Katsuobushi Production Fermentation and Smoking

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