The best rice for brewing sake has a high starch content at its core. In fact, it is too starchy for human consumption. There are over one hundred varieties of rice in use today, some with formidable names like gohyakumangoku, tamazakae, miyama nishiki, and the most widely used yamada nishiki. The climate in Japan varies from region to region, and different varieties grow in these different climates. However, unlike wine, sake is not categorized by rice varietal, but by the amount of polishing the rice undergoes. By milling away the outer layers, which contain the bran, fats and proteins, the brewer reaches a more abundant layer of pure starch.
The polished rice is then washed and soaked in very exacting times, often with a stopwatch in hand. The size of the milled rice grain will affect the soaking time, so that the steaming of the rice can be as consistent as possible. This process is called gentei kyusui, "limited water absorption." The steaming is also carefully monitored, since overcooked rice will ferment too quickly for flavor development, and undercooked rice will only ferment on the outside.
A small portion of the steamed rice is inoculated with a special mold called koji. It takes about two days to complete the inoculation, which then resembles puffed rice with an aroma of roasted chestnuts. This miraculous mold is not a dark mildew, but rather a fragrant white mold that will later break down the starch molecules of the rice, and convert them into glucose, a process known as saccharification. Koji also provides its own unique flavor and aroma to the sake.
Once the koji is made, sake brewing can begin by making a yeast starter called shubo, colloquially known as moto. Shubo means "sake mother." It is formed by combining koji, water, steamed rice and yeast. In the modern sokujo method, lactic acid is added to hasten the development of the shubo. It creates a sour, high acid environment that inhibits the development of unwanted bacteria and wild yeasts. The koji begins to make the mash thick and sweet. the shubo is ready in about two weeks. The old-fashioned yamahai and kimoto methods allow the natural development of lactic acid, which acquires flavor from ambient yeasts and bacteria, and takes twice as long to develop shubo.