Polishing is a process of removing layers in rice grains used in the production of Sake. The Polishing Ratio is the extent to which they have done it.
Brewers begin with whole grains of brown rice, which they slowly remove the outer layers (or husk) from using a rice polishing machine (this leaves them with white rice). Most producers usually continue this process on to the middle layers.
The polishing ratio is calculated as a percentage of the white rice remaining, meaning a highly polished rice grain will have a low number (like 40%), as that’s all that’s left of the grain compared to when it started.
The polishing ratio has a significant impact on the style and flavour of the sake. If a brewer only polishes away some layers, the resulting sake will be likely to have more umami flavours, whereas if they decide to remove almost all layers and getting as close to pure starch as they can - the resulting sake will most likely have a fruity and floral aroma. It also has an impact on price as that the more layers they remove, the less they can produce and therefore the higher the polish, the more expensive the sake.