How to Make Koji 7: Harvesting and the Next

Marika Groen

December 24, 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.

Now you have spent some days with your koji. How has it been? If this was your first time, I’m sure you reached out to some of your sleeping senses. In this chapter, I’ll talk about how to harvest koji, and what to do next depending on your purposes. If you are making koji from short grain table rice, your koji will most likely be ready in around 50 hours. Before you open the wrap, your nose should detect a gentle aroma of koji mold. The grains are covered in white, may have some hair grown on the surface, and taste slightly sweet when you bite them.

Another thing you can do is to check “Haze”. This is a step to check the penetration of mycelia. Pick up koji grains from a different part of the mat (top, bottom, sides) and cut them into half to see how the inner part looks like.

Two pieces of rice pinched with two fingers

Cutting the grain into half and checking the penetration

If the mycelia have made its way to the inside of koji rice, the centre part should look milky white. If transparent, that part is not eaten by koji. This may be due to the lack of moisture or unbalanced control of humidity and moisture. Remember to keep the muro humidity only to support the moisture inside of the grains. Depending on where you picked the piece, you may see a different haze pattern. Maybe the grain from the edge of the koji mat has a more transparent colour, while the one from the bottom of the mat has completely milky white on the surface. Haze check can tell you which part of your koji tray had the best condition of the mold to grow.
Six pieces of rice lined up on the edge of the wooden barrel full of rice

Comparing the result from a different part of koji bundle

You may call me an addict, but I cannot help grabbing my microscope and observing them even deeper…
Koji by looking at the microscope
If you pick the hairy one, you will have hours of enjoyment browsing through a beautiful field of koji blossom…
Koji blossom by looking with the microscope

Koji flower field – aka Conidiophores of Aspergillus oryzae

After you confirmed Haze and maybe enjoyed the microscopic fungus, you can now lift a small block and see how easily it breaks up on your hand. The more hairs are binding the grains, the harder the block is. The less hair they grow on the surface, the looser the bind is. If you worked out the optimal humidity during incubation, the mycelia grew more into the inner side of each grain, resulting in a loose mat. If you happen to encourage the humidity in your muro (meaning more moisture in the air than the grain), you may get an almost-tempeh-like koji mat. However, the hairy koji isn’t a mistake either. It would still carry some enzymes and can be used for basic applications like miso, amazake, shio-koji etc..

After making sure that your koji is ripe, you can remove all of them from a container. If you plan to transport it soon, try to break it up in small blocks and let them lose the heat. This is better than breaking everything up. If you break up all grains, there would be less space between them, which may cause “after-heat” during the transport. Breaking in rough blocks ensures enough space between blocks, discouraging the koji mold to grow back again.

Koji broken up in pieces

Breaking up in blocks to cool down

When transporting fresh koji, it is advisable to use a paper bag. The paper in the bag absorbs excess moisture from the koji, preventing condensation.
Koji in a white paper bag
If you plan to use it straight away, break up all grains and use it for amazake, short-term miso, shio-koji etc. Fresh koji are powerful, and their enzymes are hungry. The sooner you use it, the more “clean” and strong the koji is. If you know you want to use it for miso making, you can also mix it with salt at this point. Adding salt would also help contamination, making the shelf-life longer (or in a fridge if your room is warm).

Typically in winter, you may keep them at room temperature for a couple of days, but the koji will keep on working as long as the grain contains some moisture. It is advisable to take this step called “Karashi”. “Karashi” is a dehydration in Japanese, and it literally means dehydrating koji grains before storage. Usually we break up all the small lumps, and spread them on a clean and dry cloth until the koji loses the power to heat up. In the meantime, the moisture would also evaporate and koji gets semi-dry and easy to store. I normally spread it out to a thickness of about 2 cm and then draw swirling circles around it with my fingers to increase the surface area.

The swirl at the time of Karashi called Hanamichi

The swirl at the time of Karashi is also called Hanamichi

However, you may not want to leave them in the open air too long. Again, any kind of microbes are present in your environment. If you want to keep your koji pure, collect them as soon as they are cooled down and semi-dehydrated. Once stored in a bag, you can either leave it at room temperature for a couple days in a cold and dry place, though the best is to store it in a fridge for up to some weeks, or in a fridge for months. Be mindful that the longer you leave, the quality becomes less good. You may run a risk of ice growing on your koji too.

This is the end of “How to make Koji” series. I hope you enjoyed it. Remember that this is the most standard process of white rice koji making. If you opt for different substrates such as buckwheat, soybeans, barley etc, there is another story though the principle is the same. I hope you have some impression to start with, and put the essence from this series to practical use.

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.