The classification is based on how much the rice is polished and whether or not distilled alcohol is added. Seimai-buai refers to what remains of the grain after polishing. The seimai-buai is a measurement of how much the rice grain has been polished(milled). This number, expressed as a percentage, and is a parameter for classifying sake.
The smaller the grain (the more it’s polished) the more aromatic and wine like the resulting sake. The larger the grain (the less it's polished) the more subtle the aromatics, more body, umami in the resulting sake.
There are differences to be enjoyed at every level.
If the rice polishing rate is 50% or lower, the sake is called Junmai Daiginjo or Daiginjo.
If it is 60% or lower, then the sake is classified as Junmai Ginjo or Ginjo.
If it is 70% or lower, then it is called Honjozo.
Junmai sakes have no minimum polishing requirements (as of 2010).
Another distinguishing factor regarding classification is whether distilled alcohol (jozo) has been added to sake or not.
Junmai translates 'pure rice (jun = pure + mai = rice). Junmai style sakes have NO distilled alcohol added. (*right-hand column above; 4 different possible classifications: junmai daiginjo, junmai ginjo, tokubetsu junmai, junmai).
Atruten sake have a small, controlled amount of distilled alcohol (jozo) added. (*left-hand column above; 4 different possible; daiginjo, ginjo, tokubetsu honjozo, honjozo). Brewer's alcohol is added to improve flavor and aroma of sake, increase yield and to preserve quality.
These eight terms are legally classified as “Premium sake” in Japan.
Premium sake is required to indicate its rice polishing rate on the bottle. Every bottle shows the degree to which the rice has been polished. (See image here)Futuu-shuor regular sake is all of the other sake that is not classified as specially designated sake under the Japanese law. It is not allowed for regular sake to be labeled as a premium sake grade such as Ginjoand Junmai. It is like our “vinordinaire” or table wine.