Rice polishing

Whether as a food staple or a brewing material, brown rice (genmai) is generally not used as is, but after having the brown outer portion polished (milled) away to give white rice.

Compared to eating rice, the white rice use in brewing undergoes a far higher degree of rice polishing (sei-mai): in other words, a greater portion of the outer layer is removed. The reason for this is that the germ (the part from which shoots emerge as rice grows) and the outer layers of the grain are rich in proteins, fats, ash (inorganic compounds) and vitamins which accelerate the growth of koji-kin and yeast.  These can adversely affect sake balance, or become colouring or zatsu-mi components in the final sake, leading to deterioration of flavour. Rice polishing is carried out to remove these unwanted elements.

Generally, when polishing rice to 70% (removing the outer 30%), the level of various components compared to brown rice is roughly; protein: 70%; fats: 5%; ash: 20%. Furthermore, the water content of brown rice is usually 14-15% when delivery is taken, but moisture content decreases as rice polishing proceeds, and is usually reduced by two or three percent by the 70% stage.

Skillfully polished white rice has the germ and surface grooves completely removed, and, as the brown rice grain has been evenly polished in each dimension, the white rice retains the form of the original grain. Where there are deep surface grooves, and depending on the rice polishing ratio, it can be impossible to remove the grooves completely. This is undesirable, as it means that the outer portion remains where the grooves have not been removed.