Yeast has been one of those great technical advances in the sake world — one factor that separates great ginjo of today from the run-of-the-mill sake of yesteryear. Over the last 10 years or so, dozens of new yeast strains have been developed and incorporated into sake brewing.
Chemically, yeast converts sugar to alcohol and carbon dioxide. It is the heart of the creation of all alcoholic beverages. But different yeast strains will produce different things, like esters, alcohols, acids and other chemical compounds that affect the nuances of fragrance and flavor.
Each yeast will give rise to its array of chemical compounds, with scary names like ethyl n-caproate and isamyl acetate. Which esters, alcohols and other compounds are produced is highly dependent on the temperature at which fermentation takes place.
How does one strain of yeast physically differ from another? There are many ways, but not all are so obvious. When the cells of two yeast strains are set next to each other in a microscope, the average person is not likely to be able to tell the difference.
The differences are more evident in other things, such as the length of the life cycle of the yeast: How long will it work before becoming dormant, or how robust is it against alcohol and/or temperature. Another factor is which alcohols, esters and other things it tends to give off as byproducts of its life cycle during fermentation.
Although we speak of a yeast being “developed,” it is more a matter of being isolated. The process of coming up with a new, specialized yeast strain, which usually takes about three years, is actually a kind of reverse engineering.
Yeast is usually isolated by starting with a tank of sake being made. It may be one in which many strains of naturally occurring yeast strains were allowed to initiate the fermentation. Some of the thick foam on the top of the moromi (fermenting mash) is taken and analyzed. This foam has the highest concentration of yeast cells in the tank. If the sake comes out well, the strain of yeast that is most populous is isolated and reproduced for further study. If it continues to demonstrate the desired qualities, it is made available on a larger scale.
This first took place in the early 1900s. The Central Brewer’s Union would take pure yeast strains that had been isolated and make them available to kura across the nation in pure form, usually in small glass vials. These yeast strains were assigned numbers by the Central Brewers’ Union. At present, they are up to number 15.
Each one has its own special qualities. Yeasts #1 through #6 are no longer in use, as apparently the acid produced was too strong. Yeasts #7, #9 and #10 are perhaps the most important to remember these days. Yeast #7, discovered by Masumi of Nagano, is the most commonly used yeast in the country. It has a mellow fragrance and robust strength during fermentation.
Yeast #9 is the most common yeast for ginjo-shu, due to its wonderful fragrance-creating abilities, and fairly healthy constitution during fermentation. Yeast #10 produces a lower-acid, fine-grained flavor in sake, but is a bit fickle at all but the lowest fermentation temperatures. More recently, Yeast #14 (low in acidity with lots of pears and apples in the fragrance) and Yeast #15 (very fragrant but not of such robust constitution) are often seen in regional sake. They will often go by other names, such as Kanazawa Kobo and AK-1, respectively.
In addition to these publicly available yeast strains are dozens of others used on varying scales. Many are proprietary, having been developed by kura, or more commonly, developed by prefectural brewing research institutes and used by kura in that prefecture.
A few examples of this include F-701, also known as Utsukushima Yume Kobo from Fukushima, the terrifically fragrant Alps Kobo from Nagano, and HD-1 and NEW-5, which help Shizuoka sake be the wonderfully drinkable brew that it is.
And finally, no discussion of sake yeast would be complete without mention of the awa-nashi kobo, or foamless yeasts. Yeasts #6, #7, #9 and #10 all have cousins that do almost an identical job without producing the massive amounts of foam that rise and fall and breathe majestically throughout the course of the fermentation.
These are designated by adding an “01” to the number. For example, #901 is a foamless version of #9. Why foamless? This saves hours and hours of grueling cleaning time, scraping the remnants of the foam from the side of a tank before starting the next batch. Also, since a third of each tank must usually be reserved for the rising foam, more sake can be brewed with less space using such awa-nashi yeast.
Yeast is one of the newer developments in the sake world that we can all follow with interest. Although it may not make much of a difference once you are sipping, learning to identify the qualities of a yeast strain and searching for and comparing fragrance and flavor profiles can be extremely instrumental in improving your palate.
It can also simply be a lot of fun. More and more commonly, especially for decent sake, the yeast used is listed on the bottle. Also, I’ll soon upload a detailed chart on the various yeast strains on my Web site: www.sake-world.com
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Kariho (Akita Prefecture)
Seimai-buai: 55 percent
This kura is a sister kura to Dewazuru, one of the finest and most consistent brewers in Akita. Rokushu has a somewhat full body but with the rougher edges polished away, leaving a light sake with a good amount of content as a result. Leaning just a bit on the dry side, the flavor is tight but laced with various elements of nuts, rice and even a trace of richer fruit.
A charming liveliness comes out at room temperature, but a calming crispness is most apparent when cool or chilled. Versatile sake indeed.
Although Akita is the home of #10, #15 (AK-1) and other yeast strains, this sake is made using #9. Kariho and Dewazuru do, however, make sake from these other Akita yeast strains as well.
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