It is highly undesirable to use water containing iron as sake brewing water. Iron combines with ferrichromes made by koji-kin, resulting in abnormal colouration (chaku-shoku). It can also speed up the amino-carbonyl reaction, leading to a deterioration of taste and aroma or other declines in quality.

In cases such as river- or shallow well-water where there is an ample oxygen supply and oxidization leads to pH neutral water, it can be assumed that iron will exist in the form of insoluble ferrous hydroxide. It is common for natural water to contain 1-3ppm of iron. There are rare cases of iron content exceeding 100ppm.

In the case of brewing water,it is necessary to keep the level of iron below the acceptable threshold of 0.02ppm, and in cases where there is a higher level, it is necessary to treat the water to remove it. In the case of Miyamizu, iron levels are below 0.01 ppm or even undetectable, so it can be seen to be truly excellent water for brewing.

The form taken by iron in water varies easily, so it is necessary to carry out analysis quickly after sampling. Methods of analysis include atomic absorption spectrometry and colorimetry, but the method of preparation of the sample can influence the content of the given value, so it is necessary to match the method with the purpose of analysis.