Koji-kin, koji mold, Aspergillus oryzae こうじ、麹
A fungus (mold, kabi) used in East Asia to saccharify rice, sweet potato, and barley in the making of alcoholic beverages such as sake and shochu, rice vinegar and also to ferment soybeans for making soy sauce and miso. Because of its importance in Japanese food and beverage production, koji-kin has been called a "national fungus", and the Japanese kitchen a "koji kitchen." Note that while koji-kin refers to the spores of the fungus, the word "koji" is used for the rice, barley etc. after the koji has grown on it. The spores are also called "tane-koji," "seed koji."
[Aspergillus oryzae growing on rice]
Aspergillus oryzae is a type of mold that is native only to the warm and humid Southeast and East Asian regions. Koji is made by sprinkling tane koji over steamed rice and cultivating the fungus under temperature conditions suitable for its growth. As the fungus propagates, enzymes break down the grains' starch and proteins into sugars and amino acids. The enzymes produced are thus also responsible for the taste of the final product.
Different koji are employed for different fermented foods, such as sake, soy sauce, miso, vinegar, shochu, mirin, natto, and pickles.
Seed koji are distributed by seed koji dealers, centuries old family companies which possess the special skills for producing and storing seed koji. That knowledge has been kept strictly secret by each dealer, and consequently only a dozen or so dealers exist in all of Japan.
Making koji is the beginning of the brewing process. It takes place in a koji-mura, a separate room in the brewery, which is warm with high humidity. Koji for high-quality sake is always made by hand. For standard sake it can also be made by machine.
Note: koji is sometimes translated as "malted rice," but as the process of making koji is completely different from malting (which relies on germination), this is misleading.
Photos from Wikimedia Commons
Japanese Food Dictionary