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Kōji (Japanese:麹; rōmaji:kōji, also written as thekokuji糀) is a filamentous fungus, most commonlyAspergillus oryzae, which is traditionally used in Japanese cuisine for thefermentation offood, or a mixture of such a culture with wheat and soybean meal. The latter can be fried and eaten directly or processed to asauce.[1]
The termkōji in English refers specifically to the Japanese types of starter cultures. The sameChinese character (Chinese:麹; pinyin:qū, more commonly written as the homophonic曲 in simplified Chinese texts) is used in Chinese to refer to Chinese starter cultures; seejiuqu.
In Japanese, the genusAspergillus is known with the common name of kōji mold (麹黴,kōji kabi),[1] though the term is not fully limited to the genus (for example,Monascus purpureus is called紅麹黴 "red kōji mold").
Various types of kōji are used, including yellow, black, and white.[2][1] The kōji is stored for two to three days at 30 °C under high humidity to allowA. oryzae to grow.[3] In this process, thestarch from cereals such aswheat,buckwheat orbarley as well as fromsweet potato is split intoglucose, creating a sweet taste. Due to theamino acidsglutamic acid and to a lesser extent alsoaspartic acid split off from theproteins during fermentation, resulting in a strongumami taste.[4][5] Depending on the Aspergillus used, culture substrate and culture conditions (temperature, pH value, salt content, humidity), different products are created in terms of composition, flavour and odour.[6] Kōji can be freeze-dried and crushed to produce spores.[7] Dried kōji-spores can be stored and transported light-protected at room temperature.
Yellow kōji is used, among other things, for the production ofsoy sauce,[3][4]miso,[8][9]sake,[10]tsukemono,jiang,makgeolli,meju,tapai, kōji-amazake,rice vinegar,[11]mirin,shio koji[12] andnatto. Typically, for the production of soy sauce (shoyu),soybeans and sometimes also wheat are swollen in water,steamed, and possibly mixed with wheat bran roasted at 160–180 °C and ground. The enrichment with kōji creates a moistmash.[3]
There are three Aspergillus species that are used as yellow kōji:
A. oryzae has three α-amylase genes, which allows it to break down starch relatively quickly into glucose.[3] In contrast,A. sojae has only one α-amylase gene under a weakpromoter and theCAAT box has a gene expression attenuating mutation (CCAAA instead of CCAAT), but has a higherenzyme activity ofendopolygalacturonase andglutaminase.[3] A too rapid release of glucose from starch at the beginning of fermentation inhibits the growth of the microorganisms in the maturation phase.[3] For the breakdown of proteins to amino acids,A. oryzae strain RIB40 has 65endopeptidase genes and 69exopeptidase genes, andA. sojae strain SMF134 has 83 endopeptidase genes and 67 exopeptidase genes.[3] Similarly, starch-degrading enzymes (glucosidases) are more strongly expressed and protein-degrading enzymes (proteases) less strongly expressed inA. oryzae, and the odour profiles differ significantly.[17]A. sojae has 10 glutaminase genes.[18] Variousmutants ofA. oryzae with altered properties were generated byirradiation[3] or by theCRISPR/CAS method.[19][20][21] Similarly, mutants ofA. sojae with altered properties were generated by a variant of the CRISPR/Cas method[21] or chemical mutagenesis.[22]
Black kōji producescitric acid during fermentation, which inhibits the growth of unwanted microorganisms.[2] It is typically used for the production ofawamori.[2][12]
There are three Aspergillus species that are used as black kōji:[2]
White kōji (Aspergillus kawachii) is analbino variant ofAspergillus luchuensis.[6] It is typically used in the production ofshōchū.
麹 (Chinese:qū,Japanese:kōji), which means mold used in fermented foods, was first mentioned in theZhouli (Rites of the Zhou dynasty) in China in 300 BCE. Its development is a milestone in Chinese food technology, for it provides the conceptual framework for three major fermented soy foods:soy sauce,jiang/miso, anddouchi, not to mention grain-based wines (including Japanesesake and Chinesehuangjiu) andli (the Chinese forerunner of Japaneseamazake).[23][24] The process of making rice wine and fermented bean paste using molds was first documented in the 4th century B.C.[25]
In 725 AD the Japanese bookHarima no Kuni Fudoki (Geography and Culture of the Harima Province) first mentioned kōji outside of China and described that the Japanese produced kōji with fungal spores from the air.[26][27] Around the 10th century, the kōji production method underwent a change and moved from the natural sowing system in rice to the so-calledtomodane. This involved cultivatingkōji until spores were released and using the spores to start a new batch of production.[28] In the Meiji era, the integration of new microbiological techniques made it possible to isolate and propagate kōji in pure cultures for the first time. These advances facilitated the improvement of fungal culture quality and the selection of desirable characteristics.[29]
It later became known that Kōji comprises different species ofAspergillus.Aspergillus oryzae was first described in 1878 asEurotium oryzae Ahlb.[30][full citation needed] and in 1883 asAspergillus oryzae (Ahlb.) Cohn.[31][full citation needed][32]Aspergillus luchuensis was first described in 1901 by Tamaki Inui at the University of Tokyo.[33][34][35] Genichiro Kawachi isolated a colourless mutant ofA. luchuensis (blackkōji)[36][37] in 1918 and named itAspergillus kawachii (whitekōji).Aspergillus sojae was first described as a distinct species inkōji in 1944.[38][39] Initially,Aspergillus sojae was considered a variety ofAspergillus parasiticus because, unlike the other fungi ofkōji, it had never been isolated from the soil.[40]
Koji is widely used in traditional fermentation processes to create staple foods and condiments:
Recent advancements in food technology have expanded the applications of koji beyond traditional uses.
