SHINFOLD
FermentationPart 6 of 8

Why is mold grown on katsuobushi on purpose?

Why do they grow mold on katsuobushi?

By Yasuyoshi HigashideFood & Drink

The short answer

Katsuobushi is skipjack tuna, simmered, boned and smoked over wood for weeks, then deliberately cultured with mold. For honkarebushi, mold-culturing with Aspergillus glaucus and its relatives and sun-drying are repeated four or more times, leaving a block said to be one of the hardest foods in the world.

Paper-thin katsuobushi shavings drifting and curling in hot clear dashi broth, translucent pink flakes caught mid-motion, rising steam, warm backlight, close-up

Katsuobushi: Mold, 4 Times

It looks like a piece of driftwood — and it is the heart of Japanese soup stock.

Katsuobushi is skipjack tuna, simmered, boned and smoked over wood for weeks, then deliberately cultured with mold. For honkarebushi, mold-culturing with Aspergillus glaucus and its relatives and sun-drying are repeated four or more times, leaving a block said to be one of the hardest foods in the world.

The 10 beats
Block of dried bonito resting on black stone, looking exactly like weathered driftwood, hard cracked surface, dramatic low side light, dark background, macro texture

Not wood. It's fish

This is not wood. It's fish.

The block is dry enough to knock against a table and hear it ring. Nothing about the object announces that it began as a fish, which is exactly what weeks of smoking and drying are for.

Paper-thin katsuobushi shavings drifting and curling in hot clear dashi broth, translucent pink flakes caught mid-motion, rising steam, warm backlight, close-up

Shaved thin, it dances

Shaved thin, katsuobushi dances in hot dashi.

The shavings are thin enough to be translucent, and they curl as hot water hits them. That movement is the cue in a Japanese kitchen that stock is being made rather than merely heated.

Overhead view of a pot of clear amber dashi broth, faint steam over a still surface, subtle reflections, warm quiet light

The soul of dashi

That broth is the base of Japanese cooking.

Dashi sits underneath miso soup, simmered dishes, noodle broth and dipping sauces. It is not a flavor you notice on its own so much as the reason the other flavors hold together.

Rows of bonito fillets on wooden racks inside a smoking hut, thick woodsmoke filling the air, glowing embers below, moody amber light, atmospheric documentary style

Simmered, boned, smoked

The fish is simmered, boned, then smoked for weeks.

Each step removes water. Simmering sets the flesh, boning takes out what would spoil, and weeks over wood smoke drive the moisture down far enough that mold can be introduced safely.

Dried bonito blocks packed in a wooden crate in a dim curing room, soft pale blue-green mold bloom covering their surfaces, humid air, quiet muted light, macro detail

Mold, grown on purpose

Then the makers do what Japan always does: they hire the mold.

By this point the fish is already preserved. The mold is not added to keep it edible but to change it — the same move as koji, applied to protein instead of starch.

Extreme macro of powdery blue-grey mold spores blanketing the hard dark surface of dried bonito, velvety fungal bloom, razor-thin depth of field, scientific beauty

Mold pulls the water out

Aspergillus glaucus and its relatives pull the moisture out.

This is a relative of the brewing mold put to a different task. Here what matters is that it keeps drawing water out after the smoking has already stopped.

Dried bonito blocks laid out in neat rows on outdoor drying racks under strong sunlight, crisp shadows, blue sky above, repeating geometric pattern, coastal air

Four rounds of mold

Mold, then sun-drying — repeated four times or more.

That is the minimum for the honkarebushi grade, not an average. Each cycle takes the block further from a food that spoils and closer to one that has to be shaved with a blade.

Cross-section of a katsuobushi block showing glassy amber-dark interior like polished stone, sharp fractured edges, hard specular highlights, studio macro

Said to be the hardest

The result is said to be among the world's hardest foods.

The hedge is deliberate. The hardness is widely repeated but not something we could trace to a measurement, so the claim is left as it is usually stated rather than presented as fact.

Japanese fishing harbor at dawn, moored boats and low processing factories with chimneys behind, calm water, soft pink morning light, wide documentary view

Makurazaki, Ibusuki, Yaizu

Makurazaki, Ibusuki, Yaizu — three towns still make it.

The first two are in Kagoshima at the southern end of the country; the third is in Shizuoka, on the Pacific coast. All three are fishing ports, which is the condition the craft needs.

Traditional wooden katsuobushi shaving box in use, a bonito block drawn across the blade, fresh pink shavings peeling off and falling into the drawer, warm workshop light

Shave one at Yamaju, Yaizu

In Yaizu, Yamaju lets you shave one, 1,750 yen.

Shaving is done on a wooden box with a blade set into the lid, and it takes more force than it looks. Doing it once explains why pre-shaved packets exist at all.

The thread through Fermentation

Japan never fought the mold. It hired it.

Next: beans that fermented inside a bundle of straw.

Japanese terms in this story

kojimisokatsuobushihonkarebushi

Sources — primary only

  • Yamaju
  • katsuobushi producers in Kagoshima and Shizuoka
  • Yaizu
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