How Sake is Made
Rice Polishing (Seimai)
The Sake Rice (sakamai) is first polished (milled). Polishing must be gentle to avoid cracking the rice kernels, and must be done slowly to avoid too much heat being generated. The quality of the rice's polishing is based on the grade of sake it will produce. More polished for a higher grade sake.
Washing & Soaking (Senmai & Shinseki)
The rice powder (nuka) is washed from the polished rice. Nuka contains unfermentable acids and fats that produce undesirable flavors and cause hangovers. The kernels are then soaked to absorb water. The more the rice is polished, the less time it takes to absorb water. Time for soaking can range from one minute to overnight.
Steaming The rice is then put in a steaming vat (koshiki) and steam is pumped up from the bottom of the vat through all the kernels. This produces cooked rice with a harder outer shell and a softer inside. A small portion of cooked rice is removed and cooled, set aside to be seeded with a special mold spores (koji-kin).
Next koji-kin , a black powdered mold (Asperillus Oryzae), is sprinkled over the cooled, steamed rice and propagated in a warm humid room. The molded rice (koji) is stirred regularly while the temperature and humidity is strictly controlled for the 36-50 hour duration of this process. The end product will look like rice with frosting on it. The koji will now have a sweet chestnut aroma.
The Yeast (Moto/Shubo)
The koji is then mixed with the plain, steamed rice, water and pure yeast. This mixture is known as shubo. Shubo is matured for two weeks, over which time it will develop about 100 million yeast cells/teaspoon.
The shubo is transferred to a large fermentation tank where more rice, koji, and water are added in three stages over four days. Each addition roughly doubles the batch size. This is now the moromi, or the main mash. The moromi is now fermented at a closely monitored temperature for the next 18-32 days. For lower grade and honjozo sakes this is the point where alcohol would be added.
Sake Pressing The moromi is now loaded into canvas bags where it will undergo pressing (Joso). Joso will mostly be done mechanically in an accordian-like press, or more traditionally in a long wood box with a top that can be pressed down like a vise (known as a FUNE). For ultra high-end sakes the extraction of the clear sake will be achieved by letting it all drain out on its own without any pressing.
Filtering & Pasteurization (Roka & Hi-ire)
The sake is now allowed to be refined by resting. Then, the sake is drawn off of any settled, remaining sediment and filtered through charcoal. Most sake is pasteurized at this point by running it through a pipe submerged in hot water. Now, the sake is aged for around six months (depending on the type of sake and the brewery's method). Sake is usually diluted at this point from 20% alcohol down to 16% using pure, clean water. One last pasteurization now stabilizes the sake's actively fermenting enzymes before the sake is bottled.