Most meat substitutes on the market today are derived from legumes such as soybeans and peas. While these proteins can effectively replicate the taste and texture of meat, they present challenges including high agricultural land and water use, allergenic concerns, and distinct flavors that may not appeal to all consumers. To address these issues, scientists are exploring microbial fermentation as a more sustainable protein source.
There are two main methods for producing protein from microorganisms:precision fermentation, which involves engineering microbes to produce specific proteins, andbiomass fermentation, where fungi or other microbes are cultivated as whole-food protein sources. A particularly promising organism in this space is mycelium—the protein-rich, fibrous root structure of fungi.
Among microbial sources, koji (Aspergillus oryzae) is at the forefront of this innovation due to its long-standing use in food and established regulatory approvals. It is recognized as Generally Recognized as Safe (GRAS) by the U.S. FDA and classified as non-novel by the European Food Safety Authority (EFSA), facilitating commercialization.
Companies like Prime Roots, based in California, are leveraging koji to create meat alternatives with a fibrous texture similar to animal meat. Koji is cultivated in fermentation vats where it forms long, muscle-like strands. These strands are then combined with plant-based fats and natural flavors to create realistic meat substitutes.[45]
Berlin-based Nosh.bio is advancing single-ingredient koji-based meat products. In collaboration with Zur Mühlen, a leading European sausage producer, Nosh.bio is commercializing itsKoji Protein as a sustainable meat alternative.[46]
Koji's minimal processing, allergen-friendly nature, and regulatory status make it a sustainable and scalable option for the future of alternative proteins.
In addition to its use in protein innovation, koji is gaining popularity among chefs and food technologists for its powerful enzymatic properties that enhance flavor.
Modern culinary applications of koji include:[47]
These culinary innovations highlight koji's versatility beyond traditional Japanese cuisine and demonstrate its growing role in sustainable and creative cooking worldwide.
Koji provides a range of nutrients and bioactive compounds that contribute to its potential health-promoting properties. It is a source of B-vitamins, including B1, B2, B3, B6,B7 (biotin), B12, folic acid, and iron.[48] The fermentation process enhances its nutritional profile by generating enzymes, amino acids, and minerals that support physiological functions.[48] Additionally, koji is considered a low glycemic index (GI) food, which may contribute to improved blood sugar regulation.[48]
Fermentation also enhances nutrient bioavailability and introduces beneficial compounds such as bioactive peptides, polysaccharides, and glycosylceramide. Glycosylceramide, notably, resists digestion but interacts with gut microbiota in ways that may promote gut health.[49]
Like other fermented foods, koji may support digestive health due to its enzyme and probiotic content. Enzymes aid in the breakdown of macronutrients, improving digestion and nutrient absorption.[50] Koji also acts as a prebiotic, promoting the growth of beneficial intestinal bacteria such asBlautia coccoides.[49] This bacterium is associated with various health benefits and may play a role in the observed connection between traditional Japanese diets, gut microbiota, and longevity.[49]
Regular consumption of koji-fermented foods may help maintain microbiome diversity, which is increasingly recognized as important for overall health.[50]
Due to its low glycemic index, koji may help stabilize blood sugar levels and support energy regulation.[48] Sustained energy release and improved satiety may aid in weight management.[48] Some studies suggest koji-fermented soy products may contribute to improved cholesterol profiles, although further research is needed to confirm the mechanisms.[48]
Fermentation products and dietary fiber in koji-based foods may also contribute to cardiovascular health through metabolic regulation.[48]
Koji has been traditionally used for its potential skin health benefits. Nutrients such as biotin, vitamins, and amino acids may support skin elasticity, brightness, and hydration.[48] Antioxidants in koji may protect skin from oxidative damage and environmental stressors, potentially slowing visible signs of aging.[48] Furthermore, the gut-skin axis suggests that gut microbiota modulation through koji consumption may indirectly benefit skin health.[48]
Koji may support immune regulation through gut-immune interactions fostered by its probiotic and prebiotic effects.[50] A healthy gut microbiome has been associated with reduced inflammation and enhanced immune response.[50] The antioxidants present in koji may also contribute to cellular protection by neutralizing free radicals, potentially lowering the risk of chronic diseases and slowing aging processes.[48]
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