The primary grain used to make traditional Japanese sake is rice.
Sake is brewed by fermenting polished rice with water, yeast, and kōji mold. Unlike grapes in wine, rice contains starch rather than readily fermentable sugar, so kōji helps convert the starch into sugars that yeast can turn into alcohol. These processes occur together in sake’s distinctive multiple-parallel fermentation.
Brewers polish the rice to remove the outer layers, which contain more protein, fat, vitamins, and minerals. Removing them helps produce cleaner flavors and leaves a starchy core that is better suited to brewing. Premium sake often uses specialized sake-rice varieties such as Yamada Nishiki, while less expensive sake may use ordinary table rice.
Sake is sometimes called Japanese rice wine, but that comparison can be misleading: its brewing process is closer to beer because it begins with grain starch. Corn, wheat, and barley are important grains in other alcoholic beverages, but they are not the defining grain of traditional sake.