In Japan, koji has been used to make sake for hundreds of years and what might be the surprising thing is that it is actually a kind of mold. The scientific name for the mold is Aspergillus Oryzae and it creates a few different enzymes as it reproduces and these are what cause the starches in the rice to turn into sugars that feed the yeast cells that produce both alcohol and carbon dioxide. Without the addition of the koji, the Japanese alcoholic beverage of sake cannot exist. There are other beverages in Asia that have been known to use koji, but the ways they are brewed are very different.

Sake is produced differently than a beverage such as wine, so it might be useful to explain just how different the production methods are. Wine is created from fermented grapes that already have sugar (or glucose) in them and sugar is what yeast has to eat. Even though there are other kinds of sugars in existence, the yeast cannot metabolize them and so when wine is made, the yeast is put into a liquid that already has sugar in it.

Sake is brewed somewhat similar to beer, but it is not malted. It is made from steamed rice that has had its husk removed and the rice is milled in order to remove the outer covering. It is not uncommon for a rice to be washed down to 50 percent or even less of its former weight in order to get to the innermost part of the rice, which does not contain all of the proteins, amino acids, and fats that can give the sake an unwanted flavor or smell.

Aspergillus oryzae has a very powerful affect on the final product and its cultivation is taken very seriously. It is produced in a different room in the brewery that is known as the koji-muro. When it is ready, it is added to more steamed rice. Later on in the batch, it is put into a large tank where the rice, yeast, water, and koji will continue to ferment. One account says that a brewer presented a bottle of sake with an apology, saying that they had rebuilt their koji-muro the year before and that the cedar wood used in the walls was not as ready as they had thought. The cedar could be tasted and smelled in the sake.

Koji is what gives the rice its unique flavors depending upon what kind of rice it is cultivated on, the pH level of the water, the mineral content of it, and many other things are what make koji one of the most important ingredients of sake.