The fourth and final critical component to saké brewing is yeast. Like beer and wine, saké needs yeast to ferment. To create food for the yeast, rice is polished, washed, steamed and covered with Koji. By doing this we convert the hard grain into soft, sweet kernels, perfect for consumption by yeast. Not only is yeast a necessity when it comes to converting the sugars created by the Koji into alcohol, but its impact on flavor, aroma, texture and acidity levels in the final product also cannot be understated. Brewers take great care in selecting the ideal yeast strains for their styles. There are about 14 commercial yeast strains available to brewers. Each has its own aroma and flavor profile and interacts with the rice in different ways. Saké yeast strains are resilient, continuing to create alcohol beyond the point when many other brewing yeasts become dormant. This is why the alcohol content or ABV of saké can reach upwards of 20-22% during the fermentation stage.
For modern brewers, the task of identifying the right yeast has become much easier as entities like the Brewing Society of Japan meticulously cultivate and catalog a multitude of saké yeast strains, creating a trusted source for brew masters around the world. This is important as yeast is a living organism and will mutate or adapt, inherently changing the ways it will influence a given batch. Similar to the rice, water and Koji ingredients, yeast strains come in a variety and they can be mixed and matched to create many different expressions.
Let's look at some of the staples of saké yeast and how they impact saké. Brewing Society yeast is cataloged numerically.
# 1 through #8
Identified for their ability to produce a complete and successful fermentation means these yeast strains presented a lower risk option for brewers in the early-to-middle 1900s. The strong ferments of this yeast generally create saké with higher acidity and subtle aromas.
Of the original eight strains, #6 and #7 are still prominently used. #7 is particularly noted for its pleasant and subtly sweet and fruity aromas, making it the standard for many Futsu-shu saké and fairly prevalent in Honjozo and Junmai styles as well.
#9 and #10
Moving past the initial pursuit of yeast that facilitated a complete fermentation, #9 and #10 successfully brought forward the fruity and floral aromas that have become standards for the Ginjo and Daiginjo styles.
#14 and #18
Over the past forty years, the desire and efforts to push aromatic boundaries set by #9 and #10 have only intensified, yielding a new modern age of delicate, promising and smooth saké.
In many cases, saké created using #14 and #18 forego acidity in favor of pronounced fruity and floral aromas and flavor. These qualities are punctuated by a delicate and juicy texture, especially when served chilled. One of the biggest challenges for Brewmasters with these strains is to not disappoint the consumer with a flavor that doesn’t match the beautiful aromas.
These are the main yeast strains used in today's saké. As you can imagine, a deeper layer of complexity exists within this ingredient category as some breweries invest their resources in creating proprietary yeasts and yeast blends to give their products a true one-of-a-kind quality. Murai Family in Aomori and Hakutsuru in Kobe are both famed for their devoted research into proprietary yeasts. Brewers have also been creative when it comes to blending yeasts to dial in the perfect fermentation strength, aroma, and flavor to their desire. Our own Momokawa Diamond (#701, #1801) and Kasumi Tsuru’s Yamahai Junmai Ginjo (#901, #1401) are two delicious examples.
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Stay tuned in two weeks when we'll spotlight the largest Japanese saké brewery, Hakutsuru Brewing of Japan.