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Soy & legumes ferment

Miso

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Cooked soybeans, koji and salt aged for months into a savoury paste.

Temperature
15–25 °C
Cool, stable place.
Time
Sweet white: weeks to about 2 months; red: 6–24 months
Salt
Long-aged miso commonly 10–13% salt
Salt is the main safety control; low-salt sweet misos are short-aged and need refrigeration. No regulator-validated home process exists — follow a reputable miso guide and consult extension sources.

Timeline

Miso timeline

  1. Stage 1: Cook & mash beansstart · about 6 h

    Hands-on

  2. Stage 2: Packfrom 6 h · about 1 h

    Hands-on — Exclude air, salt the surface

  3. Stage 3: Agefrom 7 h · about 6 months

    Fermenting / waiting — 15–25 °C, months

  4. Stage 4: Refrigeratefrom 6 months · keeps about 6 months

    Cold storage — Fridge, months

Ingredients

  • Soybeans
  • Rice or barley koji (buy commercial koji)
  • Salt (high: commonly around 10–13% of total weight for long-aged miso)

Equipment

  • Pressure cooker or large pot
  • Food processor or masher
  • Crock or tub
  • Weight
  • Kitchen scale

Steps

  1. Soak and cook soybeans until very soft.
  2. Mash; cool to body temperature.
  3. Mix koji with most of the salt, then the beans and some cooking liquid.
  4. Pack tightly into a clean crock to remove air; salt the surface; cover directly with film and a weight.
  5. Age in a cool place (about 15–25 °C) for months; check occasionally.

Is it working?

  • Savoury, sweet-salty aroma
  • Dark liquid (tamari) pooling — normal

Smell & texture: Deep, savoury, sweet-salty; paste darkens over time.

Signs tell you it's fermenting, not that it's safe: never rely on smell or appearance alone.

Safety, mould and discard rules

Mould or yeast?

A thin white film or surface yeast on the very top is common in long ferments; traditionally the top layer is scraped away generously. Fuzzy, coloured or deep mould: discard. If unsure, discard.

Storage

Once opened or finished, refrigerate. Low-salt sweet misos must be refrigerated.

Shelf life: Refrigerated long-aged miso keeps for months; quality changes slowly.

The science: what's happening

Koji — grain grown with the mould Aspergillus oryzae — supplies enzymes that break soy proteins into amino acids (umami) and starch into sugars. Salt-tolerant yeasts and lactic bacteria slowly add aroma over months. High salt is the main safety barrier.

Organisms

  • Aspergillus oryzae (as koji enzymes)
  • Salt-tolerant yeasts (Zygosaccharomyces rouxii)
  • Salt-tolerant lactic bacteria (Tetragenococcus halophilus)

Troubleshooting

Surface growth
Better air exclusion: film directly on the surface, salt the top, weight it.

A dish (not a ferment) gone wrong? Try Recipe Rescue.

Cook with it

Learn more

Sources

Guidance changes: check the source for current wording. Original text; figures quoted from the sources where stated.