Although all that you will ever need to know about a sake is contained in one, intention-laden sip, sometimes the technical mumbo jumbo can be fun to study as well. The industry always seems to offer one more piece of information every few years, be it the amino acid level, the number of days the tank fermented, or something else. One such "something else" is kasu-buai.
Rice, rice milling and yeast are not the only things that make each sake different. Some tanks of sake are allowed to continue fermenting to (literally) the bitter end, so that every last drop can be pulled out of the fermenting rice mixture. While this will of course increase yields, it will naturally take its toll on quality. Many rough, superfluous flavors will result from this overfermenting.
Finer sake, however, is pressed earlier, so that not all the rice has completely fermented. This results in more refined, elegant flavor profiles.