THE INGREDIENTS The Rice The Yeast The Water The Koji The People Land & Weather Yeast and Sake Brewing The importance of yeast in the production of sake is extremely important, as yeast influences many elements of sake taste, most noticeably sake fragrance. And since our sense of taste is highly influenced by (if not dependent on) our sense of smell, this is crucially important. Indeed, the proper tasting of sake requires a well-developed sense of smell. There is the initial sniff, noting the general fragrance of a sake. Then comes the fukumi-ka, or the fragrance that arises as you hold the sake in your mouth and breathe. There is also the modori-ka, a third fragrance that becomes discernible immediately after you expel or swallow a sip. Yeast will affect all of these. What is Yeast? What Does it Do? 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, and acids and other chemical compounds that affect the nuances of fragrance and flavor. Each yeast will give rise to its own specific array of chemical compounds, with scary names like ethyln caproate and isamyl acetate. These will be present in varying quantities, depending on the choice of yeast and the successful progress (or lack thereof) of the fermentation. Which esters, alcohols and other compounds are produced are highly dependent on the temperature at which fermentation takes place. All of this will help to determine the character and nature of the sake. In this way, the choice of yeast also directly affects flavor. Are All Yeast the Same? How does one strain of yeast physically differ from another? There are many ways, but not all are so obvious. It is often not simply a matter of size or physical appearance. In fact, 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. Like the length of the life cycle of the yeast: how long will it work before becoming dormant, or how robust or fickle it is against alcohol and/or temperature. Which alcohols, esters and other things it tends to give off as by-products of its life cycle during fermentation is of course, another important factor. How is Yeast Developed? 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, usually takes about three years, and 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, many strains of naturally occurring (i.e. floating in the air) yeast strains were allowed to initiate the fermentation. Some of the thick foam on the top of the moromi (the 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. History of Yeast in Japan In the early 1900s, the Central Brewers Union first began taking pure yeast strains that had been isolated (usually by larger breweries from particularly good tanks of sake) and making them available to kura (breweries) across the nation in pure form, usually in small glass vials. These yeast strains have since been assigned numbers by the Central Brewers Union. Yeast Table 1 No longer used, as acidity too strong.  2 No longer used, as acidity too strong.  3 No longer used, as acidity too strong.  4 No longer used, as acidity too strong.  5 No longer used, as acidity too strong.  6 No longer used, as acidity too strong.  7 Discovered by brewer Masumi. Mellow fragrance, strong in fermentation. Most commonly used yeast in the country. 8 No longer used, as acidity too strong.  9 Discovered by Koro, a brewer in Kumamoto Prefecture. High fragrance, solid fermentation. Many ginjo yeasts are #9-based strains.  For much more on this yeast, click here.   10 Tohoku Moromi. Low acid, fine-grained flavor. Commonly used in Tohoku. 11 No longer used, as acidity too strong 12 No longer used, as acidity too strong 13 No longer used, as acidity too strong 14 Unknown. Also known as "Kanazawa Kobo." Used a lot in Shizuoka too. Low acid, pears and apples in nose.  15 Akita Moromi. "AK-1," or "Akita Hana Kobo." Very lively fragrance and characteristic nose/flavor; but needs to ferment slowly and at low temperatures.  601 "Awa nashi" or foamless version of the 6, 7, 9 and 10 yeasts. Nothing else changes, say most.  701 Same as 601 above. 901 Same as 601 above. 1001 Same as 601 above. Common Yeast Strains At present, they are up to number 15. Each one has its own special qualities. Yeast #1 through #6 are no longer in use, as apparently the acid produced was too strong. Yeast #7, #9, and #10 are perhaps the most important these days. Yeast #7, discovered by Masumi of Nagano, is the single most commonly used yeast in the country, with its 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 (which is low in acidity with lots of pears and apples in the fragrance) and Yeast #15 (which is very fragrant but not of such robust constitution) are often seen used in finer sake, especially in particular regions. There are, on top of the publicly available yeast strains, dozens of others that are used on varying scales throughout Japan. Many of these are proprietary, having been developed by kura and used only by them, or more commonly, developed by prefectural brewing research institutes and used by kura in that prefecture. Many of these are wonderful indeed, and go well with the water and rice of that region. A few examples of this include F701, also known as Utsukushima Yume Kobo from Fukushima, the wonderfully 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. Yeast #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 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.   However, some experts say that the these foamless versions are not quite what their bubbling cousins are, in terms of the final product. Fragrance and flavors are not quite as refined, although they may be 99 percent of the way there, say some. This is somewhat unsubstantiated, and may be nonsense. It is most likely affected by preconceived notions, but such talk has been heard. Often these yeasts will be given working names, names that are more romantic than a simple number. One example given above is the F701 from Fukushima, known much more commonly as the Utsukushima Yume (dream) Kobo. Others include mighty #9, also known as Kumamoto Kobo, in honor of the prefecture from which it originally comes (more specifically, from the kura that brews the fragrant and very fruity sake called Koro. Number 10 was discovered by a gent named Chikara Ogawa, so that it is often called Ogawa Kobo (especially in northern Japan, where it was isolated). Number 14 is often referred to as Kanazawa Kobo, and #15 is commonly known as AK-1,  for Akita Kobo, both in recognition of their origins. Why Pay Attention to Yeast? Yeast is one of the newer developments in the sake world that we can all follow with interest. Over the last ten years or so, dozens of new yeast strains have been developed and come into use. This 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. 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. Be sure to look for and try to identify the various special qualities of the various yeast strains. It is also interesting and important to note that yeast development is one area of progress that has taken place outside of the actual brewing process, yet still affects the final product greatly. In other words, great sake might be brewed using a brand new yeast strain, but the actual brewing process still relies on old hand-made ways that eschew modern machinery.