Miso (味噌) is a Japanese fermented paste made from soybeans inoculated with Aspergillus oryzae mould (koji), mixed with salt, and fermented from weeks to years. It is the foundational seasoning of Japanese cuisine, used in soup, marinades, glazes, dressings, and braises.
Production
Soybeans are cooked, then combined with koji (mould-inoculated rice, barley, or soybeans), salt, and a small amount of the previous batch (mizukiri). The mixture ferments in cedar vats under weighted pressure from days to years. The koji enzymes break down proteins into amino acids (umami) and starches into sugars (sweetness). Duration and koji type determine colour, flavour, and sugar content.
Major types
Shiromiso (白味噌, white miso): Short fermentation (days to weeks). High rice koji proportion. Pale yellow-white; sweet, mild, delicate. Kyoto specialty. Melts quickly; low tolerance for high heat.
Awase (合わせ, mixed miso): Blend of white and red; the most commonly used domestic miso. Balanced sweet-savoury. Stable under moderate heat.
Akamiso (赤味噌, red miso): Longer fermentation (months to years). Lower koji proportion. Dark brown to near-black; savoury, pungent, earthy. Used heavily in Nagoya (hatcho miso style). Can withstand longer cooking without losing character.
Mugi miso (麦味噌, barley miso): Barley rather than rice as the koji substrate. Less sweet; nuttier; regional to Kyushu and Shikoku.
Hatcho miso: Soybean koji (no rice or barley); fermented minimum 2 years in 6-ton cedar vats under stone weights. Extremely dark, dry, and intensely savoury. Used in the Nagoya area. Named for the distance (8 cho) from Okazaki Castle where it was produced.
Cooking with miso
Miso should never be boiled in soup — boiling destroys the live bacterial cultures and flattens the flavour. Add miso off heat (below 80°C), dissolving it in a small amount of liquid before adding to the pot.
For glazes and marinades, miso’s sugar content caramelises at high heat — the Saikyo-style miso cod (from shiromiso + mirin) and miso-glazed aubergine both exploit this caramelisation. Longer fermented red miso glazes do not caramelize as readily and need higher heat or sugar addition.
Miso does not expire in the traditional sense — it continues to ferment slowly under refrigeration. Colour darkens and flavour deepens over time. It is safe indefinitely if stored without contamination.