All rice is brown rice in husk when harvested.
The outer part of the grain, consisting of the husk and bran, must be polished away before brewing sake.
Before the rice is steamed, it is washed to remove rice bran then soaked.
The degree to which the rice is saturated with water depends highly on the product being made.
The rice is steamed to keep the outer layers of the rice gains firm while leaving their inner cores soft and moist.
The koji mold spores, which provide starch-converting enzymes, are cultivated on a small batch of steamed rice.
Yeast Starter (Shubo) Making
A yeast starter mash (Shubo) is made by mixing small portions of koji, steamed rice, yeast, and water in a small tank.
Main Mash (Moromi) Making
When the yeast starter is ready, the shubo is transferred to a larger tank, to which steamed rice, koji, and water are added over the course of four days (sandan jikomi). This mixture is called moromi, and will continue to ferment over the next 2-4 weeks. Sake is unique among fermented beverages in that the two brewing processes, saccharization and fementation, occur simultaneously (multiple parallel fermentation), naturally yielding sake with about 20% alcohol level.
Once fermentation is complete, the raw sake is run through a pressing machine which separates the Moromi into fresh sake and sake lees.
The fresh sake is filtered to remove unsolvable proteins and starch.
All sake is pasteurized after filtration, with exception of Nama (micro-filtered) sake.
Most sake is aged for 3-6 months before bottling. This allows the rough edge, typical of freshly made sake, to mellow.