What is Koji and how to make it?

What is Koji?

Koji is a crucial ingredient produced by inoculating steamed grains, such as rice, barley, or beans, with koji mold spores (tane-koji). The koji mold is then carefully cultivated under optimal temperature conditions to facilitate its proliferation.

Koji is often referred to as a "treasure trove of enzymes" due to the abundance and diversity of enzymes it contains. These enzymes play a vital role in breaking down the components of the grain.

Furthermore, the growth of the koji mold imbues the koji itself with an array of beneficial nutrients, including sugars, amino acids, vitamins, and minerals, making it a highly nutritious food.

Koji serves as the foundation for the production of many traditional Japanese fermented foods and beverages, such as sake, shochu, mirin, miso, soy sauce, vinegar, and amazake.

Rice Koji recipe
This recipe will make approximately 1440g of rice koji
Ingredients:
1200g, uncooked white rice
0.8 ~ 24g, Koji starter culture (tane-koji or koji-kin)*
*The amount will vary according to the freshness of the koji starter culture.

Steps:
Koji making is a 3 day process. Here is the breakdown of the 3 day process. 

Preparation the Day Before:

1. Wash the rice.
2. Soak the washed rice in water.

Day 1:
3. Drain the water from the washed rice well.
4. Steam the rice.
5. Sprinkle the seed koji over the steamed rice (seeding).
6. Wrap the rice in a cloth and keep it warm (pulling in).

Day 2:
7. First koji maintenance (turning over).
8. Transfer the cloth-wrapped rice to the koji incubation box (piling up).
9. Second koji maintenance (middle work).
10. Third koji maintenance (finishing work).

Day 3:
11. The koji is complete (taking out the koji).

Let's get started! 

1. Wash the rice
   Wash the rice thoroughly.
   Thoroughly remove the bran and dirt attached to the rice.
   Wash 2-3 times until the water runs clear, and discard the water.
2. Soak the washed rice in water
   Add clean water until the rice is submerged.
   The soaking time depends on the temperature - in spring and fall, soak for 6-12 hours, in summer 3-5 hours, and in winter 15-20 hours.
   Cover with a wrap to prevent contamination.
   
   In hot weather, you can also place the rice in the refrigerator's vegetable compartment and let it soak for around 20 hours.

3. Drain the water from the washed rice well
   Lift the rice in a colander and let it sit for 2-4 hours to thoroughly drain the water.
   If there is still water remaining, it will cause uneven steaming of the rice when you steam it later,
   which will result in the koji not turning out well.
   Take the time to drain the water completely.
   Changing the angle of the colander or gently mixing the rice can help the water drain evenly across the entire batch.
   
   If you vigorously shake the colander to drain the water, the rice can become broken or turn into a powdery state, which will not result in good koji.
   Instead of shaking the colander, leave it still to allow the water to drain.

4. Steam the rice (about 40 minutes)
   Place the rice wrapped in a steaming cloth into a steamer or steaming basket and steam it.
   For the first 5-10 minutes, steam the rice without the lid on.
   Confirm that steam is coming up.
   
   When steam starts penetrating the surface of the rice, and a part of the surface becomes translucent, put the lid on the steamer.
   
   The standard steaming time is 40 minutes after the lid is on.
   The key is to steam the rice with strong steam.
   The steaming condition greatly affects the final quality of the koji.
   
   When using a steamer basket to steam the rice:
   It's best to leave the bottom tier of the steamer basket empty.
   Keeping a gap between the water and the steamed rice prevents the bottom of the steamed rice from becoming soggy.
   
   If steaming in multiple tiers, to prevent uneven steaming,
   switch the tiers halfway through the total steaming time (after 20 minutes).
   
   When using a steamer basket to steam the rice:
   It's best to leave the bottom tier of the steamer basket empty.
   Keeping a gap between the water and the steamed rice prevents the bottom of the steamed rice from becoming soggy.
   
   If steaming in multiple tiers, to prevent uneven steaming,
   switch the tiers halfway through the total steaming time (after 20 minutes).
   
   Using a steamer basket is recommended for steaming the rice for koji.
   One tier of an 18cm steamer basket can steam 2 cups of rice.
   A polyester steaming cloth works well to prevent the rice from sticking.

The rice koji's nemesis - the Natto bacteria
The ideal moisture content for koji mold is around 36-38%, which has a similar texture to mochi.
This moisture level is not very suitable for the rice koji's nemesis, the Natto bacteria. The Natto bacteria requires more moisture, temperature, and humidity, so it has a hard time taking hold in the rice koji environment.

5. Sprinkling koji starter over the steamed rice (Tanekit)
   After the rice is fully steamed, spread a clean tray or cloth.
   In this case, a fabric made of Pairene material (shikinuno) is used as the spreading surface.
   (The steaming cloth should not be used for this stage, as it has absorbed too much moisture.)

