Katsuobushi (鰹節) is smoked and dried skipjack tuna, and one of the foundations of Japanese cooking. With kombu, it makes dashi, the savoury stock under almost every Japanese dish. Fully cured and dried, it is famously the hardest food in the world, and its inosinate is the umami that makes dashi sing.
This is our deep guide to katsuobushi (bonito flakes), written for chefs and curious cooks: what it is, the grades from arabushi to honkarebushi, how it is made over months of smoking and mould-curing, what a working bonito factory looks like, and how to turn it into dashi. It sits inside our Dashi and Umami Masterclass.
Katsuobushi: Japan's Smoked, Dried Bonito
Katsuobushi (鰹節) is smoked and dried skipjack tuna (Katsuwonus pelamis), the fish English-speakers call bonito. It is one of Japan's most fundamental ingredients, the primary basis of ichiban dashi (一番出汁), the clear first-pull stock that combines katsuobushi with kombu. The skipjack is caught mainly in the Midwest Pacific and Indian Ocean and processed entirely in Japan.
Two things make it special. Fully cured and dried, katsuobushi is so hard it is often called the hardest food in the world, which is why it is shaved rather than cut. And it is one of the great sources of inosinate, the nucleotide that drives umami: paired with kombu's glutamate, it produces the synergy that makes dashi far more savoury than either alone. See our umami guide for that mechanism, and kombu for its partner.
For a chef, the one-line version: katsuobushi is concentrated, shelf-stable umami, and the grade and form you buy decide how delicate or how deep that umami is.
Source: Marutomo EU specification (species, catching ground, Japanese processing) and Marutomo's producer site; the inosinate-umami link is in our umami and katsuobushi records. Facts extracted and reworded.
The Grades: Arabushi to Honkarebushi
Katsuobushi is sold as dried blocks (nodes) at increasing degrees of curing, and as the shavings cut from them. The key divide is whether the block has been mould-cured: arabushi is smoke-dried only, while honkarebushi has been through repeated mould cycles for a deeper, mellower flavour. A single skipjack yields four nodes, two dorsal (背節, sebushi) and two ventral (腹節, harubushi), split along the lateral line.
| Grade | Japanese | What it is |
|---|---|---|
| Arabushi | 荒節 | Smoke-dried node, no mould; rough, black surface, about 20% water. Lighter, more accessible flavour. Most hanakatsuobushi is shaved from this. |
| Hadakabushi | 裸節 | Arabushi with its smoke-blackened surface ground off, exposing clean flesh ready for mould. |
| Karebushi | 枯節 | Mould-cured node, about two months of repeated inoculation; mellow and deep. |
| Honkarebushi | 本枯節 | The highest grade: four or more mould cycles over roughly six months. Extremely hard, profoundly savoury. |
The everyday form most kitchens buy is hanakatsuobushi (花かつお, "flower bonito"): thin shavings that curl like petals, planed from arabushi, for dashi and as a topping. Thicker shavings (kezuribushi) give a more robust dashi. Blood-line-removed blocks give a cleaner, clearer stock.
For a chef, the steer is simple: arabushi-based hanakatsuobushi is your workhorse for everyday dashi and toppings; honkarebushi, shaved to order, is the special-occasion grade where the dashi itself is the dish.
Source: Ninben and Marutomo producer sites (the grades, node anatomy and yields) and the first-hand Kaneyo factory observation (Yaizu); grade A/B, facts extracted and reworded. The honkarebushi mould species is not pinned in our sources.
How Katsuobushi Is Made
Katsuobushi is built up in stages: a whole fish becomes smoke-dried arabushi, and the finest grade then goes through months of mould-curing to become honkarebushi. The sequence below comes from two producers' own technical pages and a first-hand SushiSushi factory visit.
Cutting and yield. A skipjack is cut into the four nodes (two sebushi, two harubushi). Flesh of about 1 to 3% fat is ideal, so near-coastal fish with tighter fat are prized; the skin is left on to stop the node breaking up during smoking.
Boiling. The nodes are simmered before smoking.
