Kombu: the kelp that seasoned a nation

Look closely at a sheet of good kombu (昆布) and you might think it has gone off. The dark, green-black surface is dusted with a fine white bloom, a little like the frost on a plum. Plenty of first-time cooks have reached for a scrubbing brush. Please don't. That powder is flavour, and it is the first lesson this remarkable seaweed has to teach.

Kombu is a large brown kelp, dried in long, leathery ribbons. On its own it looks unassuming. In water, it gives up a soft, savoury sweetness that sits underneath a great deal of Japanese cooking, from Buddhist temple food to the broth in your noodles.

A northern harvest

Most of Japan's kombu comes from the cold waters around Hokkaido, the northern island, where long blades of kelp grow in dense underwater forests. It is gathered mainly in summer, traditionally from small boats using long poles, then dried in the sun on beaches and gravel yards.

Different coasts give different kombu, and cooks can be devoted to their favourites. A few of the best known:

• Ma-kombu (真昆布), from the southern coast of Hokkaido around Hakodate. Thick and broad, it makes a clear, gently sweet, refined stock, and it has long been a favourite in Osaka.
• Rishiri kombu (利尻昆布), from the far north, including Rishiri island. Firm and slightly salty, it gives a clear, fragrant dashi and is closely associated with Kyoto's restaurant cooking.
• Rausu kombu (羅臼昆布), from the Shiretoko peninsula in the northeast. It makes a rich, deep, slightly golden stock, generous and full, if a little cloudier.
• Hidaka kombu (日高昆布), from the Hidaka coast. Softer and quicker to cook, it is often eaten as well as used for stock, and turns up in simmered dishes and oden.

I would not get too hung up on the rankings. For a home cook, any decent kombu makes a lovely stock. But it is worth knowing that the choice is real, and that a Kyoto chef and an Osaka chef might well reach for different packets.

The kombu road

Here is a puzzle. If kombu grows in the far north, why are some of Japan's great kombu cultures found in Osaka, in Kyoto and, of all places, in subtropical Okinawa?

The answer lies on the water. During the Edo period, trading ships known as kitamaebune (北前船) sailed between Hokkaido and Osaka, down the Sea of Japan coast, around the western tip of Honshu and through the Seto Inland Sea. Among their most important northern cargoes was kombu. This long trade route is often called the "kombu road" (昆布ロード).

Kombu reached Osaka in quantity, and the city built a deep culture around it: specialist kombu merchants, sweet-salty kombu preserves, and a style of dashi in which kelp leads the flavour. Kyoto, a short way inland, shaped much of its refined cooking around it too. Places along the route picked up the habit as well; Toyama, on the Sea of Japan coast, is still known for its love of kombu.

The road kept going. Kombu travelled south to Satsuma, today's Kagoshima, and on to the Ryukyu kingdom, now Okinawa, which in turn traded with China. Okinawa took kombu to heart as something to eat rather than only to steep: shredded and stir-fried with pork in kubu irichi (クーブイリチー), or tied into knots and simmered with pork. A place where kelp does not grow became one of Japan's most enthusiastic kombu-eating regions, which I think is one of the happiest accidents in food history.

That white powder

Back to the bloom on the surface. Much of it is mannitol, a naturally sweet substance that comes out onto the surface of the kelp as it dries. It is part of kombu's flavour, alongside the glutamate that gives kombu its savoury depth, the very compound that led to the discovery of umami.

So, no scrubbing and no rinsing under the tap. If a piece looks dusty or gritty, wipe it lightly with a barely damp cloth or paper towel and leave the rest. The white is part of what you paid for.

Soak, don't boil

Kombu is easy to use, but it has one firm rule: treat it gently.

The simplest stock is cold-water kombu dashi. Put a piece of kombu in a jug of water and leave it in the fridge for several hours or overnight. What you pour off is soft, clean and quietly sweet.

For a warmer extraction, soak the kombu in a pot of cold water for a while, heat it slowly, and take it out just before the water comes to the boil. Boil kombu hard and the stock can turn slimy and pick up bitter, overly oceanic flavours as the kelp releases its thickening compounds. From a gentle kombu stock, add katsuobushi and you are on your way to a classic dashi.

Even then, the kombu is not finished. Used pieces can be sliced and simmered with soy sauce and sugar into tsukudani (佃煮), a sweet-salty preserve that is excellent with rice.

Kombu also comes in forms you eat directly: shio kombu (塩昆布), salty seasoned strips for rice or salads; tororo kombu (とろろ昆布), fluffy shavings that melt into soup; and kombu-jime (昆布締め), a technique of pressing raw fish between sheets of kelp so that it firms up and takes on flavour. At celebrations, kombu often appears as a lucky food, thanks to a pun on yorokobu (喜ぶ), "to rejoice".

To me, kombu is the perfect example of an ingredient that does its best work out of sight. You rarely see it in the bowl. But take it away and almost every Japanese broth would taste a little emptier.