Step 1 - Wash the rice
Wash the rice properly, so the dirt and the bran that remains on the rice are washed off. Rinse it about 2-3 times until clean.
Step 2 - Soak the clean rice in water
Soak the rice in clean water. The period of time to soak the rice depends on the temperature; but in spring and fall it's 6 to 12 hours, while in summer it's 3-5 hours, and in winter it's 15-20 hours. When the temperature is too high such as in the summer, it's ok to soak and put it around 20 hours in the refrigerator.
Cover the bowl with plastic wrap so germs and bacteria don't enter the bowl.
Step 3 - Drain water from the rice
Drain the rice with strainer, then leave it for 2-4 hours so the water is drained properly from the rice. If there's water left, there might be unevenness in the rice grains during the steaming process. If the rice is cooked unevenly, the koji won't be good. Please take time to drain the water from the rice completely.
If you change the angle of the strainer or stir the rice gently when draining the rice, you can drain it completely. However, if you shake the strainer intensely to drain the water, the rice wil crack and become powdery. If there's cracked or powdery rice grains, there might be unevenness when you cook it. Leave the rice on the strainer as it is so the water is drained properly.
Step 4 - Steam the rice for approximately 40 minutes
Wrap the rice with a steamer cloth, then steam it with steaming basket. For the first 5-10 minutes, steam it without putting on the cover/lid. Make sure that the steam is going up. When steam penetrates from the surface of rice and the grains start to become transparent, cover the steaming basket.
Steam it for 40 minutes (recommended) after you put on the lid. It's important to steam the rice with strong steam. The steaming process here affects the finished result of koji greatly.
When using seiro/bamboo steamer
When steaming rice with a seiro/bamboo steamer, it's good to leave the bottom row empty. By keeping distance between hot water and the rice, it prevents the bottom surface of steamed rice from becoming messy. When steaming in bamboo steamer that has two or more rows, it is better to change the rows in midway of steaming process to prevent unevenness when steaming. When half of the recommended steaming time has passed (recommended steaming time is 40 minutes, so half of it will be 20 minutes), please switch the place of first and second row (if your bamboo steamer has two or more rows).
The condition of good steamed rice is that if you take one piece of rice, and pinch then crush it, its texture is just springy like a mochi. If the rice is still hard even after steaming, or if the rice doesn't seem cooked enough, you can steam the rice again.
When you steam rice for koji making, we recommend you to use a bamboo steamer. You can steam about 300g of rice with one row of 18cm-sized (diameter) bamboo steamer. Also, it will be more convenient if you use tetron cloth (tetron is one kind of polyester) when steaming, beacuse the rice won't cling to the cloth.
Step 5 - Sprinkle the koji starter on the steamed rice (Seeding)
When the rice is cooked, spread a clean dish cloth on the koji lid (wooden tray for koji making). Spread the steamed rice on it evenly. Here, we're using a cloth made from polypropylene (do not use the cloth that was just used for steaming for the next other steps, because it's too damp/wet.）
Spread the rice on the cloth-layered tray with rice paddle, so the heat flow away. Measure the temperature of the rice with a thermometer and spread the koji starter on it when the rice temperature reaches around 45℃. This steep is called "seeding". When you sprinke the koji starter (seeding), it's convenient to use tea strainer so the koji starter won't clump and spread evenly.
If you sprinkle the koji starter when the rice is still too hot, it will kill the microoganism (Aspergillus oryzae) in koji starter. But, do not leave the rice until it's completely cooled down. Fermentation process won't go well on cold rice. After estimating the timing when the temperature of rice is 45℃, quickly mix the koji starter on rice.
Untangle the clumps of rice and mix thoroughly so that the koji starter spreads all over the rice. If you didn't mix it completely, other miscellaneous germs and bacterias may grow.
Please pay more attention in winter, as the temperature drops quickly. In some cases it's better to make rice koji in a higher room temperature (you can adjust the air conditioner). If you sprinkle the koji starter properly and carefully mix the whole rice, the seeding step is successfully done.
When you finish the seeding, quickly gather the rice together and wrap it with a clean cloth. When wrapping it with cloth, wrap it roundedly as small as possible by applying force.
The amount of koji starter used
When it's made by the professional, 20g of koji starter can be used to 15kg of rice at max to make rice koji. But in the case of making small amount of rice koji (500g-2kg) at home, it's easier to succeed if you use 2 or more times of the specified amount of koji starter. This is because the koji starter will stick to and absorbed on the cloth that wraps the steamed rice, and thus the Aspergillus oryzae is difficult to reproduce accordingly. Therefore, in this recipe, the koji starter used is about six times as much as the specified amount. We attached the table of the amount of the koji starter used below. But, if you put too much koji starter on the rice, it won't affect the result of the rice koji. So don't worry.
