Metabolomics and Antioxidant Capacity Analysis of Flavonoids in the Bran of Purple-Black Kam Sweet Rice (Oryza sativa L. var. glutinosa Matsum)

Metabolomic analysis and antioxidant activity of flavonoids were investigated in the bran of deep purple-black (cv. Yadingsha), light purple-black (cv. Gounendang) and white (cv. Gouyangnong) Kam Sweet Rice (KSR). Differential flavonoid metabolites between the three varieties of KSR were determined and the correlation between flavonoid metabolites and antioxidant activity was analyzed. The results showed that a total of 139 flavonoids, belonging to 10 different classes, were identified in the rice bran. Seven, two and seven unique metabolites were found in Yadingsha, Gounendang, and Gouyangnong, respectively. Additionally, 111, 104, and 96 differential metabolites were found in Yadingsha versus Gouyangnong, Gounendang versus Gouyangnong, and Yadingsha versus Gounendang, respectively. Significant differences were observed in the contents of anthocyanins, proanthocyanidins, dihydroflavonols and chalcones among rice brans of different kernel colors. Yadingsha exhibited the highest total flavonoid content and demonstrated the strongest scavenging activity against 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical and 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) cation radical, followed by Gounendang, suggesting a positive correlation between kernel color depth and flavonoid content as well as antioxidant capacity. A total of 72 flavonoid metabolites showed significant or highly significant positive correlations with antioxidant capacity. This study provides a scientific basis for the processing and utilization of purple-black KSR bran and further research on its flavonoid metabolism.