Aspergillus oryzae, also known as , is a mold used in East Asia to saccharify rice, sweet potato, and barley in the making of alcoholic beverages such as sake and shà Âchà «, and also to ferment soybeans for making soy sauce and miso. It is one of the different koji molds used for food fermentation.
However, in the production of fermented foods of soybeans such as soy sauce and miso, Aspergillus sojae is sometimes used instead of A. oryzae. A. oryzae is also used for the production of rice vinegars. Barley kà Âji (麦麹) or rice kà Âji (米麹) are made by fermenting the grains with A. oryzae hyphae.
The technique of solid-state cultivation using rice grains, soybeans, and wheat bran to propagate fungi for use in fermented foods is believed to have originated in China. However, there are two scholarly theories regarding the country that first employed A. oryzae in the production of fermented foods: one suggests it was China, while the other proposes it was Japan. Genomic analysis has led some scholars to believe that the Japanese domesticated the Aspergillus flavus that had mutated and ceased to produce toxic aflatoxins, giving rise to A. oryzae. While the two fungi share the same cluster of genes that encode for aflatoxin synthesis, this gene cluster is non-functional in A. oryzae. Eiji Ichishima of Tohoku University called the kà Âji fungus a "national fungus" (kokkin) in the journal of the Brewing Society of Japan, because of its importance not only for making the kà Âji for sake brewing, but also for making the kà Âji for miso, soy sauce, and a range of other traditional Japanese foods. His proposal was approved at the society's annual meeting in 2006.
The Japanese word kà Âji (麹) is used in several meanings, and in some cases it specifically refers to A. oryzae and A. sojae, while in other cases it refers to all molds used in fermented foods, including Monascus purpureus and other molds, so care should be taken to avoid confusion.
Aspergillus oryzae is currently widely used in the production of fermented foods throughout East Asia. Historically, however, traditional methods of alcohol brewing varied significantly. In Japan, A. oryzae was cultivated on steamed and loosened rice for brewing purposes. In contrast, in countries such as China, the Philippines, Korea, and Thailand, it was more common to use fungi from the genera Rhizopus and Mucor, which were cultivated on a brick-like mass formed by mixing ground grains with water and kneading the mixture into a solid shape.
This difference in fermentation methods is rooted in dietary traditions. In northern China, there was a long-established custom of making a paste by combining flour from wheat or millet with water for daily consumption. This practice was adapted for fermented food production and eventually spread to other regions. In contrast, such a dietary habit did not take root in Japan for a long time, where the staple food remained cooked (steamed) rice. As a result, a uniquely complex method of sake brewing developed in Japan, relying exclusively on Aspergillus oryzae.
The following properties of A. oryzae strains are important in rice saccharification for sake brewing:
Two of the key enzyme groups secreted by A. oryzae are pectinase and peptidase. Pectinase drives starch hydrolysis by breaking down the pectin in the cell walls of plant materials like soybeans, in the case of miso and soy sauce production, while peptidases like leucine aminopeptidase cleave amino acids from proteins and polypeptides like glutamic acid, an amino acid that contributes to the characteristic umami flavor of these fermented soybean products.
A. oryzae secretes many salt-tolerant alkaline proteases which makes it particularly stable in the high-sodium conditions required for the production of miso and soy sauce. The strain A. oryzae RIB40, for example, appears to have specific salt tolerance genes that regulate K<sup>+</sup> transport.
Three varieties of kà Âji mold are used for making shà Âchà «, each with distinct characteristics.
(1883 -1948), who is said to be the father of modern shà Âchà « and Tamaki Inui (1873 -1946), a lecturer at University of Tokyo succeeded in the first isolation and culturing of aspergillus species such as A. kawachii, A. awamori, and a variety of subtaxa of A. oryzae, which led to great progress in producing shà Âchà « in Japan. Since then, aspergillus developed by Kawachi has also been used for soju and makgeolli in Korea.
Initially kept secret, the A. oryzae genome was released by a consortium of Japanese biotechnology companies in late 2005. The eight chromosomes together comprise 37 million base pairs and 12 thousand predicted genes. The genome of A. oryzae is thus one-third larger than that of two related Aspergillus species, the genetics model organism A. nidulans and the potentially dangerous A. fumigatus. Many of the extra genes present in A. oryzae are predicted to be involved in secondary metabolism. The sequenced strain isolated in 1950 is called RIB40 or ATCC 42149; its morphology, growth, and enzyme production are typical of strains used for sake brewing.
The increased number of genes in Aspergillus oryzae are responsible for the function of proteins and cellular processes such as hydrolase, transporters, and metabolism. The extensive array of secretory hydrolase and transporters allows the mold to break down or secrete various compounds effectively. Typically, when A. oryzae exposed to high concentrations of foods like rice, soybean, wheat, etc. during fermentation, its growth may be negatively affected. However, over time this may potentially allow the kà Âji to gain new transporters due to the environment's conditions.
Although A. oryzae is closely related A. flavus and A. parasiticus, which are known to secrete toxins called aflatoxins that cause severe food poisoning, the kà Âji mold has not been found to produce those toxins. Furthermore, no carcinogenic substances have been discovered in the mold. A study has shown that even when A. oryzae is put under conditions favorable to express and secrete aflatoxin, the aflatoxin genes in A. oryzae were not expressed.
Trans-resveratrol can be efficiently cleaved from its glucoside piceid through the process of fermentation by A. oryzae. "Flavourzyme", a protease blend derived from A. oryzae, is used to produce enzyme-hydrolyzed vegetable protein.
A. oryzae is hard to study due to difficulties in conventional genetic manipulation. This is because A. oryzae have cell walls that are difficult to break down which makes gene insertion/editing complicated. However, scientists have recently started utilizing CRISPR/Cas9 in A. oryzae. This increased mutation rates in the genome which was not possible in the past since the mold only reproduced asexually.
A. oryzae is a good choice as a secondary metabolite factory because of its relatively few endogenous secondary metabolites. Transformed types can produce: polyketide synthase-derived 1,3,6,8-tetrahydroxynaphthalene, alternapyrone, and 3-methylorcinaldehyde; citrinin; terrequinone A; tennelin, pyripyropene, aphidicolin, terretonin, and andrastin A by plasmid insertion; paxilline and aflatrem by co-transformation; and aspyridone, originally from A. nidulans, by Gateway cloning.