Step 6 - Keep the rice warm
Measure the temperature of the rice with a temperature. Wrap with a polypropylene cloth and press it powerfully so that rice will be rounded as small as possible. After that, tie the cloth with rubber bands.
We recommend using polypropylene cloth and bleached cotton cloth to wrap the rice（The fabric holes on polypropylene cloth is small that prevents the spillage of koji starter and let extra water flow outside.)
Wrap the cotton cloth over the polypropylene cloth from the outside. The cotton cloth around the polypropylene cloth will absorb the dampness coming out from the rice. If you're only using cotton cloth, the water from the steamed rice will be absorbed too much. In that case, the rice koji will not be finished nicely.
Put the rice that is wrapped by the polypropylene and cotton cloth in to the koji fermenter (fermentation device for koji), and keep it warm for 18-20 hours. The optimal temperature for koji starter to propagate is at 35-40℃. Keep the rice warm around this temperature.
Depending on the season, temperature of the room, and the amount of koji starter being used, the speed of rice temperature change may differ. Please set the temperature of fermentation device between 32-42℃, and keep checking the temperature of the rice as needed so it's between 35-40℃.
The ideal rate of humidity is from 70% until 80%.
Low humidity can dry out the rice during the warming process. Therefore, the koji starter won't propagate properly. The ideal rate of humidity for the koji starter to propagate is from 70% until 80%. If it's humid enough that on the front part of the koji fermenter (fermentation device) there are a lot of water drops, then it's the best environment for breeding the koji starter. However, do not let the dew condensation dripping off and wet the koji. Make sure to wipe the dew condensation that dripped off from the device's ceiling frequently.
Click here to see our Recommended Product for Rice Koji
Important point: please measure the balance of the drying speed of rice with the breeding speed of the koji starter.
This chart is the reference for the temperature. The temperature change of the rice may vary as the difference in the method of making rice koji. Also, standard temperature change may differ depending on the koji being used. The rice's temperature change will vary depending on the amount of rice and koji starter being used, environment (temperature and humidity), steaming condition of rice, etc. Please do the timing of each changes flexibly while looking at the temperature, aroma, and fermentation condition of koji.
Depending on the rice temperature, you may not need to do the first, second, and third maintenance (7th, 9th, and 10th step).
When using Yogurtia
Put the rice that's wrapped up with steamer cloth in the Yogurtia's container. The condensation that comes out when the rice is being fermented will pile up in the bottom part of the device, so please put a kitchen paper with a hole on it on the bottom of the Yogurtia's container.
Furthermore, a large amount of oxygen is required for the fermentation of koji starter. Please do not close the container tightly. Do not use the inner lid of the yogurtia, and leave the outer lid a bit opened. If you use a cooler box, please try using hot water bottles, etc. for temperature adjustment.
Step 7 - First maintenance of koji
When it's about 18-20 hours from when you started warming the rice koji, due the activity of koji starter, the temperature of the steamed rice will rise and there will be the sweet aroma of koji from the fermentation device.
When the temperature of the steamed rice rises to about 38 to 40℃, please undo the wrapping cloth and observe the appearance and aroma of the steamed rice and its humidity. If the fermentation progress is going well, the rice grains surface will become white-ish with the hyphae of koji starter (Aspergillus oryzae), and the unique sweet aroma of koji will come out.
In this state, please untangle the clumps of rice to make the temperature at the same level on all the grains. This is called the first maintenance. With this, you are also spreading oxygen to every corner of rice.
Please untangle rice one by one quickly. It's important to mix the rice quickly so that the temperature of rice does not go down too much. Once properly mixed, wrap firmly with the cloth as before. Do not forget to wipe the dew condensation on the ceiling of the device/box.
Be careful of sudden rise in the temperature of rice.
Depending on the situation of koji starter, the temperature may exceed 40℃ in several hours after the start of warming. In that case, please do the first maintenance at that point. When the rice's temperature exceeds 45℃, some koji starter microorgamism will die. Please pay attention to the rapid rise in the tempertaure of rice.
On the opposite, there are cases that even more than 20 hours have passed, but the rice temperature still haven't risen above 40℃. In that case, please wait for the temperature to exceed 40℃. If this happens, try increasing the temperature setting of the koji fermenter device/box and to raise the rice temperature.
By doing the maintenance, air will spreads throughout koji, and accelerate the breeding of koji starter. If the temperature of rice is too high, quickly mix rice and lower the temperature.
