Sake Brewing Science Vol.6 - Kimoto

Sake brewing, while certainly distinct from beer or mead, has many complex and fascinating aspects. In this column, I would like to break down these elements as logically as possible to gain a better understanding of them.

My previous column focused on "shubo" (yeast starter).
The ANTELOPE-style summary of why we make shubo for sake is:

1. Because rice lacks the ability to lower pH on its own, it was necessary to introduce external lactic acid bacteria.
2. To select yeast that can continue fermenting for a long period under such an environment, it was necessary to start with a small batch (shubo).
3. The umami components and complex flavors released as the "weaker" yeast die off during the selection process can be expressed as "complexity."

That is the gist. While "multiple parallel fermentation" is also an important point, it’s a more niche topic, so I’ll set it aside for now. To summarize, one might ask, "Why not just finish saccharification at the beginning like in beer?"—but that, too, turned out to be surprisingly profound.

Today, I’d like to introduce "shubo-making techniques."
I want to use a brewing-based approach to explain those things that you hear about often but don't quite understand. Today’s topic is the "Kimoto" method of making shubo.

Let’s get started.

"Kimoto": A Design for Fermentation

"Kimoto" is one of the traditional methods of making shubo for sake.
It is not merely an old-fashioned production method; it is a "fermentation design" that continues to hold its appeal even today (I know there are dissenting opinions, but I’m ignoring them for now).

"Kimoto" is a technique where you don't add lactic acid artificially, but rather wait for natural lactic acid bacteria to multiply, creating an environment that keeps unwanted bacteria at bay while fostering the growth of yeast.
Preparing the shubo takes about a month. Over this long period, lactic acid bacteria, yeast, and other microorganisms exist in symbiosis.

Its greatest characteristics are its strong acidity and complex flavor.
Why is it that Kimoto sake develops a unique complexity and acidity not found in other shubo-making methods (like Sokujo or Yamahai)?

Let's look into the secret behind this.

"Strong" Yeast That Survives in Low-pH Environments

In Kimoto production, lactic acid bacteria first proliferate explosively within the shubo, rapidly lowering the pH. How is this possible?

It lies in a technique called "moto-suri."
Moto-suri is the process of grinding the mixture of steamed rice, koji, and water using a wooden tool called a "kabura-gai." This is also known as "yama-oroshi."
This increases the surface area of the rice, making it easier for external lactic acid bacteria to adhere, thereby achieving rapid lactic acid production.

As a result, bacteria without low-pH tolerance and weak yeast are successively eliminated, and the only ones left surviving are yeast that is resistant to acid and possesses high fermentative power.
Through "natural selection," the yeast most suitable for sake brewing is cultivated.

Furthermore, the yeast that survives this environment multiplies and proceeds with alcohol fermentation.
Within the shubo, meanwhile, the dead yeast cells undergo autolysis, releasing umami components such as amino acids and nucleic acids.

This cycle of "life and death" of yeast gives the sake depth and complexity.

In the Sokujo-moto method, artificial lactic acid is added only in amounts sufficient to prevent the growth of unwanted bacteria, so almost no yeast selection occurs. Of course, it is possible to trigger it, but that feels like it misses the intent of the traditional technique.
Additionally, the Yamahai-moto method, which I will introduce next time, does not use "moto-suri/yama-oroshi," so the dissolution of rice is slower and the increase in lactic acid is more gradual, making it difficult to create an intensely fermentative environment like in Kimoto.

Even more interestingly, "umami from autolysis" is generally considered an off-flavor. This is because it is often a sign of poor fermentation, or if too many cells die, it produces an excess of umami that tastes like sausage.
However, in a low-pH environment like in Kimoto, the amount of dying yeast inevitably increases, and it seems like everything would end up tasting like sausage.

That is precisely why the shubo-making process of "starting with a small amount and selecting yeast that won't die" is so important. In other words, the strength of acidity and the complex flavor achievable only through Kimoto are completed by having only selected yeast survive and adding a small amount of umami produced solely during the shubo stage.

Aging Brings Out the Best in Kimoto Sake

Sake made with the Kimoto method is generally considered suitable for aging.
This is because it is said that the strong acidity and rough flavors generated during the fermentation stage mellow out through aging and transform into a deeper flavor profile.

If Sokujo-moto sake is meant to be enjoyed for its "fresh fruitiness,"
Kimoto sake is like a "vintage wine that finally reaches completion after a long period."

By the way, why does acidity become mellow over time?
It’s simply because it is turning into "something other than acid."

Actually, when "acid" and "alcohol" react, they turn into "esters," which are aroma components.
If yeast is alive, this process may advance over time, and even in the absence of yeast, ester production can occur after a long period.
That is precisely why sake with "high acidity" and "high alcohol content" is well-suited for aging.

And perhaps it is this story of long-term fermentation that makes the Kimoto shubo-making process so fascinating even today.

That’s all for now.
What I found really interesting was the realization that shubo-making is truly necessary to control the amount of "autolysis."
It is truly fascinating to see how the science hidden behind tradition is being understood in the modern day.