Koji is a funky little culinary thing: it’s a type of mold that’s responsible for some of our favorite fermented foods.
While it’s now firmly considered Japanese (and is even Japan’s national fungus!), koji was first used in China 9,000 years ago. Among the culinary delights we’d never have without koji is soy sauce, miso, sake, rice vinegar, mirin, shochu, and more. There are three “colors” of koji that are used to produce different things — yellow, white, and black.
It grows when koji spores are mixed with specially-prepared rice or barley and from there it’s used to create other foods and drinks. You can purchase this already put together in the form of rice koji, which is steamed rice that’s already been inoculated with the spores before being dried. However, you can also do the process yourself from scratch by buying koji spores, preparing the rice, and maintaining the mixture while the koji grows. This takes two days, and if you let it stay longer, the resulting koji will itself create spores that you can use in the future.
From there, you can apply koji to many different ingredients like rice, sweet potato, barley, and soybeans to create whole other foods, sauces, marinades, or drinks.
Uwajimaya sells rice koji in our grocery department.