Flavins are a cause of seishu colouring (chaku-shoku), deriving from rice, kome-koji and yeast.  Riboflavin , which is well-known as vitamin B₂, is by far the most common. New sake (shinshu) contains in the region of 20-70μg/dℓ flavins, but these can be almost completely removed by treatment with active carbon. The typical colour of new sake (pale yellow with a blue-greenish cast) is the colour of flavin.

As sake ages, melanoidin and other pigments increase, so the relative effect of flavin colour decreases. Riboflavin acts as a photosensitizing agent, speeding up colouring due to light, and is also a contributing factor to the growth of hi-ochi kin, so quality can be stabilized if it is removed using active carbon treatment.