When manganese is present in seishu, it speeds up colouring due to exposure to light, and so, like iron, it is desirable that it exist in as small a quantity as possible. Manganese exists plentifully in rice in a form that can be extracted easily in an acidic environment, so rice is the main source of the manganese in sake. It is essential for the growth of lactic acid bacteria, but, as a sufficient quantity derives from rice, there should be as little as possible in sake brewing water. Brewing water generally contains less than 0.01ppm of manganese, but it is necessary to remove it if higher levels occur.

Methods of analysis include atomic absorption spectrometry and colorimetry, but it sometimes precipitates when in suspension with ferrous ions, so it is necessary to carry out analysis as soon as possible after the sample has been taken.