The bubbles in sake come from alcohol fermentation

The appeal of refreshing sparkling sake is the fizzing bubbles that rise from the surface. There are also slightly effervescent varieties of unpasteurized sake. These bubbles are all naturally derived, but where exactly do they come from?

To explain this, let's first review how alcoholic fermentation works.
Alcoholic fermentation is a reaction in which yeast breaks down sugar to produce alcohol and carbon dioxide. The base of sake produces alcohol at the "moromi" stage, when the sugar produced by rice koji meets the yeast in the preparation tank and fermentation progresses, producing bubbles of carbon dioxide. This carbon dioxide usually disappears during processes such as pasteurization, but sake that is left in the bottle is called sparkling sake (some brands add carbon dioxide later).

Bubbles are a nuisance, but they also play an important role

These bubbles from the moromi are a nuisance for sake brewers. The preparation tank is filled with carbon dioxide, and if you accidentally fall into it, you can suffocate and it can be a life-threatening accident. Also, have you ever tried to smell the moromi during a tour of a sake brewery, but stuck your face too far into the tank and ended up with a stinging nose? This is because the carbon dioxide reacts with the water in the back of your nose to turn carbonic acid, which gives off a mildly acidic stimulant smell?

Especially at the peak of the fermentation of the mash, a large amount of rising foam (high foam) is produced, so the foam remover is rotated to suppress it so that it does not overflow from the tank. In the past, it was a lot of hard work, with people keeping watch over it all night. The reason this foam is difficult to eliminate is because the yeast adheres to the bubbles, covering them. Therefore, if the yeast overflows and is reduced along with the foam, the fermentation power will weaken, so care must be taken.

At the same time, the foam in the mash also plays an important role as a marker for fermentation. Firm foam is proof that the yeast is working well. By observing the changes in the foam from day to day, you can see how well fermentation is progressing.

Nowadays, most yeasts are non-foaming.

Currently, most sake breweries use "non-foaming yeast." This yeast does not produce thick bubbles during fermentation. A yeast mutation was discovered in 1963, and after extensive research, "Kyokai No. 701" was isolated for the first time a few years later. In fact, the first discovery was in 5, but since the scale of sake brewing in breweries was small at the time and there was enough manpower, it seems to have disappeared without attracting attention.
By the way, the "01" at the end of the Kyokai yeast name indicates that it is a non-foaming yeast, so "Kyokai No. 701" is Kyokai No. 7's non-foaming yeast. In the case of non-foaming yeast, the progress of fermentation is determined by component analysis.

The birth of the convenient non-foaming yeast has made the work of sake brewers a little easier. Although it has no direct impact on us as consumers of sake, it is still something to be happy about.

Writer/Sake Master Eriko Fujita
Expand your world to a sake specialty store in Osaka and encounter a variety of sake and sake breweries. Favorite sakes are Akishika, Ohgi, and others.
Other than drinking, my hobbies include collecting minerals and raising swallowtail butterflies.