20 minutes before using the water as a base for soups, stews or sauces. Kombu can also be served as a food wrap for appetizers, first soaking the kombu and then wrapping it around seasoned pieces of fish or various vegetables.
Store dried kombu in an airtight container placed in a dark, dry area. Cooked kombu should kept in airtight containers that are stored in refrigerated areas.