This week I thought I would talk a little bit about sake rice. Rice is one of the 4 major ingredients needed to make sake. (Water, koji, and yeast are the other three ingredients need to make sake.) What we need to understand first is that sake rice is different than table rice (rice used to eat). Sake rice is typically larger and has less fat and protein than table rice. Sake rice also has a bigger shinpaku (starchy center). Sake rice also is better at absorbing water and is not as soft was table rice.
In table rice the starch, proteins, and fats are fairly evenly. In sake rice the shinpaku (starch) is mostly concentrated in the center of the rice grain. Having more starch and having it concentrated makes it better for sake brewing. The starch of the rice is what gives sake its flavor. The starch will (eventually) be the source of sugar for the yeast which will make the alcohol.
We talked about how the starch in sake rice is mostly concentrated in the middle of the grain. This is where the Rice Polishing Ratio or seimaibuai comes into play. When the rice is polished what is happening is the outside of the grain is being removed. This exposes more of the starch which will lead to more of the floral and fruity flavors typically associated with ginjo (polished so that 60% or less of the grain is left) and daiginjo (polished so that 50% of the grain is left) sakes. When you have a higher rice polishing ratio such as a honjozo which is 70% of the grain left or a junmai (which has no minimum RPR) you will get more cereal, grainy, and lactic flavors.
You can see that the rice and how the different varieties of sake rice can have an affect on the taste. Be sure to check the bottle now for the rice type and if it is listed then you will have some idea on what the sake might be like.
Note 1 Information taken from Sake-World.com by John Gauntenr