Yeast refers to microorganisms that facilitate fermentation in food items, and they are an indispensable element in making Japanese sake from rice. Even when using the same rice and water, the taste and aroma of Japanese sake can vary depending on the type of yeast used. Most of the yeast used by sake breweries in Japan is referred to as "kyoukai koubo," which is managed by Brewing Society of Japan. Breweries typically select and purchase yeast from Kyokai Koubo 1 to 19 (there are approximately 30, including improved ones) for brewing. However, nowadays, in the industry, Kyokai Koubo 7 to 19 are commonly utilised. Even Senjo, for instance, primarily uses yeast from Kyokai Koubo 7 onwards. Sakes brewed with yeast Kyokai Koubo 1 to 6 have become increasingly rare in recent times. The first discovery of Kyokai Koubo 1 dates back over 110 years ago in 1906 (Meiji 39). Even Kyokai Koubo 6 traces its origins to 1930 (Showa 5). As societal shifts and changing preferences in taste and dietary habits occurred over time, the demand for different types of sake and the yeast used evolved. Consequently, Kyokai Koubo 1 to 6 gradually became known as "rare yeasts" amid these changes.
