How to Make Koji 6: Incubation

Marika Groen

July 17, 2022
The entire process to make koji from tsutsumikomi to finish generally takes about 48 hours, but you should not tie to this number without thinking. Some require less, some require more.

In the previous article, your koji was broken up and put back in your muro. You are now observing the process of their growth.

As I mentioned in the article, koji “wakes up” in the first 12-24 hours from tstsumikomi, and that’s when they start asking for some air. When you opened the wrap last time, likely you transferred your koji rice onto some kind of trays. Or you may have decided to keep it in the plastic bag. Either way, you will need a tray at some point, so you are able to spread out koji to give more surface area to access to air as well as releasing its own heat and excess moisture. You can also try a salad spinner (posted in another article.)

Once transferred to a tray, koji bundle is loosened and they have better access to the air. At the same time, you must be careful not to spread them too thin from the early stage. Koji mold needs moisture to propagate first before requiring the air. The tighter you pack the koji the more humid between rice grains is, which is good for the initial stage of mold growth. Then you can slightly spread the bundle in the next teire (mixing), and even more in the next one. Of course, this decision must be made according to the actual condition of the koji at that time. If you notice the weak growth, you must doubt the possibility of lack of moisture (and lack of humidity) or lack of heating support. Maybe your rice wasn’t prepared properly (not steeped or steamed long enough etc) or tane-koji used was too little or too old. Anything is possible, but if the moisture of the rice was the reason, you must wait with spreading them too early. Wait until you confirm enough signs of propagation (aroma, white spots on the grain, still-soft-texture of the rice, taste of rice is getting slightly sweeter than just after steaming etc).

Gut microbiota modulation

Breaking up lumps of koji

Gut microbiota modulation

At first, pile up koji thick enough so the moisture can be kept

If you succeed to wait and judge the timing of spreading, your koji grows more and more mycelia, which you will see visually, smell and taste the difference. So in the course of last 24 hours approximately, you will likely to spread them divided in three steps:
  1. Mori (mix and make a mound of koji rice on a tray)
  2. Naka-shigoto (mix and spread koji on a tray)
  3. Shimai-shigoto (mix and spread koji more on a tray)
Each step name refers to the look or the actual name of the step in Japanese. For example, mori means to “mound” and naka-shigoto means the second task. Shimai-shigoto means the final task, which indicates that this is the last time you touch your koji before harvesting. Between each task, the koji is likely to increase temperature of about 4 or 5 degrees. Make sure it doesn’t exceed 43C, or the mold is burned and unable to grow further. In real practice, these numbers should be taken into account after the koji’s look and feel to you. You must feel the sign of changes, observe and learn, then look at the temperature and the time. Otherwise, you ignore all the signs that koji is sending you while monitoring the numbers on all your devices.
In this shimai-shigoto, we also make some patterns on the surface. This is to increase even more surface area of koji, and the pattern could be simple straight lines or swirl pattern if your tray has round shape. The pattern itself has name in Japanese, which is “hanamachi”. The translation could be a floral trail, and I find it always beautiful when I think of the bed of koji as a flower field of koji blossoms. It feels like a celebration of the long journey, walking on the path that we crossed together.
Gut microbiota modulation

At the end increase the surface area. Making hanamachi

At the time of shimai-shiogoto, your koji is most likely generating enough heat on its own, so you can also turn off your heat setting and remove the lid on the top.
Gut microbiota modulation

First you are covering koji with a lid

Gut microbiota modulation

At the and remove the lid

The entire process to make koji from tsutsumikomi to finish generally takes about 48 hours, but you should not tie to this number without thinking. Some require less, some require more. The number of hours is given only to indicate the average, and to give you an idea that 100 hours is too long, for example. Still, the longer the more chance of contamination risk, but as long as you keep the general cleanliness, you could spend another day (72 hours) to grow koji gently.

This is all up to the koji’s growth, how you work with koji, depending on the condition of rice, type of tane-koji (which usually comes with instructions from the producer), your set-up, environmental humidity, etc. In short, practice it all year.

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