Which fungus is used to produce the Japanese alcoholic beverage sake?

Aspergillus oryzae is the fungus used to produce sake. In sake brewing, this mold grows on steamed rice to make kōji, releasing enzymes that break the rice starch into fermentable sugars.

That step is essential because rice does not naturally contain the simple sugars that yeast can ferment efficiently. After A. oryzae has converted starch into sugar, Saccharomyces cerevisiae yeast turns those sugars into alcohol and carbon dioxide. Sake therefore depends on a partnership between mold and yeast, even though A. oryzae is the characteristic fungus named in the question.

The mold is traditionally known as yellow kōji and is especially associated with Japanese sake. Related kōji molds, including Aspergillus sojae, are used in products such as soy sauce and miso, while black and white kōji are more commonly associated with shōchū.

A frequent misconception is that Saccharomyces cerevisiae alone produces sake. That yeast performs the alcoholic fermentation, but without the starch-saccharifying enzymes supplied by A. oryzae, the brewing process would not work in the same way.