Aspergillus oryzae is a filamentous fungus in the family Trichocomaceae that plays a foundational role in East Asian food fermentation systems. It is commonly referred to as the koji mold and is domesticated for safe culinary use. The genus name Aspergillus derives from the Latin aspergillum, meaning “holy water sprinkler,” referring to the shape of the fungal spore-bearing structure under a microscope, while the species epithet oryzae comes from Oryza, the botanical genus for rice, indicating its close association with rice-based fermentations. Although its wild relatives occur in soil and decaying vegetation across East Asia, Aspergillus oryzae itself is considered a domesticated lineage derived from closely related species such as Aspergillus flavus, selectively cultivated over centuries in China and especially Japan for food production.

As a food source organism, Aspergillus oryzae is not typically consumed directly in isolation but is used to inoculate grains and legumes to produce fermentation starters such as rice koji. It is central to the production of sake, miso, soy sauce (shōyu), mirin, amazake, rice vinegar, and various traditional pickled and fermented foods. In these processes, the mold is grown on steamed rice, barley, or soybeans, where it secretes a broad array of extracellular enzymes that transform complex macromolecules into fermentable or flavor-active compounds. The domestication of A. oryzae in Japan led to highly controlled koji-making techniques, making it one of the most economically important microorganisms in Japanese cuisine and fermentation technology.

Biochemically, Aspergillus oryzae produces significant enzymatic bioactive agents, including α-amylase and glucoamylase, which hydrolyze starch into glucose and maltose; acid and neutral proteases, which degrade proteins into peptides and amino acids such as glutamic acid; and lipases, which act on lipids to release fatty acids. These enzymes are proteinaceous and not heat-stable at high cooking temperatures, becoming denatured during pasteurization or cooking. Unlike some related species, domesticated strains of A. oryzae used in food production are selected for the absence of aflatoxin production, making them safe for culinary application. During fermentation, secondary metabolites such as organic acids, volatile aroma compounds, and small peptides are generated indirectly through enzymatic substrate breakdown and subsequent microbial interactions.