How to Make Koji 2: Setting up Koji Muro (Incubation Box)

Marika Groen

February 16, 2022

Last October, American food professionals visited fermented food producers to deepen their knowledge of Japanese fermentation culture during the “Hakko Tourism in Japan” tour campaign. As part of the tour, organizers held a tasting session where guests gave candid advice from the perspective of the American market to food product manufacturers looking to enter the United States market.

In the last issue, I talked about choosing the right tane-koji (starter) for koji making. In this edition, I would like to focus on the ideal environment to make koji. We will be focusing on setting up the Koji Muro (koji box/koji incubator). 

Koji fungus is fond of the Japanese climate, with our extra support in winter, and in summer it can be made almost free of energy. They prefer a warm and humid environment to thrive, a bit sauna-like condition at the beginning. I experience a great difference in koji making when I do it in Japan and some parts of Europe. Even within Europe, I see different approaches are required for local weather and the availability of the ingredient.

This is why I want to increase the number of people who can teach locally. Traditionally speaking, we have the basics in Japan but the process is still adaptive depending on in which part of the country we are making koji. This means that the distance is simply larger but the principle of adaptability is the same when we talk about different approaches in the world.

Koji is a fungus. The starter is a seed spore. So we sow this seed in the bed of soil, in this case, rice (or barley, soy) first to let it germinate. The most important requirement is moisture, which is why we prepare the substrate hydrated and the environment to culture this fungus also needs to be humid. This humidity is to ensures that the moisture in rice is not going to get lost before the fungus germinates. If you let the rice dry out first, no fungus wakes up. This is why we usually wrap the inoculated rice in a layer of cloth and/or plastic, so the rice stays moist while the spore is getting ready to sprout.

Once germinated, the fungus needs air. At this point, the mold is very weak and the substrate needs to stay moist to encourage its growth. Here we need to make sure that the rice is sitting in an extremely humid condition.
How humid? I’d say more than 90% in general, but this depends a bit on the substrate you are working with. Let’s talk about rice koji today, then you can aim as much humidity as possible in your environment.
However, it is hard to find such a natural environment. That is why we must create a sort of koji sauna. We call this room to make koji as koji muro. Muro can be a size of your living room or a small cooler, depending on the volume of koji you are making. To create the humidity, you can think of anything. In the traditional muro at breweries in Japan, often the volume of rice is bigger so the moisture from the rice itself is enough to keep them hydrated. When working with a smaller volume, you might need an extra humidifier to keep your muro humid.
Japanese muro is often made of cedar trees for its availability and adaptive character to high and low humidity. Cedar is also used for koji-buta, a container to incubate the koji in for the same reason.
assembled timbers to make the incubation box (muro) for koji, also used for koji-buta
If you live outside Japan, you can make your muro with different wood, free of strong aroma or resin, or you can even use an oven, an old fridge, or even a cooler.
Once you attain the moisture, next you can look for a heat source. The inoculated rice does not produce any heat by itself first, so you need to find something to keep it warm. If your place is over 35 degrees C by default, lucky you, you can just rely on the natural temperature, but I believe most of us need to create the condition, in which case you can use a hot water bottle, an electric blanket, a light in the oven, etc.
When koji is kept under such conditions, the germinated fungus keeps growing, eating the nutrients provided by the substrate, and extending the arm (mycelia) to thrive more.

Hope you can find or build your own muro from what is available in your country. If you come up with interesting ideas to design your muro, we would be more than happy if you share them with us 😉
incubation box (muro) for koji
Malica Ferments | + posts

Marika Groen is the head of Malica Ferments, an online platform dedicated to fermented products. As a Kojiologist, traveler, brewer, photographer, and writer, she published the book "Cosy Koji" in 2021, offering insights into the art of Koji making based on her worldwide lectures and experiences.