Unveiling diversity in amino acid stable isotope profiles for classifying rice varieties, refining types and cultivation systems.

Abstract

Isotope Ratio Mass Spectrometry (IRMS) is a promising tool in organic authentication cases. Premium-priced Italian rice varieties (Carnaroli, Arborio, Baldo) are used in cuisines worldwide for their unique qualitative properties. Organic authentication of rice by morphological assessment is unfeasible, while its market availability at different refining stages (brown, white) further increases the data variability. In this study, bulk and compound-specific (CS) - IRMS analysis of nine rice amino acids (AAs), by elemental analyser (EA) - IRMS and gas chromatography (GC) - combustion (C) - IRMS, respectively, were applied in order to explore their organic authentication potential in cases involving different rice varieties and refining types. The individual and interactive effects of the different variables were assessed on the δ¹³C_AAs, δ¹⁵N_AAs, δ¹³C_bulk and δ¹⁵N_bulk, and the sample classification was attempted by linear discriminant analysis (LDA) and decision tree analysis (DTA). Organic authentication of brown rice was achieved by CS-IRMS. Generic rice was differentiated from all Italian organic and conventional varieties (δ¹⁵N_leucine < 2.5 ‰). The δ¹³C values of glutamic acid, glycine, phenylalanine and proline, significantly contributed to the complete LDA separation of conventional Arborio, conventional Carnaroli and organic Carnaroli samples. This study showcases the interplay between refining type, variety and cultivation, which should be considered in cases of organic authentication by IRMS methods.