The fermented soybean paste used in sauces and soups served with sushi is miso.
Miso is made by fermenting soybeans with salt and kōji, a culture usually grown on rice or barley. The mold Aspergillus oryzae helps break down starches and proteins, creating miso’s deep savory, or umami, flavor. Fermentation may last from days to several years.
Miso is best known in miso soup, where it is blended with dashi stock, but it also flavors marinades, glazes, ramen, udon, dipping sauces, and vegetable dishes. Sushi restaurants may serve miso soup alongside sushi, although soy sauce—not miso—is the usual dipping condiment.
Color is a useful clue: shiro miso is generally lighter and sweeter, while aka miso is darker, saltier, and more strongly flavored. Miso should not be confused with shoyu, which is liquid soy sauce, or mirin, a sweet Japanese cooking wine.