Smoke-drying (焙乾, baikan). Smoke and heat are applied together over hardwood, cherry and oak varieties (sakura, kashi, nara, kunugi). The first firing (一番火), a water-extraction smoking, starts from the fish's fresh water content of around 68%. Smoking then continues as intermittent firings: a node takes 10 to 15 firings over about ten days, driving the water down to roughly 20%, at which point it is arabushi. Baikan also adds the smoky aroma and a natural anti-bacterial, anti-oxidant effect.
Surface grinding to hadakabushi (裸節). The blackened surface of the arabushi is ground off, exposing clean flesh ready for mould.
Mould-curing (かび付け, kabitsuke). The hadakabushi goes into a mould cellar, where a beneficial mould is grown on it and the node is then dried, in repeated cycles. About two months of cycles makes karebushi; four or more cycles over roughly six months makes honkarebushi. Each round draws out more moisture and develops flavour, and crucially builds inosinate, amplifying the umami. The result is the rock-hard, mellow block prized for the finest dashi.
The chef's takeaway: the smoke gives aroma, the drying gives concentration, and the mould gives depth and umami. That is why a honkarebushi dashi tastes so much rounder than one from a quick arabushi flake.
Source: Ninben producer site (firing counts, water percentages, mould cycles, yields) and Marutomo producer site (the baikan and kabitsuke stages, hardwoods); grade A, facts extracted and reworded. Exact per-firing durations and the precise mould-cycle schedule beyond "4+" are not in our sources.
Inside a Bonito Factory: Kaneyo, Yaizu
We visited Kaneyo (㈱カネヨ) in Yaizu, Shizuoka, a historic bonito-processing port, and watched the arabushi line end to end. It is a striking mix of heavy industry and hand craft: frozen skipjack is defrosted in aerated tanks, sectioned on a water-cooled rotary saw, then hand-filleted into sebushi and harubushi at water tables. The nodes are boiled in basket cages lowered into steam-heated vats, air-cooled on tiered racks, then smoked in brick kilns fired by split hardwood, with batches tracked by chalk marks across consecutive smoke runs until they emerge under a deep charcoal crust and ship as Yaizu katsuobushi.
Two things stood out. Mould-cured blocks were resting under a natural grey-brown mould layer before final sorting, a glimpse of the kabitsuke grade alongside the arabushi line. And a kezuri-shi (削り師), a shaving artisan, was hand-planing and wire-brushing cured blocks outdoors to remove surface fat and char, telling us "we're probably the only place that still does this by hand." That surviving manual craft, beside the machinery, is exactly why grade and provenance matter with katsuobushi.
Source: first-hand SushiSushi factory visit to Kaneyo, Yaizu (Shizuoka), documented from on-site footage and signage; scene observations grade B, stated/spoken detail grade A.
Popular Products
Four ways into katsuobushi, from whole flakes to ready dashi and the tool to shave your own:
Okan Bonito Flakes (Katsuobushi), 500g
Hanakatsuobushi, the everyday flower-bonito shavings, in the catering bag. The workhorse for making dashi and for topping okonomiyaki, tofu and rice. Light, smoky and deeply savoury.
Marutomo Katsuo Dashi Powder, 1kg
Pure bonito milled into an instant dashi powder, the fast route to a katsuo stock when there is no time to steep flakes. The katsuo dashi some starred kitchens reach for, in the catering pack.
How to Make Dashi with Katsuobushi
Katsuobushi's main job is dashi, and the standard method is quick. Bring 600ml of water to the boil, lower the heat, add 20g of hanakatsuobushi, simmer uncovered for about two minutes, then turn off the heat and let the flakes settle before straining through cloth or paper. That ratio, roughly 3.3% by weight, gives a clear, everyday katsuo dashi.
Two refinements matter in a kitchen. For ichiban dashi, the delicate first pull used where the stock is the star, katsuobushi is paired with kombu (steep the kombu first, then add the flakes off the heat) to exploit umami synergy. And the spent flakes still hold flavour: a second, longer extraction gives niban dashi, a deeper, more assertive stock for miso soup, braises and sauces where other flavours carry. Beyond stock, katsuobushi is a topping in its own right, scattered over okonomiyaki, tofu, boiled vegetables and salads, where the warmth makes the shavings appear to dance.
