(photo: the lab at Dassai)
In our last issue (ST#17) we looked at the choice of yeast in sake brewing. That article covered how much yeast contributes to aromas–more so, in fact, than just about anything else in the sake brewing process. We also talked about the main sources for yeast available to sake brewers, mainly the Nihon Jôzo Kyôkai (Brewing Society of Japan), but some others as well.
There is simply too much to say about yeast to fit into one Sake Today article, though. So let us look at another handful of interesting aspects of yeast used in making sake.
The aforementioned Nihon Jôzo Kyôkai was originally formed as a research group, but it also became involved in providing pure sources of specific yeasts to the sake-brewing industry. These yeasts are referred to as “Kyôkai Kôbo” (kôbo meaning yeast).
Kyôkai Kôbo can be easily recognized on labels because they are numerically named. For example, we often see “Nanagô Kôbo,” or “Kyôkai Number Seven” (as just one example) on labels and promotional material. Most have a lineage to them, meaning we know their family tree, so to speak. DNA analysis has furthermore found that most of them are, in fact, related.
The most commonly seen Kyôkai Kôbo are Numbers 6, 7, 9, 10, 14 and, lately, 18 and 19 (Numbers 1 to 5 are no longer around). Less commonly seen are Numbers 15, 16 and 17. Not every number has an actively used yeast associated with it. Number 7 is probably the most commonly used yeast in Japan, followed by Number 9 and its mutant versions. More recently, Number 14 is growing in popularity among brewers, as is Number 18-01.
Each of these yeast strains differs from the others in several ways, including the robustness of fermentation, stamina, and tolerance to alcohol. But more relative to style are the amount and types of acids that are created, as well as the types of esters and other aromatic compounds that are produced during fermentation. In other words, each yeast strain leads to different flavors and aromas in the resulting sake.
Basically, the higher you go in numerical nomenclature, the fruitier the resulting sake. So a 9 is fruitier than a 7, and a 15 even more aromatic than a 9 might be. In truth, it isn’t quite that simple, but as a pragmatic guideline, it works well. Also, in general, the higher the number, the less acidic the flavor, although that is an even more dubious rule of thumb. Nevertheless, these help in grasping what can be expected from the various Kyôkai Kôbo.
Another naming convention worth remembering is the -01 suffix to Kyôkai Kôbo. The presence of -01 (as in 7-01, 9-01 and 18-01) indicates a low-foaming version of the same yeast. So yeast Number 9-01 is a low-foaming version of a Number 9. They are almost identical other than that.
Why might a low-foaming yeast be advantageous? Because there is less mess, but also, yields are significantly higher. When using a normally foaming yeast, brewers need to leave the top 20 to 25 percent of the tank open so as to contain the great swaths of foam that arise. Should they overflow the tank, fermentation will significantly weaken. But with non-foaming yeast this is not a concern; they can get more fermenting mash into the tank, and therefore more sake from each tank.
Of course, they lose the visual and other sensory input that a big head of foam provides, but modern instruments make that less necessary these days. Or so say some brewers.
Again, many of these Kyôkai Kôbo have traceable ancestry. For example, Number 6 was discovered in the kura (brewery) making Aramasa in Akita; Number 7 in the kura making Masumi in Nagano; and Number 9 at Kôro in Kumamoto. While such information is for the most part relegated to sake lore and tradition, it is sometimes used in marketing efforts by the respective companies.
Remember that all the above information that can be deduced from the numerical name of a sake applies only to the Kyôkai Kôbo; other yeasts do not follow such conventions. While these are in fact the lion’s share of yeast used, there are plenty of others outside of the Kyôkai Kôbo that are significant. Many are developed by prefectural research institutes and the like, but there are other sources as well. Let us ask again: with such a good range of yeast already available, why would we develop more? If it works, why fix it, right?
There are a number of reasons, including finding the next big thing in the yeast world, local (prefectural) identity and terroir, and, of course, winning competitions. In fact, there are so many yeasts out there that it is all but impossible to keep track of them. Perhaps the best strategy is to note them when the information is available, and learn what one can along the way. Many–but not all–of these recently developed yeasts lead to sake that is fairly aromatic. Few are as balanced and grounded in flavor and body as the Kyôkai Kôbo of old.
This discussion leads to a practical theory about the world of sake yeast strains, something that one can remember easily and apply to any yeast one might encounter. In short, the world of yeasts used in sake brewing can be roughly divided into what I call “classic yeasts” and “modern yeasts”. That terminology is not universal; it is my own way of expressing a concept that is referred to in other ways in the sake world. Some say “single-digit yeasts and double-digit yeasts” or use uber-techno-speak and talk about cerulenin-resistant yeasts, for example. But the classic and modern breakdown is actually quite easy to follow.
Classic yeasts are those that produce moderate aromas–think banana, melon and grapefruit. Sake made with classic yeasts are comparatively more solid, structured, and grounded if not simply more balanced. Modern yeasts give rise to apple, pineapple, other tropical fruit, and not uncommonly licorice and anise. The acids that arise when modern yeasts are used are sharper, often lending more earthiness and bitter elements that give depth to the flavor profile.
Of course, reality is slightly more complicated than that. While each yeast has tendencies in terms of what compounds it will produce, that is all affected by countless other factors in the brewing process. In general, though, classic yeasts are more balanced and subdued, while modern yeasts are more expressive and untethered. Classics usually stand up to time in the bottle better, whereas sake with modern yeast is tastier much sooner after pressing–immediately, in fact.
Looking at the main Kyôkai Kôbo, the Numbers 6, 7 and 9 would be classic. Number 14 would be right on the border, while those yeasts numbered above Number 14–in particular, 18-01–are decidedly modern.
To reiterate, these classifications and terms are my own! But the concept is fairly widely accepted and discussed in the sake industry using different terms. Neither of these two groups are unequivocally better than the other, though they do have their strengths, weaknesses and idiosyncrasies. Having said that, there seems to be a revival among younger brewers in the use of classic yeasts, particularly Numbers 7 and 9. It’s not a huge movement, mind you, but at least a noticeable swing of the pendulum.
Not surprisingly, long ago producers learned how to blend yeasts in sake brewing. A very aromatic modern yeast can be blended with a full-bodied classic yeast, with the result being an aromatic, full-bodied and balanced sake. It happens all the time.
Discovering just how it is accomplished can be frustrating for sake geeks, though. The ratio of the respective yeasts used, the timing of their addition, and how that blending is done is all over the map. Methods vary vastly from brewer to brewer even when using the same yeasts.
Fortunately, understanding and remembering it all is not a requirement for enjoying sake. As mentioned in the issue #17 article, there are brewers who choose not to divulge the yeast or yeasts that are used so as to avoid inviting preconceptions, or letting the yeast be a distraction. As interesting as the technical stuff might be, assessing the aromas and flavors with your senses is really what sake appreciation is all about. The numbers are secondary.
