Koji & Soy · Intermediate

How to Make Natto at Home: Sticky Fermented Soybeans

Homemade natto with Bacillus subtilis natto: soaking and steaming small soybeans, inoculating while hot, fermenting at 40°C for about 24 hours, then ageing in the fridge. How to tell it worked, too much ammonia, storage, and vitamin K2.

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Sticky fermented soybeans lifted with chopsticks

Updated October 2026 · 8 min read

Natto is the polarising Japanese breakfast of sticky, stringy fermented soybeans: earthy, savoury and slightly pungent. It is made by a bacterium, Bacillus subtilis natto, rather than a mould, and it is one of the quickest of the soy ferments: a day in the warm and a day in the fridge.

What you need

500 g small dry soybeans
1 pinch natto spores, or 1 tbsp store-bought natto
~40°C for 22–24 hours

How to make natto

  1. 1
    Soak Rinse the soybeans and soak them for 12–24 hours until they have roughly tripled in volume.
  2. 2
    Steam Steam for 3–6 hours or pressure-cook for about 40–45 minutes, until a bean crushes easily between your fingers. Steaming gives better natto than boiling.
  3. 3
    Inoculate hot Dissolve the spores (follow the packet; a tiny amount covers a kilo) or 1 tablespoon of natto in 2 tablespoons of boiled water. Pour it over the beans while they are still hot and mix gently. The heat helps the natto bacteria outcompete others.
  4. 4
    Spread and cover Spread the beans 2–3 cm deep in a clean shallow dish. Cover with cheesecloth, then foil or a lid with a few holes. Natto needs some air.
  5. 5
    Ferment Hold at 38–42°C for 22–24 hours. The beans develop a grey-white film and smell earthy and slightly of ammonia.
  6. 6
    Age in the fridge Refrigerate for at least 24 hours. This develops the strings and softens the ammonia.

How to tell it worked

Good natto A thin white-grey bloom on the beans; long, sticky strings when you stir; a nutty, earthy smell with a light ammonia note.
Bad natto No strings and a sour or rotten smell, or coloured mould. It was too cool, contaminated, or the starter failed. Discard it.

Watch for

  • Very strong ammonia Too long or too hot. Ferment for less time next batch.
  • Dry, hard beans The cover let too much moisture out. Cover more closely.
  • If you take warfarin or another vitamin K antagonist, ask your doctor before eating natto.

How to eat natto

The classic stir 40–50 times with a little soy sauce and hot mustard until frothy and stringy, then pile on hot rice with spring onions.
With egg stir in a raw or soft-cooked egg yolk for a richer bowl.
Mixed in add to omelettes, fried rice, toast or noodles.

For the mould-based soy ferments, see how to make tempeh and how to make miso.

FAQ

Frequently asked questions

What do I need to start natto?
Natto spores (Bacillus subtilis var. natto) from a fermentation supplier, or a tablespoon of plain store-bought natto as a starter. Small soybeans give the classic texture.
What temperature does natto ferment at?
About 38–42°C, with 40°C ideal, for 22–24 hours. A yogurt maker, dehydrator, proofing box or oven with the light on can hold it. Humidity helps; cover the beans loosely so they do not dry out but still get air.
Why isn't my natto sticky?
It was too cool, the starter was weak, or it has not aged yet. The strings develop most during the day or two in the fridge after fermenting. Stir it vigorously before eating; that is what pulls the threads.
My natto smells strongly of ammonia. Is it bad?
Some ammonia is normal in natto and fades after a day or two in the fridge. A very strong ammonia smell means it fermented too long or too hot. Shorten the time next batch.
Is natto high in vitamin K2?
Yes, natto is one of the richest food sources of vitamin K2 (menaquinone-7). Because vitamin K affects blood clotting, people taking warfarin or other vitamin K antagonists are usually advised to avoid natto. Check with your doctor.
How long does homemade natto keep?
About a week in the fridge. Freeze it in small portions for up to a few months and thaw in the fridge.
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