Water hardness is determined by the concentration of metal ions such as calcium (Ca2+) and magnesium (Mg2+) dissolved in it.
- Soft water: Low hardness, with low levels of calcium and magnesium.
- Hard water: High hardness, with high levels of calcium and magnesium.
In Japan, water is generally considered soft, and this is primarily due to the country's small land area, short rivers, high precipitation levels, and volcanic ash soil.
The water used for making Nada's sake is called "Miyamizu," which originally stands for "Nishinomiya's water." It refers to the well water that springs along the coast of Nishinomiya City in Hyogo Prefecture. Miyamizu is a moderately hard water containing a variety of minerals like phosphorus, calcium, and potassium. This abundance of minerals serves as a source of nutrition for the koji mold and yeast during the sake brewing process, resulting in vigorous fermentation. As a result, it has been highly regarded by the citizens of Edo (modern-day Tokyo) for its "rich and sharp, clear sake," often referred to as "a man's sake."
The water used in sake brewing in Fushimi, Kyoto, is known as "Gokousui" or "Honorable Perfume Water." It is a relatively soft water with fewer minerals, and it leads to a slow fermentation process. This gradual fermentation smoothes out the roughness of the sake, resulting in a milder and less acidic taste. Especially in the case of new sake, it develops a rounded, gentle flavor, earning it the moniker of "Onna-sake" or "a woman's sake," in contrast to the "Otoko-sake" or "a man's sake."