Sake is made using only 4 ingredients - rice, water, koji (an enzyme for fermentation) and yeast. Yet, there are more than 10,000 types of sakes and each one is different and unique.
Rice - There are more than 120,000 varieties of rice but only less than 70 of these are suitable for making sake. The kernels of the sake rice has to be bigger than table rice so that it has a larger centre, called shinpaku, which contains more starch for the sake-making process. Generally, the more polished down the rice is, the better the sake grade.
Water - Other than rice, the other key ingredient in making sake is water. Each region has different water quality, therefore the sakes from different regions have different aromas and taste.
Koji - After the rice is polished, it is steamed and allowed to cool. A yellow mold called koji (Aspergillum oryzae) is then added. Selecting the koji is an important part of the process and the brew master (called the toji) is very careful about which koji he uses, and sometimes he will also combine several koji. The koji is then scattered on the steamed rice and folded in evenly in a heated room. The koji converts the starch in the rice into glucose.
Yeast - The next step is to add yeast which converts the glucose and minerals in the mixture into alcohol. In olden times, the freshly made sakes are carbonated and the bubbles are removed when the sakes are ready. However, the newer strains of yeast used today do not form bubbles.