In the old days, when sake was made for the gods, a shrine maiden would chew and spit out the steamed rice, and then the rice would be fermented into sake. The process was adding human amylase by spitting it out into a jar. The glucose produced in this way was used by the sake yeast for alcoholic fermentation. This was called Kuchikami no sake. Sake brewing used to be a sacred process.
Nowadays, the conversion of starch to glucose is done by adding Koji spores to steamed rice.
【Koji and Yeast】
In this article, I will focus on saccharification which is the conversion of starch into glucose sugar. In this process, yellow spores, a type of mold, are seeded into steamed rice. Koji is produced by propagating mold from the surface to the center of the steamed rice. Of particular importance is amylase, a starch-degrading enzyme that breaks down rice starch and converts it into glucose.
Since grapes, the raw material for wine, have sugar content naturally, wine can be made simply by leaving the grapes alone. However, if you mix steamed rice and water and leave it alone, you will not be able to produce alcohol no matter how long you leave it. The process of converting starch into sugar is very important.
You can read the current sake brewing process in detail in articles 5, 6, and 7.