Natural Fermentation (健全醗酵)
The natural fermentation is one of notions in Japanese sake production and means that, without artificial or non-artificial inhibition of simultaneous plural fermentation, the power which original sake yeast has by itself is fully utilized to complete fermentation without any inhibition.
Summary
Like Japanese liquor kimoto-kei (a natural yeast method), this has become an especially important notion in traditional production methods which utilize the power of natural yeast as it is. The reason is that, as modern processes such as Japanese liquor alcohol addition are not included, whether natural fermentation or not determines directly the quality of seisei-shu (product sake). Conversely, in case of a production method including modern processes, even if the fermentation is somewhat unnatural, there is room to make minor adjustments by including other processes based on the policies of the sake breweries.
Quality of sake made with natural fermentation
During the fermentation process some acid is produced in the Japanese sake moromi, but the acid produced by the natural fermentation is felt to have clear and neat refreshing deliciousness.
On the other hand the acid produced by the unnatural fermentation is felt to have 'sour' bitterness, harshness rather than sourness, and to give acridity or bulky discomfort.
Causes of not achieving natural fermentation
Not achieving natural fermentation is called unnatural fermentation.
Before the establishment of safe brewing in Japan, the main cause had been putrefaction by hiochi (putrefaction) bacterium (a kind of lactic acid bacterium), but in the today's brewing environment, it can be mentioned that the causes are oxidation due to the poor management of moromi and temperature, the human-induced extinction of yeast cells due to the addition of alcohol at an undesirable timing, and others.
Also, among sakes claimed to be Kimoto type (sake production based on Kimoto method), there is seisei-shu (sake product) with risky sourness which exists just before turning to putrefaction and with harsh taste which makes sake hard to drink, and it is called nezumi-watari (rat's sake) in jargon of workers at sake breweries. It means that sake just before turning to putrefaction and within sellable limit has been barely brought in.