Koji soy sauce is a soy sauce made from soybeans that have been fermented with aspergillus oryzae, also known as koji. koji is a mould that is used in the production of various foods and drinks, including sake, mirin and miso. Koji soy sauce has a milder, more complex flavour than traditional soy sauce, thanks to the fermentation processes involved in its production. It is often used to add flavour to noodle, vegetable and meat dishes.