This is edible kelp grown along the coasts of Japan, Korea and China - quite different from the giant kelp of California which is processed rather than eaten directly. The long wide fronds are dried and packaged for use particularly to make soup stock, but also for use as wrappers for various prepared foods. Almost all this kelp used for food is cultivated rather than gathered wild which explains why there are so few crunchy critters growing on it.
This kelp is normally sold as flat dried sheets cut from the fronds but is also sold as narrow cut strips, salted and bagged in the refrigerated section. In Japan it is also pickled and served as a snack to accompany green tea. The photo shows a piece of a frond as dried, and a shorter piece cut from the end after soaking.
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Kelp is very important to the cuisine of Japan, particularly for making soup stock, generally in combination with shaved bonito flakes or, for vegetarians, dried shiitake mushrooms. It is also very popular in Korea and to a much lesser extent in China.
Buying & Storing: This seaweed is easily found in large flat packages in any market serving a Japanese or Korean community and to a lesser extent in other Asian markets. It is imported from both Japan and Korea and price varies greatly depending on the company's quality image.
Keep the package sealed as much as possible and in a cool, dry place away from light and it will last a year or so.
Cooking: Do not wash - the slight amount of whitish powder on the outside is part of the product. When making soup, konbu is put in a pot with cold water which is then heated. The konbu is removed from the water just as it comes to a boil as boiling will make for an overly strong taste. The soaked kelp can then be discarded or used for some other purpose.
Some more substantial recipes do call for strips of konbu to be cooked for a much longer time and eaten as part of the recipe. To prepare for cutting into strips, soak in cool water for about 1/2 hour.