Koji · Intermediate

How to Make Koji at Home: Growing Rice Koji Step by Step

Grow your own rice koji with Aspergillus oryzae spores: soaking and steaming rice, inoculating, incubating at 28–35°C for about 48 hours, breaking up clumps, and how to tell good koji from bad. Plus how to store it and what to make with it.

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Rice koji growing on a cloth-lined tray

Updated October 2026 · 10 min read

Koji is the starting point for most of the great Japanese ferments. It is rice (or barley, or soybeans) grown with a friendly mould, Aspergillus oryzae, until the grains are wrapped in white mycelium and full of enzymes. Growing it takes about two days and steady warmth, and once you can make koji you can make miso, amazake, shio koji and more.

What you need

1 kg polished short- or medium-grain rice
~1 g koji spores per kg of rice (follow the supplier)
28–32°C incubation for about 48 hours
Rice polished white rice gives the sweetest koji. Pearled barley works for barley miso.
Koji-kin Aspergillus oryzae spores for rice koji. Dosing varies by product, so use the rate on the packet.
A steamer a bamboo or metal steamer lined with a cloth. Boiled rice is too wet for koji.
Steady warmth an oven with only the light on, a proofing box, a cooler with bottles of warm water, or a dehydrator on its lowest setting, plus a thermometer.

How to make rice koji

  1. 1
    Wash and soak Rinse the rice until the water runs nearly clear, then soak it for 6–12 hours or overnight.
  2. 2
    Drain Drain in a colander for at least an hour. The grains should feel damp but not wet on the outside.
  3. 3
    Steam Steam the rice in a cloth-lined steamer for 45–60 minutes, until the grains are cooked through but firm and separate. A grain pressed between your fingers should squash to a translucent paste without being sticky.
  4. 4
    Cool and inoculate Spread the rice on a clean cloth or tray and let it cool to about 35°C. Sprinkle half the spores over it, mix thoroughly with clean hands, then add the rest and mix again.
  5. 5
    Incubate Gather the rice into a mound in a cloth-lined tray, wrap it in a damp cloth, and keep it at 28–32°C with high humidity.
  6. 6
    First mix: 18–24 hours You will see faint white threads, the rice will start to clump and warm up, and it will smell sweet. Break up the clumps, spread the rice 3–5 cm deep, and draw furrows through it with your fingers to release heat.
  7. 7
    Second mix: 30–36 hours Break it up and re-furrow again. The rice now makes its own heat; keep it between 30 and 38°C by lowering the incubator temperature or opening it.
  8. 8
    Harvest: 40–48 hours When the grains are covered in white, velvety mould and smell of chestnuts, the koji is done. Break it up and spread it out to cool so it stops growing.

Good koji and bad koji

Good White, fluffy and velvety; grains knitted together; a sweet, floral smell of chestnuts, mushrooms or fresh bread. A little yellow-green at the edges means it is starting to spore: harvest straight away.
Bad Strong ammonia; slimy, wet or sticky grains; a sour or rotten smell; black, pink, red or grey fuzzy patches. These come from too much heat or moisture. Discard it.

Watch for

  • Overheating is the most common failure. Check the temperature inside the rice, not just the room, especially after the first day.
  • Too wet rice grows bacteria instead of mould. Drain well and steam rather than boil.
  • Too dry air gives patchy growth on the surface only. Keep a damp cloth over the rice.

Storing and using koji

Fresh koji keeps about a week in the fridge and several months in the freezer. Dried below about 45°C, it keeps in an airtight jar in the pantry.

Miso the main use. See how to make miso.
Amazake and shio koji quick projects that use koji in days. See amazake and shio koji.
Sake koji converts the rice starch for the yeast. See how to make sake.
Soy sauce uses a soybean and wheat koji instead of rice. See how to make soy sauce.

FAQ

Frequently asked questions

What is koji?
Koji is a grain or bean, usually steamed rice, that has been grown with the mould Aspergillus oryzae. The mould produces enzymes that break starch into sugar and protein into amino acids. Those enzymes are what make miso, soy sauce, sake, amazake and shio koji.
Where do I get koji spores?
Buy koji-kin (also called tane-koji) from a fermentation supplier. It is sold as a powder of spores, often mixed with rice flour, and labelled for its use: rice koji for miso and amazake, or shoyu koji for soy sauce.
What temperature does koji need?
Keep the room or incubator at about 28–32°C with high humidity. Once the mould gets going, the rice makes its own heat, so watch the temperature inside the rice and keep it between about 30 and 38°C. Above 40°C the mould suffers and bacteria take over.
How do I know my koji is ready?
After about 40–48 hours the grains are bound together by a white, velvety mould, and the koji smells sweet and floral, like chestnuts or mushrooms. Harvest before it turns yellow-green, which means it is starting to form spores.
How do I tell if koji has gone bad?
A strong ammonia smell, slimy or wet grains, or black, pink, red or grey-green fuzzy patches mean it went wrong, usually from too much heat or moisture. Discard it.
How do I store homemade koji?
Use it within a week from the fridge, freeze it for several months, or dry it at a low temperature (below about 45°C) to keep it in the pantry. Mixing it with salt as shio koji also preserves it.
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