Skip to content
FriendsPantry

Soy & legumes ferment

Natto

Advanced

Steamed soybeans fermented by Bacillus subtilis var. natto into a sticky, stringy, pungent dish.

Temperature
38–42 °C
Time
18–24 hours + 1 day in the fridge

Timeline

Natto timeline

  1. Stage 1: Soak & steamstart · about 14 h

    Hands-on

  2. Stage 2: Incubatefrom 14 h · about 21 h

    Fermenting / waiting — 38–42 °C

  3. Stage 3: Mature in fridgefrom 35 h · keeps about 24 h

    Cold storage

  4. Stage 4: Storefrom 2 days · keeps about 7 days

    Cold storage — Fridge about a week; freeze longer

Ingredients

  • Small soybeans
  • Natto spores (commercial) or a spoon of commercial natto

Equipment

  • Pressure cooker or steamer
  • Shallow dishes
  • Incubator holding about 38–42 °C
  • Thermometer

Steps

  1. Soak beans overnight; steam or pressure-cook until soft.
  2. While hot, mix in spores dissolved in a little boiled, cooled water.
  3. Spread thinly in clean dishes; cover loosely.
  4. Incubate at about 38–42 °C for 18–24 hours.
  5. Refrigerate for a day to mature the flavour and strings.

Is it working?

  • White-grey film on the beans
  • Long sticky strings when stirred
  • Pungent, savoury smell

Smell & texture: Strong, earthy, slightly ammoniac — normal for natto; very sticky.

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?

Natto should grow a smooth, white-grey bacterial film, never fuzzy mould. Fuzz or colour: discard.

Storage

Refrigerate; freeze for longer.

Shelf life: About a week in the fridge; ammonia smell strengthens with age.

The science: what's happening

Natto bacteria grow on hot, moist soybeans, producing the sticky polyglutamic acid strings and a strong, cheesy-ammoniac aroma.

Organisms

  • Bacillus subtilis var. natto (commercial spores)

Troubleshooting

No strings
Too cool or too dry; keep at 38–42 °C with some humidity.

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

Learn more

Sources

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