Step 8 - Move the wrapped rice to koji lid
After about 3 hours passed from the first maintenance, when the temperature of rice exceeds 40℃, move the rice from the cloth to the koji lid, a wooden tray made specially for koji making. Untie the cloth, then carefully put the rice (which now has become half-done rice koji, so we will call this rice koji from now on) into the koji lid. Spread it flat and put it back to the fermenter device/box.
At this moment, again, quickly untangle the clumps of rice and stir the whole of rice koji. Like before, do it quickly so that the temperature of rice doesn't go down as well.
You can not decide the timing of this step by only the timing the duration of fermentation. It's necessary to check with the naked eye around the fracture of the koji seed (Aspergillus oryzae) and decide it from the temperature of rice koji. As a guideline for the timing of this step, the rice koji's temperature must be above 40℃.
When you moved the rice koji from to cloth the koji lid, the moisture of the rice will begin to evaporate, and the drying speed of the rice will increase. It is important to balance the drying speed of rice with the breeding speed of the koji starter (Aspergillus oryzae).
For the ideal temperature change of rice koji, please refer to this chart.
If the temperature doesn't rise for a long time, don't move it to the box, please go to the next step as the rice koji is wrapped in cloth.
Step 9 - Second maintenance of koji
When 5-6 hours have passed from when you move the rice koji to the koji lid (step 8), the koji starter will further breed and the temperature of the rice koji will rise again to 40℃.
At this moment, once more, quickly untangle the clumps of rice and stir the rice koji. This time also, keep in mind to do the maintenance quickly so that the temperature of rice doesn't go down rapidly. The standard is, you can stir the rice koji until it's temperature goes down to around 38℃.
After that, it will be good to keep doing the maintenance of rice koji at any time when the temperature rises above 40℃.
If the temperature of rice koji rises too much, please lower the temperature of the fermenter device or turn off the fermenter. Depending on the situation of the koji starter, the temperature of rice may not rise above 40℃. In that case, do not do the second maintenance of koji, and just wait for the temperature to exceed 40℃.
Step 10 - Second maintenance of koji
After several hours from the second maintenance, when the temperature of the koji rico rises above 40℃, please do maintenance again.
If the fermentation is progressing, a faint sweet aroma like a chestnut will come out from the rice koji.
Keep the temperature above 40℃ for 6 hours in the second half of the progress!
About 12 hours from here until the rice koji is finished, keep the rice koji's temperature above 40℃ for 6 hours, to let sweet compound of glucoamylase works. Then, the strong sweet-tasted rice koji will be completed.
Usually, it needs approximately 45 hours from the start of keeping the rice warm until the completion of koji, and about 2-3 times of maintenance will be needed. Depending on the fermentation condition, it might be necessary to do the fourth maintenance as well.
Also, depending on the situation of koji starter, the rice's temperature may not rise above 40℃. In that case, don't do the third maintenance, and just wait for the temperature to exceed 40℃.
Step 11 - The completion of koji
Koji will be completed several hours after the process work, approximately 42 to 50 hours from sprinkling the koji starter on steamed rice (seeding process). When the whole process is done, confirm the appearance of the rice koji. Observe if the hyphae of Aspergillus Oryzae are rooted in the steamed rice and the hyphae look like to have attached itself to the rice grains. If the state of the rice koji is like that, and the scent like chestnut has come out, then the rice koji is completed.
If the koji is finished, you can take it out from the fermenter device.
Because the speed of the reproduction of Aspergillus oryzae (koji starter) differs depending on temperature and humidity, the time it takes for koji to be completed will vary. Please look and decide the time when the koji is finished from its appearance and aroma.
The signs of a completed koji: the hyphae of koji starter stretches out, the rice sticks to each other and becomes a plate shape, it is also easy to disentangle by hand. The ideal rice koji is when the Aspergillus Oryzae propagated well on the surface of rice grains as well as rooted nicely into the rice grains, and the whole body of rice is white.
Try Out The Finished Koji by Making Amazake (Sweet Sake)
Let's immediately make sweet sake from your freshly made koji. If a rice koji can be made into sweet and delicious sweet sake, it can be said that it is a good rice koji. You can make a sweet sake to check whether your rice koji is a good one or not.
How To Make Amazake (Sweet Sake)
How to make amazake (sweet sake) with rice and koji, or only with koji (Aspergillus Oryzae). This amazake recipe is made by Kawashimaya. There are only 2-3 easy steps before you can actually make your own homemade amazake. Let's try to make amazake at home!
Read How To Make Amazake Here.
Click here to see our Recommended Product for Rice Koji