As it's also called "rice wine", it's not hard to guess that sake is made from rice. But what kind of rice?

Rice grown in Japan is usually short-grained Japonica rice, both glutinous (e.g. mochi rice) and non-glutinous (table rice) varieties.

It's entirely possible to make sake from the standard short-grain rice sold and consumed as Japan's staple carbohydrate, and many futsu-shu table sakes do just that.

However, there are also special varieties of rice known as shuzō kōtekimai (酒造好適米, rice suitable for brewing sake) or sakamai (酒米, sake rice) which are grown specifically for making sake. These varieties have certain characteristics that make them more suited to the brewing process.

  • More starch: Sake rice varieties have less protein and fat than table rice. Their bounty of starch (心拍, shinpaku) is concentrated in the centre which means that other components, which influence aroma and flavour, can be removed by milling away the outside of the grain.
  • Larger grains: Big grains contain plenty of starch and stay a reasonable size after milling.
  • Water absorbance: Rice must be moist in order to grow koji mold. Water retention is crucial to good koji growth and to the production of enzymes that break carbohydrates down into fermentable sugars.
  • Water solubility: Rice breaks up and dissolves during the brewing process, increasing the liquid component and reducing the proportion remaining as solid lees (酒かす, sake-kasu).
  • Milling resistance: Tough grains stand up to milling without cracking. Cracked rice is unsuitable for growing koji as the outer layers cannot be reliably removed.

There are around 100 varieties of sake rice, the most popular are listed below.

  • Yamada Nishiki
  • Gohyakumangoku
  • Miyama Nishiki
  • Dewa San San
  • Omachi
https://hyogo-tourism-en.blogspot.ch/2011/07/japanese-sake-of-nada.html

Yamada Nishiki rice (Hyogo Tourism Blog)