Rice grain-weight dependency on carbon and nitrogen isotope fractionation

Multi-stable isotope analysis is effective for exposing the mislabeling of food production areas. However, in the case of rice, grain dry weight can influence the fractionation of stable carbon (δ13C) and nitrogen (δ15N) isotopes of grains within a production area. The relationship between these isotope fractionations and grain weight remains unclear; thus, the reason for the fractionation has not yet been elucidated.We collected Koshihikari rice grains from four production areas in Japan (Niigata, Aichi, Kagawa and Miyazaki prefecture) and analysed the effect of grain weight on the fractionation of δ13C and δ15N. δ13C was positively fractionated, depending on grain weight by translocation (grain-weight dependency) in all areas, whereas δ15N was not. The statistical analysis of δ13C adjusted by grain weight was able to distinguish between Aichi and Miyazaki, which was not the case for unadjusted δ13C. Our findings suggest that the applicability of δ13C for discrimination can be expanded by considering its grain-weight dependency.