Katsuobushi: the fish that turns to wood

Pick up a whole piece of katsuobushi (鰹節) and you would struggle to guess it was ever a fish. It is dark brown, dense and dry, shaped a little like a small canoe. Knock two pieces together and they make a sound closer to wooden blocks than to anything from the sea. It is often described, half in jest, as one of the hardest foods on earth, and once you have held one you will not be inclined to argue.

Yet this unlikely lump is the source of some of the most delicate flavour in Japanese cooking. Shaved into thin flakes, it gives dashi its smoky perfume and much of its savoury depth.

From skipjack to stone

Katsuobushi starts with katsuo (鰹), skipjack tuna, a fast, lean fish of warm seas. Making it is a long, patient sequence in which every stage drives out water and builds flavour.

1. The fish is filleted into long pieces.
2. The pieces are simmered gently, which sets the flesh.
3. The bones are picked out by hand.
4. The fillets are smoked over hardwood fires, rested, then smoked again, round after round over a period of weeks. Each cycle draws out more moisture.

The smoked result is called arabushi (荒節), roughly "rough bushi". It is already hard and intensely smoky, and much of the everyday shaved katsuobushi you see in shops is made from it.

The mould that makes it fine

For the top grades, the process keeps going. The dark, smoky surface is trimmed away, and the pieces are cultured with a specific mould. They are left while the mould grows, dried in the sun, brushed off, and the cycle is repeated. This version is called karebushi (枯節), and pieces that go through the mould cycle repeatedly are honkarebushi (本枯節), the most prized of all. The whole journey can take months.

It sounds alarming to grow mould on fish on purpose, but this is a carefully managed, beneficial mould, rather as with certain cheeses. It pulls still more moisture from deep inside the fillet and breaks down some of the fat. The flavour that results is softer, rounder and more refined than arabushi: the smoke steps back and a clean, mellow savouriness comes forward.

I like to think of arabushi as the bonfire and honkarebushi as the embers.

Katsuobushi is made in several parts of Japan, but two names come up again and again: Makurazaki in Kagoshima prefecture and Yaizu in Shizuoka prefecture, both port towns long tied to skipjack and the craft of turning it into something that could last.

The art of the shave

A block of katsuobushi does nothing until it is shaved. The traditional tool is the katsuobushi kezuriki (鰹節削り器), a wooden box with a plane blade set into its lid and a little drawer underneath. You push the block across the blade, and fine curls fall into the drawer below. Those shavings are kezuri-bushi (削り節), or simply kezuri.

Freshly shaved flakes have an aroma that packaged ones cannot quite match, because the fragrance begins to fade once the surface is exposed. Still, most households buy their flakes ready-shaved in sealed bags, and those are perfectly good for daily cooking. Thin, wide, rosy flakes sold as hanakatsuo (花かつお), "flower katsuo", are a common everyday choice.

How thinly you shave matters. Gossamer flakes give up their flavour in a minute or two, which suits a quick, elegant stock. Thicker shavings need a longer simmer and give a stronger result, the kind that shows up in noodle broths and simmered dishes.

Dancing flakes and other pleasures

You will also meet katsuobushi as a topping, and this is where it turns into theatre. Scatter paper-thin flakes over hot okonomiyaki or takoyaki and they begin to curl, ripple and sway in the rising heat, as if the food were breathing. These are the famous "dancing" bonito flakes, and I defy anyone to watch them without smiling.

A few other good homes for the flakes:

• Hiyayakko (冷奴): cold silken tofu with flakes, grated ginger, green onion and a dash of soy sauce.
• Okaka (おかか): flakes moistened with soy sauce, a classic filling for rice balls.
• Blanched greens: a pinch of flakes over spinach adds instant savour.

In the ramen world, katsuobushi is part of many seafood-leaning soups, often alongside kombu and dried sardines. Some shops, especially in tsukemen, sprinkle a powder of dried fish over the dipping broth for an extra smoky punch.

Buying it and keeping it

If you want to try katsuobushi at home, start with a bag of flakes. Once it is opened, keep it sealed tight and somewhere cool, because the aroma fades and the flakes happily absorb moisture. If you fall hard, a whole block and a box plane make a lovely souvenir, and the block keeps for a very long time.

Most of all, notice it. Next time you sip a clear Japanese broth and catch that faint campfire note, think of the fish that spent weeks in the smoke, and sometimes months in the sun and the mould, to get there. Few ingredients work so hard to taste so gentle.