The primary function of rice in sake is to provide the starch that is turned into fermentable sugar to achieve alcoholic fermentation.  But of course it does more than that, and there is more to a grain of rice. In addition to starch, rice also contains proteins and fats that can give both distinctive character and/or off flavors to to sake.  The concentration of these three components (starch, protein, fat) is probably the most important aspect of what makes a good grain of sake rice. After that, how well the rice can be milled and how well it dissolves in water are other critical factors.

In sake production there are three main categories that any kind of rice might fall under:

1. Rice that is rarely or never used for sake production
2. Food rice that is also used to make sake
3. Rice that is ONLY used to make sake

Rice that is rarely or never used to make sake would be most any kind of long grain varietal.  They typically are high in unwanted protein and fat, difficult to polish (this is another way of saying “milling”) due to their shape, and the starch is more evenly distributed in the grain instead of being concentrated in the center.

Food rice that is also used to make sake would be a large number of the japonica varietals and japonica/indica hybrids such as Calrose.  Their rounder “short grain” shape makes them easier to mill down to lower percentages. However they are still high in protein and fat. This makes for more nutritious and flavorful food rice, but it means this rice is more likely to contribute rough flavors to sake.  It should be noted that this is not always the case, and some food rice also makes excellent sake.

Rice that is ONLY used for sake is a subset of japonica varieties that have been cultivated for decades or centuries for the sole purpose for making sake.  There are some exceptions such as Omachi and Wataribune that were used for both sake and food, but at some point fell out of favor on the food side. Each grain of good sake rice has lower amounts of fat and protein, high amounts of starch, and that starch is usually concentrated in the center so it can remain after fats and proteins are milled away.