[[File:Koji.jpg|thumb|350px|left|Uses of Koji]]
Koji is not actually a yeast, as many people mistakenly believe. Koji is cooked rice and/or soya beans that have been inoculated with a fermentation culture, Aspergillus oryzae. This naturally occurring culture is particularly prevalent in Japan, where it is known as koji-kin, which explains why so many Japanese foods have been developed over the centuries using it. It is used to make popular foods like soya sauce, miso, mirin and sake.