Our steer: keep hanakatsuobushi and kombu for your base ichiban dashi, a dashi powder or shiro dashi for speed on service, and treat the spent flakes as niban dashi rather than waste. (The fine-dining ichiban ratio can differ from the standard 3.3%; tune to taste.)
Source: Marutomo EU specification (the 20g per 600ml method and toppings); the ichiban/niban distinction and the kombu pairing are established practice. Facts extracted and reworded.
Is Katsuobushi Good for You?
Katsuobushi is one of the most protein-dense foods in the kitchen and very low in fat, which is part of why a little adds so much savoury depth. As a concentrated dried fish it is naturally rich in protein and the amino acids behind its umami, while the export hanakatsuobushi we stock is additive-free with no declared allergens. Used as a seasoning and stock base rather than eaten in bulk, it is a clean way to build flavour without much fat or salt.
Source: Marutomo EU specification (nutrition, additive-free, allergen status). Note: producer figures for katsuobushi's exact protein percentage vary between sources, so we describe it qualitatively as exceptionally protein-rich rather than pin a contested number.
Frequently Asked Questions
What are bonito flakes?
Bonito flakes, or katsuobushi, are thin shavings of smoked and dried skipjack tuna. The everyday form, hanakatsuobushi, is planed from smoke-dried blocks into delicate petals used to make dashi stock and as a topping for dishes like okonomiyaki and tofu. They carry inosinate, the umami compound that, paired with kombu, gives Japanese dashi its savoury depth.
What is the difference between arabushi and honkarebushi?
Arabushi is bonito that has been smoke-dried only, with a rough black surface and a lighter, more accessible flavour; most everyday flakes are shaved from it. Honkarebushi is the highest grade, made by grinding the arabushi clean and then mould-curing it through four or more cycles over roughly six months, which deepens the flavour and builds umami. Honkarebushi makes the finest, mellowest dashi.
How do you make dashi with bonito flakes?
The standard method: boil 600ml of water, lower the heat, add about 20g of hanakatsuobushi, simmer uncovered for two minutes, then turn off the heat, let the flakes settle and strain. That gives a clear everyday dashi. For the finest first-pull (ichiban) dashi, pair the flakes with kombu, which combines bonito's inosinate with kombu's glutamate for far greater savouriness.
Why is katsuobushi so hard?
Fully cured katsuobushi is repeatedly smoke-dried and then mould-cured over months, which drives its water content right down and concentrates the flesh into a dense, rock-hard block, often called the hardest food in the world. That is why traditional katsuobushi is shaved on a plane (kezuriki) rather than cut, producing the thin flakes used for dashi.
Is katsuobushi the same as bonito?
Bonito is the fish (skipjack tuna), and katsuobushi is what you get after that fish is filleted, boiled, smoke-dried and, for higher grades, mould-cured. So "bonito flakes" is the common English name for shavings of katsuobushi. Strictly, bonito is the raw material and katsuobushi is the finished, preserved ingredient.
Where to Buy Katsuobushi in the UK
If you are looking to buy katsuobushi and bonito flakes in the UK, we keep a range with fast delivery across the UK, from everyday hanakatsuobushi and katsuo dashi powder to premium shiro dashi and the slicer for shaving your own block. Start with a bag of flakes and a piece of kombu and make a dashi.
For more, browse the Dashi and Umami Masterclass, or read about kombu, katsuobushi's partner in dashi, umami, the science behind it, and soy sauce.
Technical data sourced from the producer sites of Ninben and Marutomo (the grades, the baikan smoke-drying and kabitsuke mould-curing process, yields and dashi method) and a first-hand SushiSushi factory visit to Kaneyo in Yaizu, Shizuoka (the arabushi line and the surviving hand-shaving craft), with nutrition from Marutomo's EU specification. Skipjack is caught in the Midwest Pacific and Indian Ocean and processed in Japan. Producer claims reflect those makers' documented practice; the exact mould species and protein percentage are not fixed across our sources.
