Black koji is made using black koji mold, a type of starter koji from Kojiya Sanzaemon.

The biggest feature of black koji mold is its abundant production of citric acid. Depending on how it is cultivated, the sourness and color of the koji can easily reflect the maker's individuality.
The resulting koji has a strong sour taste, so the possibilities for using it in dishes that take advantage of the sourness are endless.

What you need

*1: Measure the temperature inside the storage facility and the rice.
*2: Measure the temperature of the hot water you put in the plastic bottle. This is not necessary if you have a water heater that can adjust the temperature.
*3: Since the tray containing the rice will be placed on top of the lying plastic bottle, we recommend using a square plastic bottle to make it more stable.
*4: A container large enough to hold the plastic bottle and tray. To prevent the koji mold from being starved of oxygen, make a hole about the size of a 10 yen coin on the side.

The appropriate temperature will vary depending on the season and room temperature, so please adjust as necessary.
It is recommended to keep the temperature lower in summer and higher in winter.

Disinfecting tools

Wash all equipment before use.
While using the product, be sure to disinfect your tools and hands with alcohol or other disinfectants as necessary to keep them clean.

Soaking the rice in water (soaking)

Soak the thoroughly washed rice in water and place it in the refrigerator to absorb the water for about 10 hours.

Drain the rice

Drain the rice in a colander for about 1-2 hours.
Making a depression in the center will help the rice drain evenly. Mixing the rice several times during the process will help the rice drain evenly.

Warming up the storage room (koji room)

The temperature inside the rice insulation cabinet is pre-heated so that it is 35-38°C when the rice starts to grow.
Here, we fill a plastic bottle with 50°C hot water, place it in a styrofoam box, and then lay a bath towel on top of it to heat the inside of the box.
If you place the styrofoam directly on the floor, the cold air from the floor will cool the inside of the cabinet, so be sure to place a mat underneath it or keep it off the floor.

Steaming rice (steamed rice)

Use a steamer and steam for about an hour until the rice is a little harder than regular cooked rice. If there is still a white core, keep an eye on it and increase the steaming time.
*Rice cooked in a rice cooker contains too much moisture to make koji, so be sure to steam it.

Take a small amount of rice and crush it until it becomes soft like mochi. *Rice is very hot right after steaming, so be careful not to get burned.

Cool the steamed rice (cooling the steamed rice)

Place a hand towel on a tray that has been sterilized with alcohol or other disinfectant, and then place the steamed rice on top. Mix the rice with a rice scoop, quickly lowering the temperature of the rice to just under 40°C, and spread the rice evenly across the entire tray.

Spreading the koji starter (seed cutting)

Sprinkle about 1/3 of the required amount of koji starter over the entire surface of the rice from a slightly elevated position and mix thoroughly with clean hands. Repeat with the remaining koji starter, cutting it into three batches in total.
*Please work quickly so that the rice temperature does not drop below 30°C.

Keeping deseeded rice warm

Before the rice cools below 30°C due to the outside air, place both ends of the hand towel over the rice to wrap it up. Place the wrapped rice and tray inside the styrofoam, wrap it in the bath towel that was used to keep it warm inside the refrigerator, and close the lid.

Temperature Check

After the warming process starts, check the temperature inside the cabinet from time to time and keep it at around 35°C. If the temperature inside the cabinet drops, refill the plastic bottle with 50°C hot water to warm up the inside of the cabinet.

When the rice temperature reaches about 37-40℃, whitish spots appear on the surface of the rice as the koji mold begins to grow. The rice feels a little hard and clumped to the touch.
Gently loosen any clumps of rice, and when the temperature of the rice reaches about 35°C, wrap it again in the hand towel or bath towel and return it to the Styrofoam.

Temperature Check

After cleaning, check the condition inside the storage unit to make sure the rice temperature does not exceed 42°C.
If the rice does not heat up or the temperature inside the refrigerator falls below 30°C, replace the hot water in the plastic bottle. Add 40°C hot water to prevent the temperature inside the refrigerator from rising too much.
If the rice temperature is over 42°C, remove the plastic bottle or bath towel to adjust it.

The surface of the rice will become even whiter due to the mycelium. After the first cleaning, the temperature of the koji will rise to nearly 40°C again, so loosen the rice clumps again and lower the temperature to 35-38°C. Once the rice is spread evenly on the tray, return it to the styrofoam as in the first cleaning.

Temperature Check

After cleaning, check the condition of the inside of the storage unit. Keep the temperature inside the storage unit at 30-35°C to prevent the rice temperature from exceeding 42°C.
If the temperature inside the cabinet falls below 30°C, replace the hot water in the plastic bottle. To prevent the temperature from rising too much, fill it with hot water at 40°C.
If the rice temperature is over 42°C, remove the plastic bottle or bath towel to adjust it.

As with the first and second maintenance, loosen the rice. When the rice turns whitish and it appears that the koji mold is growing, lower the rice temperature to 32-33℃ and keep that temperature until the end.
When you try it, you will notice a slight sourness. By lowering the temperature at this stage, more citric acid is produced and the koji turns black.

Temperature Check

Adjust the temperature of the rice so that it remains at 30-32°C. If the temperature of the rice is about to exceed 33°C, lower the temperature inside the cabinet as necessary.

You can see the black spores around the rice, and when you eat it you can clearly taste the sourness of the citric acid. The koji slabs can be easily broken down by hand, so please gently break them up before using.

If you are not going to use the koji right away, store it in the freezer and use it up within a month.