Aspergillus sojae is a species of fungus in the genus Aspergillus.
In Japan, it is used to make the ferment (kōji) of soy sauce, miso, mirin, and other lacto-fermented condiments such as tsukemono. Soy sauce condiment is produced by fermenting soybeans with A. sojae, along with water and salt.
Soy sauce (sometimes called soya sauce in British English) is a liquid condiment of Chinese origin, traditionally made from a fermented paste of soybeans, roasted grain, brine, and Aspergillus oryzae or Aspergillus sojae molds. It is recognized for its saltiness and pronounced umami taste.
Soy sauce was created in its current form about 2,200 years ago during the Western Han dynasty of ancient China. Since then, it has become an important ingredient in East and Southeast Asian cooking as well as a condiment worldwide.
Aspergillus oryzae, also known as kōji mold (Japanese: ニホンコウジカビ (日本麹黴), Hepburn: nihon kōji kabi), is a mold used in East Asia to saccharify rice, sweet potato, and barley in the making of alcoholic beverages such as sake and shōchū, and also to ferment soybeans for making soy sauce and miso. It is one of the different koji molds used for food fermentation.
However, in the production of fermented foods of soybeans such as soy sauce and miso, Aspergillus sojae is sometimes used instead of A. oryzae. A. oryzae is also used for the production of rice vinegars. Barley kōji (麦麹) or rice kōji (米麹) are made by fermenting the grains with A. oryzae hyphae.