How Koji is made ?
From Gomi-Shoyu, Koji Manufactrer in Yamanashi

A long time ago there used to be koji shops everywhere in Japan, and people used to buy koji and make their own miso. Since people buy miso at supermarkets nowadays, small koji shops are having a difficult time. However, thanks to a seasoning called shio-koji, which became popular a couple years ago, the koji shops that were nearly going out of business have been recovering, and there is a slow movement for people to make homemade miso these days.

This time, I would like to introduce you to a soy sauce shop in Kofu city, Yamanashi Prefecture called “Gomi-shoyu,” a family owned business that sells their own homemade miso. Here is the report of my last visit to them to see how they produce their koji:


Step1: Soak the rinsed rice for about 15 hours. It is like a huge bath!



 Step2: Drain the water before steaming the rice.gomi_2

 They can produce 250kg of koji at a time. It is the weight of one and half sumo wrestlers. Heavy and huge! If you steam all the rice at a time, it won’t be cooked equally; therefore, they add rice little by little, mix and steam it. The steamed rice is a bit firmer than the regular cooked rice. If you press the rice grain, the surface should be non-sticky and the inside should have mochi-like texture.


Step3: Transfer rice to the steamer by hands. Such hard work!


Step4: Gently mix the rice little by little.


Step5: Steaming the rice with high pressure




Shiny rice with a non-sticky surface and mochi-like texture inside.


Step6: Stir and loosen hot steamed rice by hands and let cool.

Transfer a portion of the steamed rice to the large tub. There is a net under the white cloth for the air to go through and let the rice cool. It is the handmade tub they built themselves.

They stir and loosen hot steamed rice by hands and let cool.  Though it must be very hot, they do so by bare hands in order to check its temperature and make koji mold uniformly mixed with the steamed rice. Their hands sometimes get burned as it is too hot.


Looks very hot, but they loosen the rice into small pieces so that it is mixed well with the koji in the next step.


Step7: Add koji mold !

Once it is cooled down to 32℃, toss a portion of the steamed rice to coat it with koji mold and do the same for the rest of the rice.  Then, they use a machine to stir and loosen the rice, pack it back together, and let it rest on a koji bed called Tokobune in the koji room called Koji-muro. After letting rest a while, they sprinkle koji mold again, mixing well, and loosen the rice by machine. After double coating with koji mold, they put it altogether again and let rest on the bed.

7-1: After letting cool down, transfer a part of loosened steamed rice  in another container and sprinkle koji mold.


7-2: Put the rice with koji mold back to the tub and mix it well with the rest of rice.


7-3: The machine called Gara-gara, the name refers to the noise that the machine makes. It loosens and aerates the rice.gomi_11

It’s being done again and again to produce 250kg of koji!


 7-4: let it rest for a while…..gomi_13

7-5: Koji mold is sprinkled again and mix well!


Now, here we have comfortable temperature, humidity, and starch for koji to grow its spores. See the next page to see the finishing phase.

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