Koji is a fungus formed from cooking rice or soya beans with a fermented culture. It is vastly used in Japanese cuisine as an additive in many sauces and drinks such as miso, soy sauce, and sake.

On top of enhancing the umami flavors in food, koji is a probiotic source that aids in digestion and helps to improve acne. The fermentation process also releases amino acids that increase the levels of isoflavones, or compounds that may prevent cancer.