Japan (nationwide; Nada, Fushimi, and Niigata sake brewing centres as professional koji knowledge centres)
Koji (麹, Aspergillus oryzae) — the filamentous fungus grown on rice, barley, or soybeans that underpins virtually all of Japan's fermentation traditions — is experiencing a renaissance of home production as understanding of its extraordinary enzymatic capabilities becomes more widespread. Rice koji (kome-koji) is made by steaming short-grain rice, inoculating with dried tanekoji (koji spores) at 40°C, and incubating in a warm, humid environment (30–40°C, 90% humidity) for 40–48 hours while carefully monitoring temperature and turning the developing culture every 6–8 hours. The growing mycelium threads generate significant heat through metabolic activity — careful heat management prevents over-heating that kills the mould (above 45°C) while maintaining the warm humid environment that promotes even growth. Well-made rice koji appears white and fluffy with deep mycelium penetration throughout each grain, smells of chestnuts and mushrooms, and tastes sweet and slightly alcoholic. The resulting koji can immediately be used to make shio koji (salt koji), amazake, miso (combined with soybeans and salt), sake, mirin, or doburoku (home-brewed sake). Tanekoji is commercially available from sake brewing suppliers and specialist fermentation retailers. The craft home fermentation movement in Japan (koji revival) has brought this previously professional skill back into home kitchens.
Fresh koji: chestnut-sweet, lightly alcoholic, earthy fungal aroma; enzymatic transformation unlocks extraordinary sweetness and umami in subsequent applications
{"Steaming rice (not boiling) essential — surface moisture must be controlled for even mycelium penetration","Incubation temperature: 30–35°C start, naturally rises to 40°C as metabolism increases — must not exceed 45°C","Turn and break up clumps every 6–8 hours — prevents localised overheating and uneven growth","90% humidity in incubation environment — too dry stops growth; excess moisture causes bacterial contamination","White fluffy mycelium throughout each grain indicates success; yellow-green indicates over-mature or wrong strain"}
{"Insulate the incubation box with towels — yogurt maker, cooler box, or wooden koji-buta tray all work","White sweet smell with chestnut notes at 24 hours indicates healthy growth — no sourness or ammonia","Dry koji at 50°C in oven for 4 hours after completion — extends shelf life to months vs days for fresh","Home koji making is deeply satisfying — the living texture and aroma of fresh koji inspires further fermentation projects"}
{"Over-heating during incubation — metabolic heat plus ambient heat can kill mould above 45°C","Insufficient humidity — dry conditions halt mycelium spread even if temperature is correct","Wet steamed rice rather than properly drained — surface water prevents mycelium penetration","Not turning frequently — localised hot spots develop and kill koji in patches"}
The Art of Fermentation — Sandor Katz; Koji Alchemy — Jeremy Umansky and Rich Shih