How koji is made

Koji is steamed grain colonized by a mold that turns starch into sugar, the first step in sake, miso and soy sauce.

stations: 5
things upstream: 3
raw sources: 3
step from the ground: 1

The short version

Koji is grain grown over with the mold Aspergillus oryzae. Makers steam rice, soybeans or wheat, seed it with the mold, and keep it warm so the fungus spreads through it. The mold makes enzymes that break starch down into sugar and proteins into amino acids, which feed later fermentation.

Steam the grain

Rice is steamed and loosened so each grain is soft but not mushy. Soybeans and wheat are also used for different foods.

Goes in
Rice Soybeans Wheat
Sprinkle on the mold spores

Once the grain has cooled, spores of Aspergillus oryzae are dusted over it and mixed in.
Keep it warm while it grows

The inoculated grain rests for two to three days in a warm, humid room as the mold grows through each kernel. Solid-state cultivation of this kind is thought to have started in China.

two to three days at 30 °C
Turn and cool the batch

Workers turn the mass so it does not overheat, since the growing mold gives off heat. Even temperature gives an even growth.
Break it up and use it

Finished koji is broken apart and mixed into steamed rice for sake, or into soybeans for soy sauce and miso. The mold's enzymes saccharify the starch.

End of the lineKoji Next it goes into soy sauce, sake.

Worth repeating

Aspergillus oryzae is used in East Asia to saccharify rice, sweet potato and barley for sake and shochu.
For soy sauce and miso, a related mold called Aspergillus sojae is sometimes used.