Koji, a micro-organism, plays a most important role in the fermentation process. Koji is prepared by multiplying micro-organism from Miso's main ingredients, such as rice, barley or soybeans, and then adding koji seeds. These micro-organisms decompose, then compose to convert into a completely new substance. This whole process is called "fermentation". It takes several days for the grain starch to break down into sugar. The resulting mold-convert grain in referred to as koji.

The quality of Miso depends on the quality of the koji

The quality of Miso depends on the quality of the koji and the appropriate fermentation process. The fermentation process combined with koji bacillus creates an enzyme. With this enzyme, all starchs and protein found in grains, are converted into sugar. Adding the yeast (kobo) which uses suger as a nutrient facilitates the fermentation process.

Making koji requires a lot of attention and care

Making koji requires a lot of attention and care. It is very similar to wine- or yogurt-making. Rice koji is a very important ingredient in sake and soy sauce, as well.