It is a kelp, with thick, wide leaves several yards (metres) long. They are dark brown to greyish-black; the darker the colour the better.
The kelp is harvested, sun-dried, and folded into sheets. When dried, a whitish powder appears on it.
Over 90% of Kombu sold is farmed.
Kombu can be used as a wrap around foods, or pickled in small strips. Small pieces of it can appear in oden.
It is one of the two key ingredients in the broth that Japanese call “Dashi”, along with bonito (dried tuna) flakes.
When sold shredded, it is called “Oboro kombu” or “nalto kombu.”
Kombu is naturally rich in MSG.
Don’t wash kombu under running water or in water; instead, wipe any dirt off it with a cloth. The whitish powder adds flavour; that is why you don’t want to wash it away.
It needs to be soaked at least 20 minutes before use, unless simmering it for dashi.
If eating as a garnish, simmer dried kombu for 1 to 2 hours to soften.
Also spelt “konbu” in English.