Semi-targeted metabolomic analysis provides the basis for enhanced antioxidant capacities in pigmented rice grains

In the present study, polar metabolites including primary metabolites were analysed from red, black and white rice grains using gas chromatography mass spectrometry (GC-MS). Quantitative as well as qualitative differences in metabolite profiles of red, black and white rice were observed. Principal component analysis (PCA) of the data obtained from semi-targeted metabolite profiling showed a clear separation of grains with different pericarp colour. In the PCA scores plot, PC1 separated pigmented rice and non-pigmented rice. While, PC2 separated red rice and black rice. Biplot generated from metabolite profile of the rice grains indicated that vanillic acid, protocatechuic acid and glycerol-3-phosphate differentiate black rice from red and white rice. Erythritol and ribonic acid are present only in red rice causing its separation from black and white rice. Additionally, total phenolic content (TPC), 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2 '-azino-bis-(3-ethylbenzothiazoline-6-sulphonicacid) (ABTS) and the reducing potential of extracts in terms of ferric reducing ability of plasma (FRAP) were carried out. DPPH and ABTS radical scavenging activities of pigmented rice are higher than that of white rice, possibly due to the presence of high TPC in their grains. However, reducing power in terms of FRAP is highest in black rice and is comparable in red and white rice. Furthermore, correlation analysis of antioxidant activities with the metabolites was done to identify the possible primary metabolites contributing to antioxidant capacities of pigmented and non-pigmented rice. This study provides a premise for integration of pigmented rice in our daily diet owing to the potential health benefits of the compositional metabolites.