Once grains have cooled, gently and evenly sprinkle 1/3 of the tane koji over the bed. Stir the grains well. Repeat with another 1/3, stirring well, and finally the remaining 1/3. Stir for a total of 3-4 minutes to thoroughly incorporate the koji spores.
If you are using the pure spores, you can use a flour sifter to gently disperse the spores. They will seem lighter than air, so observe carefully as the spores fall onto the rice bed.
Cover the grain bed with the towel. If needed to cover the bed, you can also use the towel you used to steam them. Just ensure that you wring out the steaming towel first to remove excess water.
Add tray(s) to your incubating chamber. Ideal temperature range during the first 24 hours is 86° to 91°F/30° to 33°C, with 75% humidity. If you have a temperature probe, ensure it is clean, then stick it in the middle center of the grains bed.
After 24 hours, you should see signs of the mycelium beginning to form all over the grains, and it should be fairly fragrant.
Break up any clumps or mats to encourage more mycelium to form, then create several lengthwise furrows in the bed, which will provide more surface area for those oxygen-loving microbes to form mycelium.
Check again at 36 hours, ensuring the temperature doesn't climb too high. The mass of koji will begin generating its own heat, so you may even be able to lower or turn off the heat source, so it doesn't overheat.
Koji should be finished between 40 and 48 hours. If it begins to turn splotchy colors like dark olive, yellow, it has begun to sporulate (possibly because it was too warm). Make a decision as to whether enough mycelium has formed if this occurs.
Remove tray from incubator, then allow to cool down. Break up any clumps to get to as close to individual grains as possible.
Use koji immediately, or refrigerate in a tightly sealed container for up to 3 days in the refrigerator.