Open fermentation (Kaiho hakko)

Open fermentation (kaiho hakko) refers to a method of fermenting sake while the mash is exposed to the outside air. With this method, the yeast and other microorganisms active during fermentation are more easily influenced by the surrounding environment, which can give rise to new flavors and aromas. It is an important technique for bringing out the distinctive character of sake, but it requires careful attention to temperature and hygiene control. In contrast, closed fermentation (mippo hakko) is carried out in a completely sealed tank, which keeps outside influences to a minimum. This makes it easier to produce sake with a more stable flavor. Open fermentation is often adopted by sake breweries that place particular value on traditional methods, and in brewing processes that aim for a more individual, characterful taste.