Kōji (food)

Kōji (Japanese: 麹; rōmaji: kōji, also written as the kokuji 糀) is a filamentous fungus, most commonly Aspergillus oryzae, which is traditionally used in Japanese cuisine for the fermentation of food, or a mixture of such a culture with wheat and soybean meal. The latter can be fried and eaten directly or processed to a sauce.^[1]

Koji grown on rice

The term kōji in English refers specifically to the Japanese types of starter cultures. The same Chinese 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; see jiuqu.

In Japanese, the genus Aspergillus is known with the common name of kōji mold (麹黴（コウジカビ）, 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").

Characteristics

Four Aspergillus species in a Petri dish. The bottom two are strains of A. oryzae

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 allow A. oryzae to grow.^[3] In this process, the starch from cereals such as wheat, buckwheat or barley as well as from sweet potato is split into glucose, creating a sweet taste. Due to the amino acids glutamic acid and to a lesser extent also aspartic acid split off from the proteins during fermentation, resulting in a strong umami 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

Aspergillus sojae on soybeans and wheat

Yellow kōji is used, among other things, for the production of soy sauce,^[3][4] miso,^[8][9] sake,^[10] tsukemono, jiang, makgeolli, meju, tapai, kōji-amazake, rice vinegar,^[11] mirin, shio koji^[12] and natto. 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 moist mash.^[3]

There are three Aspergillus species that are used as yellow kōji:

• Aspergillus flavus var. oryzae^[13] (キコウジキン / 黄麹菌 ‘ki kōji-kin’). The growth range of this species includes pH values from below 2 to above 8, a temperature optimum of 32 – 36 °C, a temperature minimum of 7 – 9 °C and a temperature maximum of 45 – 47 °C.^[14] The colony color is initially yellow-green, later more or less brown.^[14]
• Aspergillus sojae^[15][16] (醤油麹菌 ‘shōyu-kōji-kin’)
• Aspergillus tamarii^[3][4]

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 weak promoter and the CAAT box has a gene expression attenuating mutation (CCAAA instead of CCAAT), but has a higher enzyme activity of endopolygalacturonase and glutaminase.^[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 65 endopeptidase genes and 69 exopeptidase genes, and A. 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 in A. oryzae, and the odour profiles differ significantly.^[17] A. sojae has 10 glutaminase genes.^[18] Various mutants of A. oryzae with altered properties were generated by irradiation^[3] or by the CRISPR/CAS method.^[19][20][21] Similarly, mutants of A. sojae with altered properties were generated by a variant of the CRISPR/Cas method^[21] or chemical mutagenesis.^[22]

Black & white kōji

Aspergillus niger on MEAOX-Agar

Aspergillus tubingensis on Czapek-Agar

Black kōji produces citric acid during fermentation, which inhibits the growth of unwanted microorganisms.^[2] It is typically used for the production of awamori.^[2][12]

There are three Aspergillus species that are used as black kōji:^[2]

• Aspergillus luchuensis (synonym Aspergillus awamori, Aspergillus inuii, Aspergillus nakazawai and Aspergillus coreanus, クロコウジキン / 黒麹菌 ‘kuro kōji-kin’)
• Aspergillus niger (synonym Aspergillus batatae, Aspergillus aureus or Aspergillus foetidus, Aspergillus miyakoensis and Aspergillus usamii including A. usamii mut. shirousamii)
• Aspergillus tubingensis (synonym Aspergillus saitoi and A. saitoi var. kagoshimaensis)

White kōji (Aspergillus kawachii) is an albino variant of Aspergillus luchuensis.^[6] It is typically used in the production of shōchū.

History

麹 (Chinese qū, Japanese kōji) which means mold used in fermented foods, was first mentioned in the Zhouli (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, and douchi, not to mention grain-based wines (including Japanese sake and Chinese huangjiu) and li (the Chinese forerunner of Japanese amazake).^[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 book Harima 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-called tomodane. This involved cultivating kōji 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 mushroom culture quality and the selection of desirable characteristics.^[29]

It later became known that Kōji comprises different species of Aspergillus. Aspergillus oryzae was first described in 1878 as Eurotium oryzae Ahlb.^[30] and in 1883 as Aspergillus oryzae (Ahlb.) Cohn.^[31][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 of A. luchuensis (black Kōji)^[36][37] in 1918 and named it Aspergillus kawachii (white Kōji). Aspergillus sojae was first described as a distinct species in Kōji in 1944.^[38][39] Initially, Aspergillus sojae was considered a variety of Aspergillus parasiticus because, unlike the other fungi of Kōji, it had never been isolated from the soil.^[40]

Traditional Uses

Koji is widely used in traditional fermentation processes to create staple foods and condiments:

• Miso: A fermented soybean paste that is a cornerstone of Japanese cuisine. Its production involves combining koji (usually rice-based) with cooked soybeans and salt, followed by fermentation for weeks or even years. Enzymes in the koji break down complex proteins and carbohydrates in the soybeans, creating miso's rich and nuanced flavor. The fermentation time, type of koji, and additional ingredients all contribute to a wide variety of miso types, from sweet white miso to robust red miso. Miso adds depth and complexity to dishes such as soups, stews, marinades, and sauces.^[41][42]
• Sake: In sake production, koji converts rice starch into fermentable sugars. Unlike beer brewing, where saccharification and fermentation occur sequentially, sake brewing integrates these stages in a parallel fermentation environment. This dynamic interplay contributes to sake’s unique flavor profile. Koji not only facilitates starch conversion but also develops nuanced aromas and flavors in the final product.^[43]
• Soy Sauce: A fermented condiment derived from soybeans and wheat, with koji initiating enzymatic breakdown. Koji is cultivated on roasted wheat and soybeans and then mixed with salt water to create a brine called moromi, which ferments for months. Koji enzymes break down proteins and starches, contributing to soy sauce's umami flavor. After fermentation, the moromi is pressed, pasteurized, filtered, and bottled.^[44]

Modern Applications

Recent advancements in food technology have expanded the applications of koji beyond traditional uses.

Meat Alternatives

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, and biomass 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 its Koji 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.

Flavor Enhancement

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]

• Dry-aging meats: Applying koji spores to meat breaks down proteins, accelerating the aging process and resulting in more tender, flavorful cuts.
• Fermenting vegetables: Koji is used to ferment vegetables like cabbage, carrots, and beets, producing umami-rich flavors while preserving crisp textures.
• Vegan condiments: Koji enables the production of soy-free, plant-based sauces that deliver deep, savory flavors without the use of animal products.

These culinary innovations highlight koji's versatility beyond traditional Japanese cuisine and demonstrate its growing role in sustainable and creative cooking worldwide.

Nutritional and Health Benefits

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, B12, biotin (vitamin H), 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]

Digestive and Gut Health

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 as Blautia 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]

Metabolic Health

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]

Skin Health

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]

Immune Function

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]

Literature

• H. Kitagaki: Medical Application of Substances Derived from Non-Pathogenic Fungi and -Containing. In: Journal of fungi. Band 7, Nummer 4, März 2021, S. , doi:10.3390/jof7040243, PMID 33804991, PMC 8063943.
• J. E. Smith (6 December 2012). Aspergillus. Springer US. pp. 46ff. ISBN 978-1-4615-2411-3.