Kombu: Seaweed for Dashi, How to Steep It Right, and Iodine Content

What is kombu?

Kombu, in Japanese 昆布, mostly comes from Saccharina japonica, formerly called Laminaria japonica. Most of the Japanese harvest comes from Hokkaido. Fishers harvest the broad blades in summer, dry them in the sun and cut them into sheets. In stores you get dark green to black, stiff pieces.

The white coating on the surface is not mold. It consists of mannitol and salts and carries a lot of flavor. So only wipe kombu with a damp cloth if sand clings to it, and don't wash it.

Flavor and umami

Kombu tastes mildly salty and of the sea, and in broth above all savory. It contains a lot of free glutamic acid. In 1908 the Japanese chemist Kikunae Ikeda found the glutamate while studying kombu dashi and gave the taste the name umami. You find more on how glutamate works together with inosinate from bonito and guanylate from shiitake under Umami.

Types

• Ma-kombu: broad and thick, gives a clear, slightly sweetish broth. Counts as the standard for fine dashi.
• Rishiri kombu: harder, gives a clear, salty broth. Kyoto cooking prizes it.
• Rausu kombu: softer, gives a strong, slightly cloudy broth.
• Hidaka kombu: soft and quick to cook. Good to eat along in braises and oden.

Health food stores often carry kombu from the Atlantic. It usually comes from other Laminaria species and tastes a little different, but works for dashi too.

Making kombu dashi

As a guideline, use 10 g kombu per 1 liter of water, which is a piece of about 10 by 10 cm (4 by 4 in).

1. Steep cold: Put the kombu in cold water and steep for at least 30 minutes, better 3 hours or overnight in the refrigerator. For a pure kombu dashi that is enough.
2. Heat gently: For more flavor, warm water and kombu slowly, about 10 to 15 minutes to just below a simmer. When small bubbles rise from the bottom of the pot, take the kombu out.
3. Continue: For Awase dashi you now add bonito flakes. For a vegan broth you combine kombu with dried shiitake as in the Shiitake dashi.

If you let kombu boil hard, slimy and bitter compounds are released. The broth turns cloudy, slippery and smells sharply of seaweed. You find the basics on instant and homemade stock under Dashi and dashi powder, and on the second main ingredient under Bonito flakes.

Other uses

• Hot pot: A piece of kombu on the bottom of the pot forms the broth for Shabu shabu and Yudofu.
• Oden: In Oden, soaked kombu strips tied into knots simmer along and are eaten.
• Rice: A small piece in the pot when cooking rice gives sushi rice a fine seasoning.
• Kombujime: You lay raw fish between two dampened kombu sheets for a few hours. It loses water and takes on flavor.
• Leftovers: Cut spent kombu into strips and simmer it with soy sauce and mirin into tsukudani.

The fermented drink kombucha has nothing to do with kombu. In Japan a hot drink of kombu powder is called kombu-cha, hence the confusion of names.

Iodine content

Kombu is among the foods richest in iodine of all; the content varies greatly with origin and processing. Part of the iodine passes into the broth while steeping. Watch the iodine information and the recommended maximum amount on the package, and don't eat spent kombu in large amounts. If you have a thyroid condition, talk to your doctor before eating it regularly.

Buying, substitutes and storage

You get kombu at an Asian grocery store, a health food store and online, usually in bags of 50 to 100 g. Good quality is thick, evenly dark and dry. As a substitute for dashi purposes only other seaweed such as wakame in a small amount works, or you reach for dashi powder. Stored dry, airtight and dark, kombu keeps for more than a year. Once steeped, use the kombu within two to three days from the refrigerator.