Spread the steamed rice on the tray. Be careful not to get burned.
Use a rice paddle to gently spread and mix the rice to allow the moisture and heat to dissipate from the surface.

The freshly steamed rice is still very hot, and if you immediately add the koji starter, the high temperature can kill the koji mold.
Use a thermometer to monitor the temperature, and first bring the rice down to around 45°C (113°F).

Once the temperature reaches around 45°C, you can begin the koji seeding process. A tea strainer works well for evenly distributing the koji starter.
Thorough and even distribution of the koji starter is key, as it greatly impacts the quality of the final koji.

Here is a convenient storage container for preserving and dispensing the koji starter.
Gently break up any rice clumps to ensure the koji spores reach all the rice evenly. Inadequate mixing can lead to uneven mold growth.

In the winter, the rice cools down quickly, so extra care is needed. You may want to work in a warmer environment, such as a room with air conditioning, to maintain the ideal temperature.

Repeatedly sprinkling the koji starter and gently mixing the rice is the technique for proper inoculation.

About the amount of koji starter to use:
When professionals make koji, they can use 20g of koji starter to make koji from up to 15kg of rice.

However, when making a smaller batch of koji (500g to 2kg) at home, it's better to use 2-10 times the standard amount of koji starter. This is because some of the koji spores get absorbed into the cloth used to wrap the steamed rice, making it harder for the koji to proliferate.

Therefore, in this recipe, about 6 times the standard amount of koji starter is used, and the quantity range is provided to account for this.

There is no problem with using a larger amount of koji starter - the final result will not be adversely affected.

6. Maintaining temperature (curing)
   To monitor the temperature of the rice, insert a thermometer into the rice.
   Tightly wrap the rice in piles of cloth, making the rice as small and round as possible.
   Once the thermometer shows the desired temperature, tie the opening of the cloth wrapping with a rubber band or similar.

After the seed koji (tane-koji) is finished, quickly gather the rice into one mass and wrap it in a clean cloth.

The recommended cloths are a piled cloth (hirasen) and a cotton muslin cloth (sarashi-fu).
(The hirasen has a fine mesh that prevents the koji spores from escaping, and allows excess moisture to escape.)

Then, wrap the hirasen-wrapped rice in the cotton muslin cloth.
The muslin cloth around the hirasen will absorb the appropriate amount of moisture released from the rice.
Using just the cotton cloth alone can cause the rice to dry out too quickly during the curing process, resulting in improper maturation.

Place the rice wrapped in the hirasen and muslin cloth into an incubation chamber and maintain the temperature for 18-20 hours.

The temperature of the incubating rice is called the "hinon" (ideal temperature).
The optimal hinon for koji growth is around 35-40°C. We should try to maintain this temperature.

The speed of temperature changes can vary depending on the season, room temperature, and amount of koji starter used.

Set the incubation chamber temperature between 32-42°C, and constantly monitor the hinon to keep it at 35-40°C.

This temperature profile chart is just a reference example. The temperature profile can vary depending on the koji making method and the intended use of the koji.

The ideal temperature profile will greatly depend on the rice, koji starter amount, environment (temperature and humidity), and steaming conditions of the rice. Adjust the timing of each step based on monitoring the koji temperature, aroma, and fermentation progress.

Depending on the temperature, steps like turning the koji or mid-stage work may be skipped.

 

7. First Koji Maintenance: "Tori-gawashi" (Turning)
   
   After about 18-20 hours of maintaining the temperature during the curing (hiki-komi) process, the germinated koji mold becomes active, causing the temperature of the steamed rice to rise and a sweet aroma of the koji to start wafting in the incubation chamber.

When the temperature of the steamed rice reaches around 38-40°C, untie the wrapped cloth and closely observe the appearance, aroma, and moisture content (this is called the "jōbō" state) of the steamed rice.

If the fermentation is progressing well, the rice will appear whitish due to the growth of koji mold filaments, and a distinctive sweet aroma of the koji will be present.

In this state, loosen the mass of the temperature-maintained rice to make the temperature uniform. This is called "tori-gawashi" (turning). Performing tori-gawashi allows oxygen to reach all parts of the rice.

Quickly separate the rice grains one by one. The key is to mix quickly to prevent the temperature from dropping. Once thoroughly mixed, wrap the rice back up tightly in the cloth. Don't forget to wipe off any condensation on the ceiling.

By performing koji maintenance, air can circulate throughout the koji, promoting the proliferation of the mold.

If the rice temperature is too high, quickly mix the rice to lower the temperature.

Be cautious of rapid temperature spikes
Depending on the condition of the koji mold, the temperature can sometimes rise above 40°C within a few hours of starting the temperature maintenance. In that case, perform the tori-gawashi at that point.
If the temperature exceeds 45°C, some of the koji mold may die off. Rapid temperature spikes require close attention.

Conversely, if more than 20 hours have passed since starting the temperature maintenance and the temperature has not risen above 40°C, wait for it to exceed 40°C. You can also try increasing the temperature setting of the incubation chamber to raise the rice temperature.

