Koji is not a yeast, but rather a mold called Aspergillus oryzae. This mold is specifically grown on a medium of steamed rice, resulting in the formation of koji. It is important to note that koji is not the mold itself, but rather the rice that has been inoculated and fermented by the mold.
As a sommelier and brewer, I have had the opportunity to work with koji in various applications, particularly in the production of sake, soy sauce, and miso. In these traditional Japanese fermentations, koji plays a crucial role in breaking down the starches present in the rice or soybeans into simpler sugars that can be metabolized by other microorganisms, such as yeast or bacteria.
The process of koji cultivation involves steaming rice to make it more conducive for mold growth. Once the rice has cooled to an appropriate temperature, it is inoculated with spores of Aspergillus oryzae. These spores are typically propagated on a starter culture known as “koji-kin,” which is a mixture of rice, water, and the mold spores. The spores then germinate and grow into a network of filaments, known as mycelium, which permeate the rice grains.
During the growth of the koji mold, enzymes produced by Aspergillus oryzae break down the complex carbohydrates in the rice, such as starch, into simpler sugars like glucose. This process is known as saccharification and is a crucial step in the fermentation of various traditional Japanese products. Saccharification makes the sugars more accessible to other microorganisms involved in the fermentation process, such as yeast or lactic acid bacteria.
In the context of sake production, koji is mixed with steamed rice and water, along with yeast, to initiate fermentation. The koji breaks down the rice starches into sugars, which are then fermented by the yeast to produce alcohol. Without the presence of koji, the yeast would not be able to utilize the starches in rice as an energy source, as yeast lacks the necessary enzymes to perform saccharification.
Similarly, in the production of soy sauce and miso, koji is mixed with soybeans and salt to create a fermentation mash. The koji mold breaks down the carbohydrates in the soybeans into sugars, which are then fermented by lactic acid bacteria to produce organic acids and other flavor compounds. These fermentation processes rely on the activity of both the koji mold and other microorganisms to achieve the desired flavors and textures.
To summarize, koji is a mold, specifically Aspergillus oryzae, that is grown on a medium of steamed rice. It plays a crucial role in the saccharification of complex carbohydrates present in the rice, breaking them down into simpler sugars that can be utilized by other microorganisms involved in fermentation. Koji is not a yeast itself, but rather a mold that enables yeast and other microorganisms to metabolize the sugars produced during the saccharification process.