Widely Targeted Metabolomics Analysis of Compositional Differences between Purple Black Rice and Common Rice at Different Harvest Periods

In this study, widely targeted metabolomics based on ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) was used to analyze the types and contents of differential metabolites between purple black rice and common rice produced in Guangxi and to illustrate the significantly differential metabolic pathways. A total of 667 metabolites were identified, including lipids, flavonoids, phenolic acids, alkaloids, and organic acids. Principal component analysis (PCA), cluster analysis and overall metabolite profiling showed that 239 of the 278 differential metabolites identified in mature rice samples were up-regulated, and 235 of the 267 differential metabolites identified in the 20 day earlier harvest group were up-regulated when compared with common rice. The metabolite composition of purple black rice at different harvest periods was similar, indicating that porridge made from purple black rice harvested in advance will have soft and glutinous texture as well as complete nutritional structure. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that significantly up-regulated compounds such as rhamnose-3-O-glucoside, hesperidin-7-O-glucoside, homoeriodictyol, eriodictyol, hesperidin, catechin, quercetin, naringin and chalcones in purple black rice were involved in the biosynthetic pathways of flavonoids and flavonols, regulating the contents of flavonoids and flavonols, and improving the nutritional quality and antioxidant effect of purple black rice.