We've discussed sake rice, water, and koji, the mold that makes sake possible. But there's one important sake ingredient left: the yeast. While yeast is necessary for the fermentation of every alcoholic drink, sake yeast is a bit different from the yeast that ferments wine and beer.
Why bother knowing about yeast? Because the strain of yeast that a brewer selects—as well as the way the fermentation is carried out—has a direct affect on the sake's aroma and flavor. Knowing a little about yeast can help you pick out flavors in sake, and guide you toward discovering which types of sake suit your palate best.
While wine and beer are usually fermented at temperatures between 64° and 85° Farenheit (with red wines on the hot end, and lager cooler, fermenting between 45 to 55° F) premium sake is fermented at much colder temperatures (often 32° to 48°) Sake brewers believe that a slow, cold fermentation will elicit the most favorable aromas from the sake yeast. Because the cold temperature prevents the yeast from acting too quickly, fermentation of sake takes quite a while: usually between 18 to 32 days once transferred to the tank.
Sake fermentation is also almost always done in an open tank; sake brewers fear that a lid on the tank would prevent precise control of the temperature in the tank as fermentation occurs. The fermentation of the yeast starter creates a layer of carbon dioxide that helps to protect the mash from wild yeast and bacteria. This layer of gas is so powerful, there are accidental deaths every year from brewery workers sticking their heads too far into tanks. Lactic acid (added or occurring naturally) also protects the mash from unwanted microorganisms.
But sake breweries must be kept extraordinarily clean and sterile as a precaution against contamination. Brewery workers and guests are required to wear hairnets, and constantly wash their hands and spray them with alcohol. Many breweries also prohibit their staff from eating natto (soybeans fermented with bacteria; a major staple in the Japanese diet), for fear that the bacteria will affect the fermenting sake.
How much does temperature really matter? I've tasted two sakes side by side, where the only difference was temperature during fermentation. The colder-fermented sake was considerably more fragrant than the other. The brewmaster explained to me that this was achieved by keeping the temperature so cold that the yeast almost died, and then warming it up so that the yeast recover and propagate. By doing this, the yeast became much stronger and developed more aroma in the finished sake.
But the goal is not always to develop the highest levels of aroma; that depends on what style you're aiming for.
While many prefer a bold, fruity style of sake for a first glass of an evening, it's often the more subdued styles that are appropriate for drinking throughout a meal. For Kyoto style kaiseki dining, chefs often prefer sakes that are far more subtle and delicate.
Although quite a few breweries pride themselves on developing their own yeasts, the majority of sake brewers rely on sake yeasts that have been cultivated and tested over time. Some popular yeast strains used today include:
#7: The most commonly used sake yeast, developed by Miyasaki brewery in Nagano. Sake brewers rely on this yeast for its consistency and stability in fermentation, as well as its subdued, soft aromas. This yeast is commonly used in complex sakes like junmai and honjozo.
#9: This yeast was discovered at Koro brewery in Kumamoto, and is prized for strong, slow fermentation resulting in fragrant sakes. It's mostly used for elegant sakes with balanced fruit and floral notes, like ginjo and daiginjo.
#1801: The '01' signifies that this yeast is non-foaming, which is preferable for some sake brewers because it's cleaner, and sake can be brewed up to the top of the tank, increasing efficiency. (Brewers I've asked about this yeast assured me that there's no quality difference between foaming and non-foaming yeast.) This yeast was discovered fairly recently, but it has become very popular, due to its strong floral and fruity aromas. #1801 is mostly used in sakes with sweet, fruity aromas, like daiginjo.
Yeast isn't everything; temperature of fermentation is essential in shaping the finished product. But as a general guideline, if you tend to prefer fruitier, more wine-like sake, you should look for sakes brewed using expressive yeasts such as #9 and #1801.
If you're looking for subtler, more earthy rice aromas, sakes brewed with yeasts like #7 are a better way to go. Many importers have started listing types of sake yeast used on labels to help consumers navigate through the many sake choices. Identifying sake yeast strains when tasting is a great way to hone your sake tasting skills.