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Varieties of kōbo (yeast starter culture): sake brewing and Kyōkai kōbo


For sake brewing, kōbo (yeast starter culture) is indispensable. It’s because of the kōbo that fermentation proceeds and aroma and flavor are developed in the brew.


Even if the same water and rice are used as ingredients, if the kōbo is different, it will be noticeable in the sake. This has much to do with the fact that kōbo brings forth aromas and produces alcohol.


Utilizing high-quality Kyōkai kōbo
There are vast amounts of yeast, of many varieties, in the natural environment. Among these yeasts are those that are suited to the brewing of alcoholic beverages. For example, there are particular yeasts for making wine, and others for brewing beer. Similarly, there are yeasts that are suited to making sake.


In the past, breweries had to use the yeasts that happened to inhabit the brewery. No doubt there was a lot of random variation in the quality of the fermentation produced by this sort of yeast.


However, in 1906 the Brewing Society of Japan (Nihon Jōzō Kyōkai) was founded, and at the same time, high quality yeast starter cultures were collected and propagated. The starter culture produced in this way, referred to as Kyōkai kōbo, was then sold to breweries. Nowadays, many breweries use Kyōkai kōbo.


There are several varieties of Kyōkai kōbo, and each one is assigned a number. Naturally, the characteristics of the sake depend on which kōbo is used for the fermentation.


Type of Kyōkai kōbo


Kyōkai kōbo No. 6 Produces a less aromatic brew with a mellow flavor.
Kyōkai kōbo No. 7 Aromatic, suited to being served warm. Very popular.
Kyōkai kōbo No. 9 Produces a refreshing, fruity aroma. Very popular.
Kyōkai kōbo No. 10 Low in acid. Smooth and refreshing.
Kyōkai kōbo No. 11 Low in amino acids. Produces sake with a high alcohol content.
Kyōkai kōbo No. 14 Low in acid. Produces sake with a mild aroma.


Which kōbo to use is up to the individual brewery. Typically, the decision would be made after a discussion between the head of the brewery and the chief brewer. The most popular choices are Kyōkai kōbo No. 7 and Kyōkai kōbo No. 9.


The yeast varieties shown in the table above are called awa ari kōbo, meaning that they tend to produce foam in fermentation. When the ferment is progressing vigorously, the foam can rise up quite high. For this reason, various precautions need to be taken, such as using a tall vat for the fermentation, and keeping an eye on the ferment through the night. Some varieties of kōbo that produce less foam are now available. They are referred to as awa nashi kōbo.


Fermentation with awa ari kōbo proceeds at a leisurely pace, so tiny bubbles accumulate. On the other hand, with awa nashi kōbo, because the fermentation proceeds rapidly, the bubbles are large and they burst. That is why foam does not build up with awa nashi kōbo. There is less trouble and effort involved when awa nashi kōbo is used.


Of course, since the fermentation proceeds rapidly with awa nashi kōbo, the fermentation process needs to be regulated to ensure that it does not go too far.


Kōbo for producing ginjō-ka
Kōbo is important for the process of producing sour flavors and aroma. For example, some varieties of sake have an aroma of apple or nashi pear. The kōbo plays a large role in producing such aromas and flavors.


To produce ginjō-shu, as much as possible of the outer layer of the rice grain, containing protein and other nutrients, is removed. Thus, the nutrients available to the kōbo are reduced. In addition, the brewing of sake takes place in the cold of winter.


Fermenting under these conditions, the kōbo is placed under considerable stress. This is what causes the ferment to produce various esters, giving it its fruity aroma. This type of aroma is called ginjō-ka.


Matters relating to Kyōkai kōbo are handled by a specialized agency, and each year stable, high quality kōbo varieties are made available. However, there are some breweries that use a locally sourced kōbo. They produce unique varieties of sake whose characteristics are influenced by the kōbo of the region.

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