Sake Brewing Science Vol. 5 - The Significance of Making Yeast Starter (Shubo)

Sake brewing is, of course, different from brewing beer or mead, possessing many complex and fascinating facets. In this column, I would like to break down these elements as logically as possible to help us understand them.

This time, we will look at the significance of the sake starter, or shubo.

What is shubo in the first place?

Shubo (also known as moto) is a "yeast starter" prepared during sake production to multiply yeast and stabilize fermentation. In beer brewing, this is comparable to using a starter to acclimate yeast from a small quantity to an environment, gradually scaling up to the main batch.

In sake production, there are many unique methods for creating shubo, each with its own interesting characteristics.

The purpose of making shubo

Fermentation does not start simply by adding yeast.
An appropriate environment is necessary for yeast to multiply reliably and for fermentation to proceed stably.
The role of creating that environment falls to the shubo.

This seems to be the sake-centric way of thinking. This "environment" is necessary in terms of sugar concentration, pH levels, and various other conditions. This makes sense for a natural fermentation approach, but I hope to explore the significance of continuing to create shubo in the modern day, where that is not necessarily the case.

From a beermaking perspective, if the cell count is low, you could simply add a larger amount; the reason to cultivate from a small amount usually relates to costs or using the best generation of yeast.

In sake brewing, there seems to be a culture of using shubo to suppress the growth of undesirable bacteria and ensure smooth fermentation. This can be considered a fermentation design unique to sake, distinct from beer or wine.

Germinated brown rice, malted barley, and the sake fermentation environment

Taking a slight detour to talk about brown rice.

Beer uses germinated barley (malt).
Malt contains saccharification enzymes, which convert starch into sugar, allowing it to prepare the conditions for yeast to produce alcohol from the grain all on its own.

So, if you germinate rice, can it ferment just like malt without using koji?

The answer is leaning heavily toward "no."

While germinated brown rice sees an increase in proteolytic enzymes, it does not generate enough saccharification enzymes. This seems related to the different survival strategies of wheat and rice, as well as the sugar composition of their starches (the proportion of amylose).
For this reason, the method adopted for sake is not to rely solely on germinated brown rice for saccharification, but to use koji mold to drive it.

Another major difference is the way the fermentation environment is prepared.
Wine naturally uses acidic fruit juice, and beer utilizes the antibacterial properties of hops.
Conversely, before fermentation begins, sake uses shubo to create an acidic environment that suppresses the growth of spoilage bacteria, thereby stabilizing fermentation.

It is difficult to create an acidic environment with germinated brown rice alone, and because it is rich in amino acids, it is vulnerable to bacterial contamination. "Rice" required shubo to prepare the environment.

Designing the fermentation environment

The role of shubo is to multiply the yeast that supports fermentation and suppress spoilage bacteria.
Therefore, a strongly acidic environment is required for shubo.

In the modern-looking sokujo-moto, an acidic environment is prepared in a short period by adding artificial lactic acid. Conversely, in kimoto, lactic acid bacteria are naturally cultivated to create an environment where yeast can multiply stably.

In kimoto, lactic acid bacteria proliferate in the initial stages of fermentation, lowering the pH of the shubo. In this process, non-resistant bacteria and wild yeast are culled, and only acid-tolerant yeast survives. As these are cultivated, they can slowly ferment even in a highly acidic environment.

Moreover, quite interestingly, yeast that cannot tolerate the low-pH environment undergo autolysis, releasing amino acids and nucleic acids. This self-decomposition is called "autolysis," and it has the potential to lead to the complex flavors found in sur lie in wine, long-aged beers, and, in the case of sake, the unique profile of kimoto.

By the way, I will explain later why this is so interesting, but I remember slapping my knee in excitement when I realized, "So that’s why shubo was significant!"

Shubo as preparation for fermentation

Just as malt provides saccharification enzymes to drive sugar conversion, in sake, koji creates sugar and shubo prepares the fermentation environment.

Shubo is not merely a process of multiplying yeast; it is also a process of determining which yeast will be utilized and which bacteria will be eliminated. While sokujo manages the environment through artificial lactic acid, kimoto leaves it to the natural fermentation process, adding unique elements to the sake's quality. This is easier to understand in the context of "natural wine" or "naturally fermented beer," which are even more complex.

The direction of fermentation is already determined at the shubo stage.
The environment created here seems to significantly change the final outcome of the sake.

Next time, we will discuss kimoto.