Aspergillus oryzae, also known as kōji, is a microscopic, aerobic, filamentous fungus of the class Ascomycetes, which belongs to the "noble" molds. This species has been used for millennia in Chinese, Japanese and other East Asian cuisine, especially to ferment soybeans and rice.
The fungus A. oryzae has been cultivated for food for more than 2,000 years by the Chinese, who called it qū or qü (ch 'u) (Barbesgaard et al. 1992). In the medieval period the Japanese learned from the Chinese and called it kōji.
By Yulianna.x, from Wikimedia Commons
At the end of the 19th century the western world became aware of this fungus; German professor Herman Ahlburg, who had been invited to teach at the Tokyo School of Medicine, analyzed the kōji ferment used in making sake.
He identified a mold on kōji which he named Eurotium oryzae (in 1876) and was later renamed in 1883 by the German microbiologist Ferdinand Julius Cohn as Aspergillus oryzae.
Taxonomy
- Domain: Eukaryota.
- Fungi kingdom.
- Phylum: Ascomycota.
- Subphylum: Pezizomycotina.
- Class: Eurotiomycetes.
- Order: Eurotiales.
- Family: Trichocomaceae.
- Genus: Aspergillus.
Morphology
Initially, the culture of the fungus shows a whitish color, then it turns yellowish-green. Sexual reproduction has not been seen in this mold, but asexual spores (conidia) are easy to distinguish and are released into the air.
The conidiophores are hyaline and mostly have rough walls. Some isolates are predominantly uniseriate, others predominantly biseriate. The conidia are large and smooth or finely rough. The optimal growth temperature is 32-36 ° C.
Compared to A. flavus, the mycelium of A. oryzae is more flocculent, usually turning olive or brown with age, while the colonies of A. flavus retain a bright greenish-yellow color.
Sporulation of A. oryzae is rarer and conidia are larger, with a diameter of 7 µm or more compared to 6.5 µm for A. flavus. The two species are easy to confuse; to distinguish them accurately, several characters must be used simultaneously (Klich and Pitt 1988).
Genetics
The sequencing of the A. oryzae genome, shrouded in mystery for decades, was finally published in 2005 by a team that included 19 institutions in Japan, including the Brewing Association, Tohoku University, the University of Agriculture and Technology in Japan. Tokyo (Machida et al. 2005).
Its genetic material, with 8 chromosomes of 37 million base pairs (104 genes), has 30% more genes than A. fumigatus and A. nidulans.
These additional genes are believed to be involved in the synthesis and transport of many secondary metabolites that are not directly involved in normal growth and reproduction, and were acquired throughout the domestication process.
Comparison of various Aspergillus genomes revealed that A. oryzae and A. fumigatus contained similar sex-like genes.
Biogeography
Koji is primarily associated with the human environment, but outside of this area, it has also been sampled in soil and decaying plant material. Apart from China, Japan and the rest of the Far East, it has been reported in India, USSR, Czechoslovakia, Tahiti, Peru, Syria, Italy and even in the United States and the British Isles.
However, A. oryzae has rarely been observed in temperate climates, as this species requires relatively warm growth temperatures.
Traditional uses and biotechnology industry
Traditionally A. oryzae has been used to:
- Make soy sauce and fermented bean paste.
- Sacarify rice, other grains, and potatoes in the making of alcoholic beverages such as huangjiu, sake, makgeolli, and shōchū.
- Production of rice vinegars (Barbesgaard et al. 1992).
Historically, it has been easily cultivated in various natural (carrots, cereals) or synthetic (Raulin liquid, among others) environments.
Since the raw material for sake is finely ground rice with low amylose content, low gelatinization temperature and white heart, these characteristics have been used by the Japanese, as they facilitate the penetration of the mycelium of A. oryzae. The steamed rice is mixed with the kōji to be hydrolyzed in two to three days.
In China, the traditional ferment of A. oryzae is used to provoke the fermentation of cereals and give various cereal wines (huangjiu, 黄酒). Also to ferment soybeans, prepare soy sauce (Jiangyou, 酱油), miso (weiceng, 味噌) and Tianmianjiang sauce (甜面酱).
Advances in genetic engineering have led to the use of A. oryzae in the production of industrial enzymes. Since the 1980s, the first industrial uses have included the use of its enzymes as laundry detergents, cheese production, and cosmetic enhancement.
Currently biotechnological processes include the production of certain commercial enzymes such as alpha-amylase, glucoamylase, xylanase, glutaminase, lactase, cutinase and lipase.
Faced with the problem of greenhouse gas emissions from fossil fuels, many research centers are oriented towards the development of biofuels from biomass, by biotechnological methods inspired by the industrial production of rice starch sake, using A oryzae and its enzymes.
Some people with a low tolerance to milk sugar (or lactose) may benefit from the manufacture of low-lactose milk, where the hydrolyzing enzyme lactose (or lactase) can be prepared from A. oryzae, which is considered a sure mold.
Bibliography
- Barbesgaard P. Heldt-Hansen HP Diderichsen B. (1992) On the safety of Aspergillus royzae: a review. Applied Microbiology and Biotechnology 36: 569-572.
- Domsch KH, Gams W., Anderson TH (1980) Compendium of soil fungi. Academic Press, New York.
- Klich MA, Pitt JI (1988) Differentiation of Aspergillus flavus from A. parasiticus and other closely related species. Trans Br Mycol Soe 91: 99-108.
- Machida, M., Asai, K., Sano, M., Tanaka, T., Kumagai, T., Terai, G.,… & Abe, K. (2005) Genome sequencing and analysis of Aspergillus oryzae Nature 438 (7071): 1157-1161.
- Raper KB, Fennell DI (1965) The genus Asperoillus. Williams and Wilkins, Baltimore.
- Samson RA, Pitt JI (1990) Modern concepts in Penicillium and Aspergillus classification. Plenum Press, New York.