One is in Iwate, northern Japan, and the other in Kofu, Yamanashi.
And the both owners were so gracious to give me a private tour of the koji-gura, while explaining the process of koji making.
First, the rice is steamed. (the rice was not as soft as I expected to be)
Then the rice is mixed with koji mold.
They let it sit for a half day so the mold and rice blend well together.
Now the mixture is put in wooden boxes with the cover, and placed in a room called koji muro.
In koji muro, the temperature and humidity are strictly controlled so the mold can grow on rice.
It felt like I was in sauna when I got in the koji muro.
For the next few days, the mold starts to sprout and flower while root deep into each grain of rice.
According to the owner of koji maker in Iwate, it usually takes about 2 to 3 days for the koji to be complete, but in his place, they let it sit for 4 days so that the koji can become more active in fermenting food.
Now koji is ready to be used to make fermented foods.
At the koji maker in Iwate, they make amazake drink and some pickles with the koji, while the one in Kofu make miso.
*Koji won't die in freezer, so you can store them in it.