Bean Koji (Mame-koji)
Bean koji
Japanese: 豆麹（まめこうじ）
Koji made by growing koji mold directly on soybeans. Used for bean miso such as Hatcho miso.
Bean koji is made by shaping steamed soybeans into balls and cultivating koji mold on their surface. The soybeans serve as the substrate, without rice or barley.
Preserved mainly in the Tokai region centered on Aichi. Hatcho miso is made exclusively from bean koji. Protein degradation dominates over saccharification, yielding modest sweetness but deep umami and astringency. Many varieties age for two to three years. The longer koji-making process rewards patience with exceptional richness. Ideal for those who enjoy long-term aging.