The Japanese fermentation starter used to make sake, miso, and soy sauce is koji.
Koji is most commonly a culture of the filamentous mold Aspergillus oryzae grown on steamed rice, barley, or soybeans. Its enzymes break starches into sugars and proteins into amino acids, supplying the raw materials that fermentation organisms need.
In sake, koji converts rice starch into fermentable sugar while yeast produces alcohol—a simultaneous process unlike ordinary beer brewing. For miso, koji is combined with cooked soybeans and salt; for soy sauce, it is cultivated on soybeans and wheat before being mixed into a salty moromi brine.
Koji is sometimes confused with the finished fermented food. Mirin is a sweet rice wine, natto is fermented soybeans made primarily with Bacillus subtilis, and tempeh is an Indonesian soybean cake usually made with Rhizopus mold. The word is also written kōji, with the macron marking the long vowel.