Kome suibun

Water content of rice

The moisture content of brown rice is closely related to the efficiency of rice polishing and to the solubility of rice in the fermenting moromi mash. It is set below 15% in the Agricultural Products Inspection Code, but has been relaxed for brewing rice in certain regions, and has been set at below 16% for ippan-mai for the foreseeable future.

The moisture content of brown rice affects the rate of water absorption and the rigidity (a measure of the level of resistance to deformation, or the (lack of a) tendency to alter shape). If the moisture content is low, rigidity is high and the rate of water absorption increases. If, on the other hand, the moisture content is high, then rigidity is reduced, leading to cracking during rice polishing、and an increase in the level of broken grains. Further, since the moisture content of the white rice is higher after polishing, the absorption rate of the steamed rice will be lower, and the solubility of the rice (its ability to dissolve in the fermenting mash) will be reduced.

Because of moisture dissipation occurring during rice polishing, the moisture content of white rice after polishing will be 2% or so lower than the original brown rice (for rice polished to 70% of original size).

The “water absorption rate” (kyu- (long “u”) sui ritsu (吸水率)) is the weight of the water absorbed during soaking, divided by the weight of the white rice. There is a close correlation between the absorption rate and the enzyme-related solubility of the steamed rice: the higher the absorption rate, the more the steamed rice will dissolve. When making ginjo-shu, it is usual to keep the absorption rate to a minimum in order to prevent rice from dissolving excessively.