Sake is in the same category of fermented liquor as wine and beer
Fermented beverages are alcoholic drinks made when yeast ferments alcohol from the agricultural crops used. That fermentation process differs for each type of drink, and a unique characteristic of sake is that it uses a sophisticated production method called multiple parallel fermentation.
Polishing the rice
The brewing process begins with polishing hulled rice, the main ingredient. As it passes through a special polisher, the proteins and bran that can produce off flavors in the sake are removed.
Washing, steeping and steaming
The polished rice is washed in water to remove the bran and is left to steep in water. When the grain has absorbed 30% of its weight in water it is steamed. One batch of steamed rice may be used to make koji, yeast starter, and to feed the moromi mash.
Spores of the aspergillus oryzae mold (koji-kin) are added to the steamed rice, which is then incubated to produce koji. The koji is added to the yeast starter and the moromi mash to help convert the rice starch into glucose.
This is made by mixing steamed rice, water, koji, and yeast. It contains large amounts of yeast, which promotes the moromi fermentation process.
Preparing the moromi
Koji, steamed rice, and water are added to the shubo and then left to ferment.
Here (during the moromi preparation stage), a process unique to Japanese sake brewing takes place. It’s a three-step fermentation process known as sandan shikomi. On the first day, koji, steamed rice, and water are added to the yeast starter (this addition is called hatsuzoe). The mixture is left to stand on the following day to allow the yeast to slowly multiply (this step is called odori). On the third day, the second batch of koji, steamed rice, and water is added to the mixture (this addition is called nakazoe). Then finally on the fourth day, the third batch is added to the mixture (this addition is called tomezoe) to complete the three-part process.
Multiple parallel fermentation
From this point, the koji will convert the starch in the rice into glucose, which the yeast will then use to create alcohol and carbon dioxide. The conversion of starch to sugar and sugar to alcohol takes place in parallel all in the same tank. This is known as "multiple parallel fermentation," and is a process that is entirely unique to sake.
Once the moromi is completely fermented, it is passed through a press to separate out the sake lees. The sake is then filtered, pasteurized, and placed in cold storage where it matures before being bottled.