— The Presence That Designs the Aroma of Sake —
It is often assumed that the taste and aroma of sake are determined by “differences in rice.” Certainly, sake rice and the polishing ratio called 精米歩合（せいまいぶあい / seimai buai） – rice polishing ratio – are important factors. However, even with the same rice and the same polishing ratio, sake can sometimes be born with completely different aromas and flavors.
One of the biggest factors that creates those differences is 酵母（こうぼ / kōbo） – yeast.
Yeast is the microorganism responsible for alcoholic fermentation in sake brewing. 麹（こうじ / kōji） – koji (rice cultivated with mold for saccharification) – breaks rice starch down into sugars, and yeast converts those sugars into alcohol and carbon dioxide. This flow proceeds simultaneously within the sake-specific mechanism known as 並行複発酵（へいこうふくはっこう / heikō fuku hakkō） – multiple parallel fermentation.
But the role of yeast is not only to produce alcohol. During fermentation, yeast generates various by-products. Among them are aromatic compounds that determine the individuality of sake.
For example, the gorgeous apple-like aroma felt in ginjō sake comes from a compound called カプロン酸エチル（かぷろんさんえちる / kapuronsan echiru） – ethyl caproate – while the banana-like aroma is produced by 酢酸イソアミル（さくさんいそあみる / sakusan isoamiru） – isoamyl acetate. These are generated mainly through yeast metabolism.
In other words, yeast is not only in charge of fermentation but also “a presence that designs aroma.”
What Is Kyōkai Yeast?
In the world of sake, 協会酵母（きょうかいこうぼ / Kyōkai kōbo） refers to sake yeast strains distributed by the Brewing Society of Japan.
Since the Meiji era, with the aim of improving sake quality, a system was established to isolate superior yeast strains in pure culture and supply them to breweries nationwide. This was the beginning of Kyōkai yeast.
Kyōkai yeast is identified by “No.” numbers. Strains such as No. 6, No. 7, and No. 9 have long been known, and there are also derived numbers such as No. 701, No. 901, and No. 1801.
These numbers are not mere identification codes. Each yeast strain has different characteristics.
- Fermentation strength
- Amount of acids produced
- Tendency to generate aromatic compounds
- Foaming behavior
Breweries choose yeast strains according to their target style and equipment conditions. Even with the same ingredients, different yeast leads to different aromas and impressions.
The Battle Against Foam and “Non-Foaming Yeast”
In sake fermentation, 高泡（たかあわ / taka-awa） – high foam – was once a major problem. As fermentation progressed, large amounts of foam could form and threaten to overflow the tank. Because of this, the actual batch size often had to be smaller than the tank’s capacity.
That is why 泡なし酵母（あわなしこうぼ / awanashi kōbo） – non-foaming yeast – was developed.
Representative examples are No. 701 and No. 901. These strains have the trait of not forming high foam. As a result, even with the same tank capacity, it became possible to brew about 20–30% more.
This was not merely an efficiency improvement. It was a major technological innovation for the brewing industry because production volume could be increased without additional capital investment.
The key point is that “non-foaming does not mean lower quality.” Foam characteristics reflect differences in fermentation behavior and are not directly related to superiority or inferiority in sake quality.
No. 1801 and Ginjō Aroma
In recent years, one yeast that has drawn particular attention in ginjō-style brewing is No. 1801 yeast.
This yeast is said to have a tendency to produce high levels of ethyl caproate and isoamyl acetate, key components of ginjō aroma. It makes it easier to express a bright, fruity fragrance, and it is often used for ginjō and daiginjō sake.
However, the important point here is that “yeast alone does not determine the taste.”
If the fermentation temperature is high, aroma can be suppressed. The quality of koji, the way the raw rice is handled, and the storage conditions can also change the final profile. Yeast is a crucial design element, but it is only one part of sake quality formation.
The Significance of Kyōkai Yeast
The greatest significance of Kyōkai yeast lies in “quality stability.”
If you rely on wild yeast, fermentation stability may decrease and sake quality may vary. Kyōkai yeast is cultivated in pure culture, and its characteristics are clearly defined. This enables highly reproducible brewing.
Behind the fact that modern sake is both high-quality and stable is the existence of this yeast distribution system. The management by numbers and the clear organization of characteristics also show that sake brewing has evolved not only from “experience and intuition,” but toward scientific control.
Key Points to Remember for the Exam
For exam preparation, the key points to organize regarding Kyōkai yeast are as follows.
- Yeast converts sugars into alcohol and CO2
- Yeast also generates aromatic compounds
- Kyōkai yeast is distributed by the Brewing Society of Japan
- It is identified by “No.” numbers
- No. 701 / No. 901 are non-foaming yeasts
- No. 1801 has high ginjō-aroma production ability
In particular, “non-foaming yeast” and “the characteristics of No. 1801” are points that tend to be asked.
Conclusion
Kyōkai yeast refers to distributed yeast strains used in sake brewing, and each numbered strain has clearly defined characteristics.
Yeast not only produces alcohol but also acts as a designer that shapes the aroma of sake. Non-foaming yeast born from the battle against foam, and No. 1801 that enhances ginjō aroma— the evolution of sake brewing is also the evolution of yeast.
Yeast is often not written on the label. Yet behind a single bottle, there is a carefully selected yeast strain.
Look at the rice. Look at the polishing ratio. And if possible, also pay attention to which yeast is used. With that perspective, sake begins to look less like a “drink,” and more like a “designed work.”
▶ Next: Organizing the Differences Among Representative Yeasts
So far, we have looked at the role of Kyōkai yeast and its basic characteristics.
So in practice, what differs between No. 701, No. 901, and No. 1801?
How aroma appears, acid levels, fermentation strength, and the overall impression of the sake.
By comparing these three representative yeasts, we will organize their individual personalities.
▶ What Are the Differences Between Kyōkai 701, 901, and 1801?
コメント