Sake Yeasts

Yeast: a single cell fungus whose activities have been known to man for far longer than we have known about yeast itself. We have evidence of yeast being used as far back as four thousand years ago in Egypt. They used yeast for both baking and brewing. Wine was also present in this period.

In 1857 Louis Pasteur proved that fermentation was the results of living yeast rather than a chemical reaction. In this work, Pasteur showed that as oxygen is added the growth of the cell count increases and fermentation slows. Not only did this show the significant of yeast but also its two distinct modes of operation: the aerobic and the anaerobic. In the aerobic mode, yeast reproduce by budding, a process of a child cell being created and split off from the parent cell. The anaerobic mode proceeds with little to no growth in the number of cells but with increased alcohol and CO₂ production. Alcohol and CO₂ are produced in equal amounts based on the following formula:

C₆H₁₂O₆ -> 2 C₂H₅OH + 2 CO₂

In addition to alcohol and CO_2, over 500 compounds are produced. These compounds contribute flavors and aromas to the ferments they produce. Some of these compounds are esters which create many of the taste and aroma differences between strains of yeast. The metabolic pathway that creates all this is pretty complex. The pamphlet, “The Fungus Among Us, gives the following pathway:¹

You may be thinking, “Well, this is all well and good but where does the ruber meet the sake?” It all starts with the moto. The moto is where the yeast put on some weight and build up their cell walls to prepare for a massive reproduction campaign to increase they numbers.  This is the sage, aerobic, where the number of yeast cells is increased to the optimal level. The open environment and frequent stirring help to supply needed oxygen for cell growth. This is the most crucial time for the yeast. Enzymes produced by the koji works on the rice to convert the rice starch into sugars for the yeast to work with. During the moto the yeast need both the oxygen and sugar but this is not all. Yeast needs other nutrients too.

Yeast needs Free Amino Nitrogen (FAN). No less than 130mg/l are needed for the yeast not to struggle during fermentation; levels of 300-500mg/l are ideal. In addition to this yeast needs the following:

Since rice, koji and water are not the best sources for these we generally add them, or something close to them like commercially packaged yeast nutrients.

Once the oxygen in solution starts to run out, anaerobic, the yeast will move from reproduction to fermentation. This is not to say that reproduction will stop, it won’t. However, it will slow to a trickle. The same is true for fermentation during the aerobic phase; there is a trickle. Fermentation will continue until all the sugars have been transformed into alcohol or at the point the sugar/alcohol/CO₂ mixture in the ferment overwhelms the yeasts resistance to their toxic effects.

What yeast strains are used for brewing sake? Well, back in the early 1900s the Central Brewers Union in Japan started collecting pure strains from particularly good batches of sake and made these pure strains available to all breweries. Eventually these pure strains where given numeric designations. So far there are from #1 to #15. Many breweries now use these yeast strains but there is a re-insurgence in the use of private strains.

In addition to these 15 strains there are some variations that have been modified to produce less foam during fermentation. These strains, where they exist, carry the numeric designation of the parent strain numeric value times 100 plus 1. For example strain #9’s low foaming strain is #901 = #9 * 100 + 1.

For home brew sake here in the US we have two of these strains available; #7 and #9. These stains are available from two different suppliers. Yeast #7 is available from White Labs and yeast #9 is available from Wyeast Laboratories. The details on these yeasts from their respective web sites are as follows:

The information these suppliers are providing is of little use to the sake home brewer. Sake brewing generally pushes into much lower temperatures and produces alcohol levels much higher than those given on the supplier web sites. Despite this both these suppliers produce very good yeast.

This article has gotten too long, so I will stop here. Later we should cover more on some of the specific yeast strains.