Shubo (yeast starter)

Shubo (also called moto) is a cultured starter of yeast that plays a particularly important role in sake brewing, forming the foundation for the fermentation of the moromi (fermentation mash). It is made by mixing steamed rice, rice koji, and water, then adding a superior sake yeast and cultivating it. This process is called moto-zukuri or shubo-zukuri (making the yeast starter). There are several types of shubo, including kimoto, sokujo, and yamahai, each of which cultivates the yeast by a different method. In kimoto, naturally occurring lactic acid bacteria are used to suppress the growth of unwanted microbes while the yeast is cultured over a long period. In sokujo, by contrast, lactic acid is added in advance so that the shubo can be prepared in a shorter time. Shubo is the source of ethyl alcohol production in sake brewing, and its quality and characteristics greatly influence the flavor and aroma of the finished sake. The choice of shubo and the way it is made are therefore essential factors in successful sake brewing.