Spores
1g
Incubate
48 hr
Temp
30°C
For 1 kg dry polished white rice (~1.8 kg soaked)
8 hr
45 min
30°C / 86°F
48 hr
Grain absorbs water during soaking, roughly doubling in weight.
Inputs
Result
White rice = 1g spore/kg × 1 kg = 1g. Soaked weight = 1 × 1.8 = 1.8 kg. Soak = 8 hr, steam = 45 min. A. oryzae = 30°C for 48 hours.
Inputs
Result
Soybeans = 1.5g/kg × 2 = 3g spores. Soaked = 2 × 1.8 = 3.6 kg. Soak = 18 hr, steam = 90 min. A. oryzae = 30°C, 48 hours.
Use about 1 gram of koji spore powder (tane koji) per kilogram of dry substrate for white rice. Brown rice and barley need slightly more (1.2g/kg) due to tougher bran. Soybeans need 1.5g/kg because their higher protein content requires denser colonization. For a 1 kg batch of white rice, that is 1 gram of spore powder, which is about 1/4 teaspoon.
| Substrate | Spore / kg | Soak Time | Steam Time |
|---|---|---|---|
| White Rice | 1g | 8 hr | 45 min |
| Brown Rice | 1.2g | 12 hr | 60 min |
| Barley | 1g | 10 hr | 50 min |
| Soybeans | 1.5g | 18 hr | 90 min |
Aspergillus oryzae (the most common strain for sake and miso) grows best at 86°F (30°C) with 80%+ humidity. Maintain this for about 48 hours. A. luchuensis (for shochu) prefers slightly warmer conditions at 90°F (32°C) for 42 hours. A. sojae (for soy sauce) prefers cooler 82°F (28°C) for 52 hours.
The simplest DIY method is a cooler with a heating pad and a damp towel. Set the heating pad to low inside the cooler, place a wire rack over it, and spread the inoculated grain on trays on the rack. The damp towel provides humidity. An oven with just the light on can also maintain around 85°F. Dedicated koji incubation chambers offer the most precise control.
Finished koji should be completely covered in dense white mycelium with a sweet, chestnut-like aroma. The grains should stick together in a cake-like mat. It typically takes 42–52 hours depending on the strain. If you see green or black spores, the koji has begun sporulating — it is past peak but still usable. Harvest when white and fragrant, before sporulation.
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Last Updated: Mar 19, 2026
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