Kombu

Kombu is unusually rich in free glutamate, the amino acid responsible for umami, which is why it deepens flavour without adding a flavour of its own. Combined with the inosinate in dried bonito or the guanylate in dried shiitake, the savoury effect multiplies rather than adds, which is the principle behind pairing kombu with katsuobushi in dashi.

The synergy is the reason a stock made from two weak-tasting dried things tastes stronger than either could alone.

It also contains alginates, soluble fibres studied for their effect on digestion and cholesterol, and fucoidan, a compound that has attracted research interest for immune and metabolic effects, though most of that work remains preliminary and done outside the human body.

The most practical consideration is iodine. Kelp is among the most iodine-dense foods on earth, and regular large servings can push intake well past what the thyroid needs. Using kombu as a stock ingredient a few times a week poses no such issue; eating sheets of it daily is a different matter.

Kelp has been gathered along the coasts of northern Japan for well over a thousand years, and Hokkaido still supplies the great majority of it. The Ainu harvested and dried it long before it became a commodity, and by the Edo period a sea route known as the kombu road carried it south to Osaka and Kyoto, where it entered the refined stock-making of Japanese temple and court cooking.

Okinawa, at the far southern end of the trade route and thousands of kilometres from any kelp bed, ended up with the highest kombu consumption in Japan.

In Korea the same kelp is dashima, used alongside dried anchovy for the everyday stock base that underpins soups and stews. In China it is haidai, more often eaten as a vegetable, simmered or shredded cold with vinegar and sesame, than treated as a stock ingredient.

The Japanese approach, where kombu is steeped and then removed, reflects a cuisine built on clarity. The Chinese approach, where it is eaten, reflects one built on texture.

Kombu has one dominant job: dashi, the stock under miso soup, noodle broths, simmered vegetables and most of the savoury liquid in Japanese cooking. Steeped alone it gives kombu dashi, a clean vegetarian stock; with katsuobushi added off the heat it becomes ichiban dashi, the version most recipes assume.

Add a postcard-sized piece to the pot when you cook dried beans and they soften faster and sit easier.

Beyond stock, the spent sheet from a batch of dashi should not be thrown away. Simmered down with soy sauce, mirin and sugar it becomes kombu tsukudani, a dark, intense relish eaten in small amounts with rice.

Korean cooks use it with dried anchovy for the stock behind doenjang jjigae and noodle soups. A strip dropped into a pot of rice, or into water for pickling, lends background savouriness that nobody can identify and everybody notices.

Do not wash it. Wipe the surface lightly with a barely damp cloth if it looks dusty, and leave the white bloom alone.

For cold-brew dashi, put a 10cm piece in a litre of cold water and leave it in the fridge overnight. This is the easiest method and the hardest to ruin.

For the stovetop version, steep the kombu in cold water for twenty to thirty minutes, then bring it slowly toward a simmer and pull the kelp out just before the water boils. Boiled kombu turns the broth bitter and faintly slimy, which is the single most common mistake.

Score the sheet with a couple of shallow cuts if you want faster extraction. Keep the dried sheets in a sealed bag away from light, where they last a year or more.