Sake is produced by the multiple parallel fermentation of rice. The rice is polished to remove the protein and oils from the exterior of the rice grains, leaving behind starch. A more thorough milling leads to fewer congeners and generally a more desirable product.
Newly polished rice is allowed to "rest" until it absorbs enough moisture from the air not to crack when immersed in water. After this resting period, the rice is washed clean of the rice powder produced during milling and is steeped in water. The length of the soak depends on the degree to which the rice was polished, from several hours or even overnight for an ordinary milling to just minutes for highly polished rice.
After soaking, the rice is boiled in a large pot or it is steamed on a conveyor belt. The degree of cooking must be carefully controlled; overcooked rice will ferment too quickly for flavors to develop well and undercooked rice will only ferment on the outside. The steamed rice is then cooled and divided for different uses.
Some of the steamed rice is taken to a culture room and inoculated with kouji mold (麹, Aspergillus oryzae). The mold-laden rice is itself known as kouji and is cultivated until the growth of the fungus reaches the desired level. This takes about two days.
When the kouji is ready, the next step is to create the starter mash, known as shubo (酒母), or colloquially, moto. Kouji rice, water, and yeast are mixed together, and in the modern method, lactic acid is added to inhibit unwanted bacteria (in slower traditional methods, lactic acid occurs naturally). Next, freshly steamed rice is added and the yeast is cultivated over 10 to 15 days (in the modern method).
When the starter mash is ready, steamed rice, water, and more kouji are added once a day for three days, doubling the volume of the mash each time. Staggering things this way allows the yeast to keep up with the increased volume. The mixture is now known as the main mash, or moromi (醪).
The main mash then ferments. This takes two to six weeks. With high-grade sake, fermentation is deliberately slowed by lowering the temperature to 10°C (50°F) or less.
Unlike malt for beer, rice for sake does not have the necessary amylase to convert starch to sugar and so must undergo a process of multiple fermentation, in which starch is converted to sugar by the kouji, and sugar is converted to alcohol by yeast. With sake these two processes happen at the same time, not as separate steps, so sake is said to be made by multiple parallel fermentation.
After fermentation, sake is pressed to separate the liquid from the solids. With some sake, a small amount of distilled alcohol, called brewer's alcohol (醸造アルコール), is added before pressing in order to extract flavors and aromas that would otherwise stay in the solids. With cheap sake, a large amount of brewer's alcohol might be added to increase the volume of sake produced. Next, the remaining lees (a fine sediment) are removed, and the sake is carbon filtered and pasteurized. The sake is allowed to rest and mature and then it is usually diluted with water to lower the alcohol content from around 20% to 15% or so, before finally being bottled.
The Three Types of Special Designation Sake
Honjouzou-shu (本醸造酒), in which a slight amount of brewer's alcohol is added to the sake before pressing, in order to extract extra flavors and aromas from the mash. This term was created in the late 1960s to distinguish it, a premium sake, from cheaply made liquors to which large amounts of distilled alcohol were added simply to increase volume. Sake with this designation must be made with no more than 116 liters of pure alcohol added for every 1,000 kilograms of rice.
Junmai-shu (純米酒), "pure rice sake," made from only rice, water and kouji, with no brewer's alcohol or other additives. Before 2004, the Japanese government mandated that junmai-shu must be made from rice polished down to 70% or less of its original weight, but that restriction has been removed.
Ginjou-shu (吟醸酒), made from rice polished to 60% or less of its original weight. Sake made from rice polished to 50% or lower is called daiginjou-shu (大吟醸酒). The term junmai can be added to ginjou or daiginjou, resulting in junmai ginjou and junmai daiginjou. However, as distilled alcohol is added in small amounts to ginjou and daiginjou to heighten the aroma, not to increase volume, a junmai daiginjou is not necessarily a better product than a daiginjou made with brewer's alcohol.