Preparation info

  • Difficulty


  • Makes /

    1 kg

Appears in


By Peter Gilmore

Published 2014

  • About

Koji is the starting point for Japanese staples such as miso, sake and shoyu. Aspergillus oryzae mould spores are grown on rice. Through fermentation, they convert hard-to-digest proteins, starches and fats into easily absorbed amino acids and simple sugars.



  • 1 g (1/32 oz) dehydrated koji spores (see glossary)
  • 4 g ( oz) rice flour
  • 1 kg (2 lb 4 oz) Japanese koshihikari (sushi) rice


Wash the rice in cold water several times to remove as much starch as possible.

Soak the rice in a couple of litres of cold water for 8 hours or overnight. Pour off all of the water and rewash the rice several times in changes of cold water to remove as much milky, starchy water as possible. This is an important step to ensure the rice does not stick together when steaming.

Wearing a face mask to avoid breathing in the spores, carefully mix the koji spores and rice flour. This process helps to spread the spores more evenly through the rice.

Line a steamer tray or basket with a double layer of muslin (cheesecloth). Put the washed and drained rice in the basket. Cover with muslin and steam on high for 1 hour. Once the rice has steamed, remove from steamer and open up the muslin. Allow the rice to cool slightly then break up the rice with your hands to separate all the grains. Using a digital thermometer, check the temperature of the rice. Once it cools to 35°C (95°F), carefully sieve the koji spore and flour mixture over the rice. Mix thoroughly through the rice.

Take a clean double layer of muslin, wet it and wring it out. Place the koji rice in the centre of the cloth in a wide mound about 5 cm (2 inches) thick all over. Wrap the rice in the muslin and then wrap with two clean, dry tea towels (dish towels).

Now you can put the rice in a small styrofoam container with a lid or in an incubator set at 35°C (95°F). If using the styrofoam container, place it in a warm area of the kitchen with an outside temperature of 22–26°C (72–79°F). In colder climates you may need to place a 35°C water bottle inside the styrofoam container and replace it every 6 hours. The object, whether using the incubator or the styrofoam container, is to maintain the internal temperature of the rice, for the next 24 hours, at 32–38°C (90–100°F). This temperature range is critical for the mould to grow properly: too hot or too cold could kill the koji spores.

The koji rice will eventually start to generate its own heat. I would suggest checking the internal temperature of the rice with a temperature probe, every 4 hours. Monitor the rice closely, once it starts to generate its own heat you may need to decrease the temperature on the incubator or remove the water bottle or lid of the styrofoam box.

After 24 hours, unwrap the rice. You should see the beginning of the white mould forming around the grains of the rice. The koji rice should have a sweet bakery aroma. If the mould is any other colour than white discard the batch as you may have an inedible type of mould growing or it may indicate the koji rice has overheated and begun to spore. Start again from scratch.

If the mould is white and the smell is sweet, continue the incubation period for a further 24 hours; however, you should spread the rice into a thinner 2 cm (¾ inch) layer and rewrap in the muslin. This time do not wrap in the tea towels as the rice will now continue to generate its own heat. Return the rice to the incubator or styrofoam container and maintain the temperature at 32–38°C. After the second 24-hour period the koji rice should be ready. This time the rice should be coated in fluffy white mould and smell very sweet. At this point you can put the koji rice into airtight containers, cover and refrigerate for up to 4 days.

Alternatively, you can freeze the koji rice and store it for up to 3 months.