03 Koji Rice

Grapes for making wine contains sugars, which ferment in the presence of yeast, but with beverages made from grains, such as sake and beer, it is first necessary to use enzymes to break down the starch in the grain to convert it to sugar before yeast fermentation. The enzymes play a number of roles, finely shredding the starch to convert it into sugar, breaking down protein, and producing peptides and amino acids. 

In beer brewing, malt is used as the source of these enzymes, but for making sake, a substance called koji is used. Koji rice is made by cultivating koji fungi on steamed rice. Koji rice may simply be called koji. The koji funjus (Aspergillus oryzae) is a beneficial and safe variety of mold that is also used in the production of traditional Japanese seasonings, such as miso and soy sauce. 

The first step in making the koji for use in sake brewing is to inoculate steamed rice with the spores of koji-fungi, called tane-koji. After awhile, the spores germinate and start to spread their fungal filaments. In about two days the steamed rice is entirely covered with koji-fungi. As the koji-fungi grow, they produce enzymes, which accumulate within the koji. 

Koji-fungi are most active at a temperature of around 36 degrees Celsius, but case all activity at a temperature above 45 degrees Celsius. For this reason, the process is carefully controlled in a room in the brewery called a koji-muro, where the temperature is kept at around 30 degrees Celsius and the relative humidity maintained in the range of 50%-80%. 

The polished rice to make koji is called koji-mai. Koji enzymes are highly efficient and the ratio of koji-mai in the polished rice used to make sake only has to be in the 15%-25% range for the enzymes to perform their role.