Koji – the only one domesticated fungi: characterization, enzymology, and use

Koji mold (Aspergillus oryzae) is the only known domesticated fungal species, playing a central role in traditional East Asian fermented foods such as soy sauce, miso, and sake. This filamentous fungus is highly valued for its exceptional enzymatic capabilities, as it secretes diverse hydrolytic enzymes, including proteases and amylases. A. oryzae possesses 65 endopeptidase and 69 exopeptidase genes, supporting efficient protein degradation. In contrast, A. sojae contains 83 endopeptidase and 67 exopeptidase genes. Key enzymes include alkaline protease (optimal at pH 9.0 and 40 °C), neutral protease I (broad specificity), and acid protease (optimal at pH 3.7, 39 kDa). α-Amylase and glucoamylase production are also prominent; the former exhibits thermal stability up to 75 °C, while the latter displays optimal activity at 60 °C and has a molecular weight of between 60–70 kDa. Genomic studies reveal that A. oryzae contains three amylase genes, in contrast to only one in A. sojae, which correlates with its superior saccharification performance. Importantly, A. oryzae is genetically incapable of producing aflatoxin due to critical mutations in the aflR regulatory gene and deletions within the aflatoxin biosynthesis cluster, particularly in Group 1 and 2 strains that dominate industrial use. Koji mold is traditionally cultivated through solid-state fermentation, primarily on steamed, polished rice, under controlled conditions (30–35°C, 90% humidity). This review provides a comprehensive overview of the enzymology, genomic adaptations, and fermentation technologies associated with A. oryzae, emphasizing its unique domestication, safety profile, and industrial relevance in enzyme production and sustainable food biotechnology.