When you are looking for sake, you might find a percentage on the bottle label typically ranging from 20% to 90%. This is not the alcohol level but the “Rice Polishing Ratio”(Seimaibuai), one of the distinguishing features of a sake brew.

But what does it mean to “polish” rice and why is it important?  

Rice polishing is one of the most important steps in sake brewing. Seimaibuai indicates how much of the rice remains. For example, 60% Rice Polishing Ratio means 40% of rice is polished away and 60% of rice remains. Brewers decide how much of the rice should be broken down and which elements to pursue to compose the sake’s flavor and aroma. Rice polishing is one of the important processes to make more premium sake because rice brans and other external elements need to be polished away in this process.

Rice polishing removes the exterior portion of the rice which is made up of protein and fat, leaving the interior which is made up of starch. Starch is used in saccharification (the conversion of starch to sugar) which is necessary to brew alcohol. However, exessive amount of protein and fat give sake less desirable taste like bitterness or sourness. Premium sake, which has a lower percentage of rice polishing ratio, tends to be a clearer sake.

Vertical rice polishing machines are used in this process. A few breweries have their own machines and others hire polishing companies to do the task. These machines allow rice to fall down two large, spherical, spinning grindstones. Then, the rice is returned to the top by conveyor belts. This process is continued until the desired polishing ratio is achieved.

For 70% polishing ratio, it takes one day. (cheaper sake)
For 30% polishing ratio, it takes three days. (more premium sake)