What is Koji-Kin Doing on Your Rice?

Marika Groen

May 30, 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 previous article, I went through the general growing process of koji. Now I want to illustrate what koji-kin is doing on your rice to transform it into koji rice.

Illustration of the process on how rice becomes koji
Take a look at the drawing above. Start from the top left. That’s the status of your rice and koji starter/tane-koji at the beginning. You have just inoculated tane-koji on rice, and nothing is happening until they wake up in the optimal moisture and temperature level in the next drawing. Once they wake up, they are sort of sprouting and they start to root on the rice. On the third drawing, they notice that they have actually been sleeping on their potential food, which is ready to eat. Lucky them.
It is therefore important that the rice is cooked properly. It should be hydrated and gelatinized by steaming. If there is any uncooked part left, koji-kin is unable to eat it.
Once they know they are served with freshly prepared rice, they will start eating them. Essentially this is when they start growing their mycelia. With mycelia they are able to melt the nutrient of the rice and digest it. While doing so, they generate some heat, which is why you will have to open the wrap and give hands (teire) to mix them so they can cool down a bit and also access to some air. From mycelia they are producing enzyme, just like we humans produce enzyme while we are chewing something in our mouths. Koji-kin and we are not too different, but the difference is that koji-kin leaves that enzyme in rice and that is what we humans benefit from koji making to make something else like miso, sake, amazake etc…
As long as there is enough moisture content in the rice, koji-kin keeps eating the rice. Without moisture, they are unable to grow mycelia nor produce enzyme. This is why we need to make sure that the rice doesn’t get dried out in the first stage of incubation. Mycelia seek for moisture. This is a key. Since koji-kin is able to determine what kind enzyme to produce at the moment they wake up on the substrate, you need to make sure where you guide them to grow mycelia towards to. The type of the enzyme you often expect from koji rice is amylase. Amylase is carbohydrate breaking down enzyme. With this enzyme, you are able to make sweet amazake, or sweet miso, or any other application you can utilize the feature of its ability to break down the carbohydrate into a simple sugar monocle. So where can koji-kin produce amylase? That’s where most carbohydrate is, which is inside of the rice grain. Now remember that mycelia seek moisture. If you let the rice retain enough moisture inside the grain, koji-kin automatically starts growing mycelia towards the centre of the grain. This is usually the most preferred case of rice koji making. If you are growing koji-kin on soybeans, that’s another story.
Illustration of koji making process
Take a look at the drawing above. The left indicates that the moisture content is sufficient inside the grain, and koji-kin is able to eat the rice by growing mycelia, generating enzyme to melt and chew the nutrient. On the right side, the humidity around the rice is higher than what’s left inside the grain. This way, koji-kin is not able to grow mycelia into the grain. It is too dry. Instead, they seek moisture in the air – which is outside of the grain. In this case, mycelia rather grows on the surface of the grain, and this is what’s happening when you have fluffy or hairy koji. Of course, there are cases that the mycelia did grow inwards and finish eating whatever they could, and then start growing hairs outside the grain too, but in case there was not enough moisture in the grain, they will start doing that from the beginning. This means they don’t really need to produce enzyme because there is no carbohydrate in the air. The tricky part is, if you keep the environmental humidity too low, the rice grains themselves may start losing moisture too much. This could be prevented by just covering another damp cloth on the top (make sure that doesn’t touch the rice directly), or balancing the adequate humidity in the air, so it would stay relatively dry but not too dry that the rice could stay hydrated. Essentially, you would want to decrease the environmental humidity lower and lower towards the end of the process, as koji rice tries to sweat out, which would make your muro sweat as well. This is where you need to gain some experience. Now, look at the first drawing again. The bottom left is the end of the process. Can you see a little flower blooming on the heads of koji-kin? This in fact carries spores to the next life, because koji-kin finished their one round of life, and are ready to pass it to the next generation. This flower is often what you see as the fluffy top coating of your koji, and depending on how humid you keep the environment/muro, your koji is full of this flower field all over it. There is no definite answer whether you should see this or not, as what you want to care most is whether they grew mycelia inside of the grain or not. As long as that’s done, the flower field is up to your choice.
Koji looked through a microscope
When you look at them through a microscope, this is what you see. It’s beautiful.
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.