About the purity of the water
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The water used for brewing sake is almost always drawn from a well that is often even within the enclosure of the kura.
That the water source is so close and that it does not have to travel in outside pipelines gives it a constant temperature. This consistency makes it possible to design a reliable brewing process based on the invariability of the water temperature.
The water from the source is usually only minimally filtered.
Groundwater is generally pure thanks to the filtering effects of the soil. So, water drawn from very deep can be expected to contain less haze-forming substances, fewer bacteria and other living organisms, and less organic matter than shallow groundwater does.
On the other hand, deep water is likely to contain much more carbonate ions and other soluble mineral substances.
The mineral content is the determining factor in the taste of sake. Iron and manganese induce unpleasant odours and flavours.
On the other hand, potassium, magnesium and certain phosphorus compounds will nourish the koji and the yeast which will activate and spread all the better to invade the brew more quickly reducing the possibilities of it being contaminated by other harmful organisms.
Also, the less the water is saturated with minerals, the better it can dissolve the flavourful elements of rice and koji. This explains why the harshness-free waters of Shizuoka, Hiroshima and Niigata produce sake so fragrant.