The Japanese fermented soybean paste used in miso soup is miso.
Miso is made by fermenting soybeans with salt and kōji, a culture commonly involving the mold Aspergillus oryzae. Rice, barley, or other grains may also be included. The fermentation breaks down proteins and starches, producing the savory amino acids and aromas associated with miso’s deep umami flavor.
In miso soup, the paste is usually dissolved into dashi, a Japanese stock often made from kombu and bonito flakes. Miso itself is not the soup stock; it is the fermented seasoning added to the stock. This distinction explains why miso soup can vary significantly depending on both the miso and the dashi.
Common varieties include white miso, red miso, and awase miso, a blend of different types. Flavor depends on the soybean-to-grain ratio, salt, fermentation time, temperature, and kōji used. Edamame is immature soybeans, tofu is coagulated soy milk, and natto is whole soybeans fermented into a sticky food.