Hard water (kosui)

Hard water (kosui) is water that contains a relatively high amount of mineral components such as calcium and magnesium ions. Specifically, water with a concentration of calcium or calcium carbonate of 120 ppm or more is generally considered hard water. Because of these components, hard water has a high water hardness, whereas water with fewer of these minerals is called soft water. In sake brewing, water quality has a major influence on the taste of the sake, so the choice between hard and soft water is important. Hard water can help bring out the umami of the sake, but it can also affect the activity of the yeast, so breweries are expected to understand the characteristics of the water they use. Removing the mineral components contained in hard water is called "softening" (nanka).