Because potassium is indispensable for the growth of yeast and koji-kin , and fermentation, it is, along with phosphoric acid, considered an important component of brewing water . It is generally present in large quantities in water which gives a strong fermentation, existing bonded to chloride, sulphate and carbonate ions.
Most of the quantity required by yeast comes from rice. In rice grains, it exists in free form, so it can easily be washed away during soaking . For that reason, kake-nagashi (pouring water in from the bottom of the rice soaking vessel and allowing it to overflow from the top) is sometimes used as one technique to adjust fermentation.
Generally, it is measured using flame photometry or atomic absorption spectrometry. In this case, co-existing interfering substances such as sodium, lithium, barium, free acids, phosphates, boric acid, oxalic acid, silica, glucose and so on often cause positive or negative error, so care must be taken. Miyamizu has a high potassium content compared to average wells.