Kumamoto Yeast

The birth of Kumamoto Yeast: A transition from drinking to tasting

One of the greatest accomplishments of the Kumamoto Prefecture Sake Institute was the isolation and culturing of Kumamoto Yeast at the hands of Nojiro Kinichi. Previous yeasts had been focused on making consistent sake, but Kumamoto Yeast, with its mild acidity and rich aroma, was something new. It also has excellent fermenting power, meaning that even with the same ingredients, Kumamoto Yeast can make both a dry, light sake or a sake with a sweet, robust nose, letting the master brewer create a variety of styles according to his/her intent. In recognition of its quality, the Brewing Society of Japan assigned this yeast the moniker of Kyokai Yeast #9 and began regulating its distribution. Once nationwide distribution began in 1968, brewers around Japan were able to make very aromatic sakes, thereby kicking off the ginjo sake boom.

Kumamoto Yeast helped sake transition from something people drank to something people tasted, meaning Kumamoto Yeast inspired a great leap forward in sake appreciation. Over 60 years have passed since its discovery, but even now, Kyokai #9 Yeast is still one of the most popular yeasts used in Japan.

To this day, the Kumamoto Prefecture Sake Institute stores, maintains, and distributes its original culture of Kumamoto Yeast separate from the Kyokai #9 Yeast culture kept by the Brewing Society of Japan.

During the Kumamoto Earthquakes, brewers all over the country were worried about their prized Kumamoto Yeast. Thanks to the strict control systems in place designed to survive even the worst disasters, the yeast was safe.

The importance of yeast in sake-making — an explanation of Kyokai Yeast

 The four ingredients required to make sake are rice, water, koji, and, of course, yeast. Up until the mid-1800s, sake breweries utilized what they’d call “brewhouse yeast,” which was essentially yeast microbes that were alive in the air and on the sake equipment. There were many problems with this, as while some yeasts would make good sake, others wouldn’t end up with such positive results, and so brew masters had a difficult time creating consistent sake. Kyokai Yeast is what eventually fixed this problem. Superior yeast strains from around Japan were scientifically cultivated and distributed to breweries throughout the country by the organization that would become the Brewing Society of Japan, making it so breweries were finally able to craft sake of a stable quality.

Left/Ampules of Kyokai #9 Yeast distributed by the Brewing Society of Japan
Right/The white areas in the agar media are colonies of yeast

Main Steps in Brewing Sake

(processes differ depending on brewery or type of sake being brewed)

1. Hon Arai

Preparation for brewing involving careful washing of tank for holding rice and water.

2. Seimai

Rice is polished to different degrees depending on the style of sake to be brewed, e.g., junmaishu, ginjo, daiginjo, etc.

3. Senmai & Shinseki

Rice is washed with brewing water and soaked in water according to degree rice was polished.

4. Mushi

Rice is moved to an enormous steaming basked called a koshiki and then steamed.

5. Seigiku

Once steamed rice has cooled enough, a portion is inoculated with koji yeast and left to rest in the koji room. This results in the koji that is integral in making the rice fermentable.

6. Shubo-zukuri

Yeast and steamed rice are added to a tank containing koji and water. Yeast is allowed to grow from around 10 days to 3 weeks to create the shubo, or fermentation starter.

7. Shikomi

Koji, steamed rice, and water are added to the shubo to start the brewing process. Three-step shikomi involves adding koji and steamed rice in three additions.

8. Shibori (Joso)

Shibori is where the sake mash is separated into sake and sake lees. This can be performed through a variety of methods, e.g., mechanical sake presses, etc.

9. Oribiki & Roka

Residual sediment is sake is called ori, and Oribiki and Roka steps are where sake is purified and filtered to remove ori.

10. Hiire (Chozo Hiire)

Sake is pasteurized to deactivate enzymes and preserve quality.

11. Chozo

Sake is stored at 15-20°C at the brewery. Temperature is carefully controlled as improper aging can cause the sake to change color or introduce off-flavors.

12. Chogo & Warimizu

Different sakes are blended to achieve the desired flavor of sake. To provide a consistent level of quality, sakes are blended when necessary and water is added to pure sake to adjust alcohol content.

13. Hiire (Binzume Hiire)

Another round of pasteurization. Sake that has only been pasteurized before aging is called nama-zume; sake that has only been pasteurized after bottling is called nama-chozo; and sake that hasn’t been pasteurized at all is called nama-zake.

14. Binzume

After the finished sake has been bottled, the bottles are labeled and shipped out.

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