Aspergillus oryzae is a filamentous fungus integral to food production for centuries, particularly within East Asian cuisine. Called koji mold, it is an organism used to create iconic fermented foods such as soy sauce, miso, and sake. This mold is a domesticated species, selected over generations for traits beneficial to food fermentation.
Identifying Koji Mold
Aspergillus oryzae is a multicellular fungus belonging to the Aspergillus genus, specifically within the Flavi section. Its common name, koji (้บน), is Japanese and refers to the mold-inoculated grain, where it is called the “national fungus.” When grown on a substrate like steamed rice or soybeans, it appears as a dense, fluffy mat of mycelia. This growth is white, though it can develop a light yellowish-green hue as it produces spores.
The history of A. oryzae is one of domestication from its wild relative, Aspergillus flavus. This process began thousands of years ago in China and was later refined in Japan. Over centuries, producers selected strains that were effective at breaking down starches and proteins for food production.
Role in Traditional Fermentation
The primary function of Aspergillus oryzae is to break down complex macromolecules in raw ingredients, preparing them for fermentation steps that involve yeast and bacteria. This initial process, known as koji making, is foundational to the final flavor, aroma, and texture of many products.
In the creation of soy sauce (shoyu), koji is grown on a mixture of steamed soybeans and roasted wheat. The mold’s enzymes break down proteins and starches, creating precursors for the long brine fermentation that follows. For miso paste, koji is cultivated on rice or barley, then mixed with cooked, mashed soybeans and salt before aging. The type of grain used for the koji influences the final character of the miso.
A. oryzae is also used in making alcoholic beverages. For sake, the Japanese rice wine, koji is cultivated on steamed rice grains. The mold’s enzymes convert the rice starch into fermentable sugars, a process called saccharification. This sugar-rich rice is then ready for yeast to convert into alcohol. Mirin, a sweet rice wine used in Japanese cooking, and rice vinegar also depend on this initial starch conversion.
The Enzymatic Process
The transformative capability of Aspergillus oryzae lies in its remarkable ability to secrete a diverse and powerful set of enzymes. This fungus acts as a highly efficient cell factory, producing and releasing large quantities of enzymes into the substrate on which it grows, such as rice or soybeans. These enzymes function like microscopic scissors, systematically deconstructing large biological molecules into smaller, more accessible units that are fundamental to the fermentation process.
Two main classes of enzymes produced by A. oryzae are particularly important. The first are amylases, which target complex carbohydrates like starch. Amylases hydrolyze these starches, breaking them down into simple sugars such as glucose. This action is necessary in products like sake, where the resulting sugars become the fuel for yeast to produce alcohol.
The second group consists of proteases, which are responsible for breaking down complex proteins into smaller peptides and individual amino acids. In the fermentation of soybeans for miso and soy sauce, proteases release amino acids like glutamic acid, which is a primary source of the savory flavor known as umami. This enzymatic breakdown not only creates flavor but also makes the nutrients in the original ingredients more digestible.
Safety Profile and Distinction from Pathogens
The “Aspergillus” name can be concerning, as the genus includes species known to be harmful to humans. However, Aspergillus oryzae has a long and established history of safe use in the food industry. Regulatory bodies, including the U.S. Food and Drug Administration (FDA), have designated it as Generally Recognized as Safe (GRAS), confirming its non-toxic status. Strains used in commercial food production are consistently verified to be free of harmful compounds.
It is important to distinguish A. oryzae from its pathogenic relatives. Its closest relative is Aspergillus flavus, a species notorious for producing a group of potent carcinogens called aflatoxins. While the two fungi are nearly identical at the genomic level, A. oryzae is considered a domesticated form of A. flavus that has lost the ability to produce these toxins. Genetic mutations, accumulated over centuries of selection for food production, have disabled the gene cluster responsible for aflatoxin synthesis.
Another related species is Aspergillus fumigatus, a common mold that can cause serious infections in immunocompromised individuals. Unlike A. oryzae, which is adapted for food fermentation and is not considered a pathogen, A. fumigatus thrives at human body temperature and possesses specific traits that allow it to cause disease. The clear genetic and functional differences between A. oryzae and its harmful relatives underscore its safety for consumption.