The choice of yeast in sake brewing exerts marvelous leverage on the aroma and style of the final product. And, while creativity and diversity lead to better sake over time, things can indeed get out of hand. Today, there are so many different yeasts -- and ways of combining them -- that it almost ceases to be worth the effort to try and follow developments.
Without yeast, there is no alcohol. Yeast takes the sugars converted by the koji mold from the starches in the rice and breaks it down into alcohol and carbon dioxide. Yeast also creates an array of chemicals that give sake its body, fragrance and, to some degree, its flavor.
Over the past 100 years or so, the Japan Central Brewers' Association has isolated these yeasts as they were discovered and made them available to breweries. The first half-dozen or so that were discovered are no longer being used, but at present there are still about a half dozen association yeasts that are commonly seen. These various yeasts are identified by number, such as Association Yeast No. 9 (kyokai kyu-go). Each has unique qualities, and inherent strengths and weaknesses.