Katsuobushi production documented in Tosa Province (Kochi Prefecture) from the Muromachi period; the mould inoculation step (producing hon-karebushi) developed in the late Edo period in Makurazaki (Kagoshima) and Yaizu (Shizuoka), which remain the dominant production centres
The production of authentic katsuobushi (本枯節 — hon-karebushi) is a months-long process involving cooking, smoking, mould inoculation, and sun-drying repeated in cycles. Step 1: skipjack tuna (katsuo) fillets are simmered in graduated water temperatures (60°C rising to 85°C) until internal temperature reaches 75°C — then cooled and bones removed by hand. Step 2: smoking in cherry or oak smoke chambers (20–30 sessions over 2 weeks), building the characteristic smoke crust (ko — smoked node). This produces arabushi (荒節 — rough katsuobushi) used in home cooking. Step 3: for hon-karebushi, arabushi is inoculated with specific Aspergillus glaucus moulds which penetrate the fish over 2–4 weeks; the mould digests surface fat (responsible for fishy off-notes) and produces additional amino acids including glutamates. The fish is then sun-dried, and the mould cycle is repeated 3–5 times over 3–6 months. The result is the hardest naturally occurring food in the world (denser than many woods) — shaved to tissue-thin sheets with a specialised plane (katsuobushi kezuri). The mineral-rich flakes reveal translucency and dance in steam.
The multiple mould cycles are the key to hon-karebushi's superior dashi performance: Aspergillus digests trimethylamine (fishy odour compound), increases inosinic acid (IMP — nucleotide that synergises with glutamate for compound umami), and develops complex volatile aromatic compounds including dimethyl sulfide and pyrazines from the smoking and drying interactions
Multiple smoking cycles build smoke penetration without burning; mould cycles reduce fat and increase amino acids; each mould cycle adds complexity to the flavour compound profile; the hardness is a structural consequence of progressive drying; shaving with the grain produces translucent flakes, against the grain produces powder.
Shave katsuobushi immediately before use for maximum aroma; the kezuri-bako (shaving box) contains the shavings for use as needed; hon-karebushi whole blocks keep for years in a cool dry space; the dancing of shaved katsuobushi over hot dishes (topping tofu, okonomiyaki) is a physical response to steam currents — aesthetically pleasing and confirms the fish is properly shaved thin.
Using arabushi as a substitute for hon-karebushi in ichiban dashi — valid but produces a smokier, less complex stock; buying pre-shaved katsuobushi in bags — oxidises rapidly after shaving; improper storage after shaving (room temperature oxidises aromatic compounds within hours).
Tsuji, Shizuo — Japanese Cooking: A Simple Art; Ono, Tadashi — Japanese Soul Cooking