8. Transferring the rice wrapped in cloth to the koji tray (mori)
   
   About 3 hours after the tori-gawashi (turning), when the temperature has exceeded 40°C, transfer the rice from the cloth to the koji tray. The process of dividing the koji onto the koji tray is called "mori".
   
   Untie the cloth and carefully transfer the koji inside to the koji tray. Spread it out evenly and place it in the incubation chamber.

The timing for the "mori" (transferring to the koji tray) cannot be determined by elapsed time alone.

It is necessary to visually confirm the growth of the koji mold, and judge the timing based on the temperature as well.

In the beginning, use the rice temperature exceeding 40°C as a guide for the timing of the mori.

When the mori is performed, the moisture in the rice starts to evaporate, and the drying speed of the rice increases.

Balancing the rice drying speed and the proliferation rate of the koji mold is the key to making good koji.

Please refer to the reference for the ideal temperature profile.

If the temperature does not continue to rise, do not perform the mori, and proceed to the next step while the rice is still wrapped in the cloth.

9. Second koji maintenance "Naka-shigoto" (Middle work)
   After 5-6 hours have passed since the "mori" (transferring to the koji tray), the koji mold will proliferate further, and the temperature of the steamed rice will rise again to around 40 degrees.

At this timing, quickly break up the clumps of steamed rice with your hands and stir the entire mixture.
In this process, let's perform the maintenance quickly so that the temperature of the rice does not drop too much.
The target is to bring the temperature down to around 38 degrees.

After that, it is a good idea to continue the koji maintenance whenever the koji temperature exceeds 40 degrees. If the temperature rises too high, lower the temperature of the incubation chamber or even turn off the power if necessary.

Depending on the state of the koji mold, the temperature may not rise above 40°C. In that case, do not perform the middle work, and wait until the temperature exceeds 40°C.

10. Third koji maintenance "Shimai-shigoto" (Finishing work)
    
    A few hours after the middle work, when the koji temperature has reached 40 degrees or more, perform the koji maintenance again.

In this process as well, let's perform the maintenance quickly so that the rice temperature does not drop.
Aim to maintain the temperature around 38 degrees during the maintenance.

If the fermentation has progressed, you may sense a faint sweet aroma like chestnuts from the rice.
Observe the condition closely and incorporate it into your future koji making.

In the latter half, maintain the temperature above 40°C for 6 hours!
Out of the approximately 12 hours until the unloading of the koji, maintain the temperature above 40°C for 6 hours to ensure the glucoamylase, which contributes to sweetness, works effectively. This will result in a koji with a strong sweetness.

Normally, the maintenance is performed about 2-3 times.
Depending on the fermentation progress, a 4th maintenance may be necessary.

Depending on the state of the koji mold, the temperature may not rise above 40°C. In that case, do not perform the finishing work, and wait until the temperature exceeds 40°C.

11. Completion of the koji "Dekoji" (Unloading the koji)
    
    A few hours after the finishing work (approximately 42-50 hours after the initial seeding), the koji will be complete. Let's check the rice grains for "Hassei" (penetration).
    
    "Hassei" refers to the state where the koji mold hyphae have rooted into the steamed rice, appearing as if they have eaten into the grains.
    
    If the "Hassei" is good and a chestnut-like aroma is present, the koji is ready.

Furthermore, the act of taking out the finished koji from the room or fermenter is called "Dekoji" (unloading the koji).

The time it takes to reach the unloading stage can vary depending on the temperature and humidity, as they affect the proliferation of the koji mold.

The timing of the unloading is determined by observing the appearance and aroma.

The ideal koji is one where the mycelium has spread, and the rice grains have stuck together forming a flat, board-like structure that can be easily broken apart by hand.

The surface of the rice grains should be well-covered with the white koji mold, and the interior of the grains should also be penetrated by the mold, which is the desirable outcome.

Let's check the proliferation of the koji mold.
The proliferation of the koji mold is referred to as "Hassei-mawari" (penetration coverage).
If there are no issues with the penetration coverage, the koji is complete.

The ideal koji is one where the mycelium has spread, and the rice grains have stuck together forming a flat, board-like structure that can be easily broken apart by hand.

Make amazake to check the completion of the koji
Let's go ahead and make amazake (sweet rice drink) using the finished koji. The recipe is here.
If the amazake turns out sweet and delicious, it means the koji has been successfully produced.

Let's check the proliferation of the koji mold.
The proliferation of the koji mold is referred to as "Hassei-mawari" (penetration coverage).
If there are no issues with the penetration coverage, the koji is complete.

The ideal koji is one where the mycelium has spread, and the rice grains have stuck together forming a flat, board-like structure that can be easily broken apart by hand.

Make amazake to check the completion of the koji
Let's go ahead and make amazake (sweet rice drink) using the finished koji.
If the amazake turns out sweet and delicious, it means the koji has been successfully produced.