What is Koji?
Koji is fermented rice grains and is used to produce Miso, Soy sauce, Sake, Rice vinegar and Mirin. It was even declared as the "National Culture" of Japan in 2004! Koji is made by inoculating steamed rice with Koji culture (Aspergillus oryzae) and fermenting for 50 hours.
Koji culture produces enzymes that break down starch and proteins in the culture medium (e.g. the soybeans) turning them into digestible glucose and amino acids which also keep the culture alive - hence why it is a "living food".