The Power of Koji

August 8th is Koji Day こうじの日, a celebration created by a miso maker based in Ina City, Nagano. The date comes from a clever bit of wordplay. The character for Koji 麹 contains the element 米 (rice), and when “rice” is broken down, it becomes 八十八 which means 8+8, making 8/8 the perfect symbolic choice.

Koji is the quiet force behind Japan’s most beloved seasonings. By cultivating koji mold on rice, barley, or soybeans, producers create a fermentation starter rich in natural enzymes. These enzymes transform ingredients, turning starch into sweetness, proteins into savory amino acids, and fats into aromatic compounds. Thanks to this process, everyday staples like miso, soy sauce, sake, mirin, and vinegar gain their depth and character. Incidentally, today is also Fermented Food Day. 

Koji’s history stretches back more than a millennium, and its cultural importance was formally recognized in 2006 when Aspergillus oryzae was named Japan’s national fungus. Today, koji has moved beyond traditional brewing, appearing in home kitchens through salt‑koji marinades (pictured) and nutrient‑rich amazake.