Identification and quantification of free, esterified, and insoluble-bound phenolics in grains of hulless barley varieties and their antioxidant activities

To study the phenolics in grains of hulless barley varieties harvested in the Qinghai-Tibet plateau region, free, esterified, and insoluble-bound phenolic fractions were extracted from grain of fourteen varieties. The phenolic components and antioxidant activity of the fractions were studied. Identification of phenolics profile in QB35 variety was studied using ultra-high-performance liquid chromatography-diode array with quadrupole time-of-flight and tandem mass spectrometry (UPLC-QTOF-MS/MS), which revealed fourteen phenolics in the free form, four in the esterified form, and seventeen in the insoluble-bound form. Total phenolic contents (TPC) of the fractions were tested finding significantly (p < 0.05) higher phenolics content and greater antioxidant capacity (via DPPH and ABTS free radical scavenging assays) in insoluble-bound fraction than that of free and esterified fractions for the same hulless barley variety. In terms of phenolic components, ferulic acid was the dominant phenolic in all varieties. Chrysoeriol-7-O-glucuronide and chrysoeriol were identified and quantified in hulless barley varieties for the first time. This study also showed TPC, ferulic acid and luteolin significantly contributed to the antioxidant activities (p < 0.01). The fourteen hulless barley varieties could be classified depending on the geographical locations and the color of their grains via hierarchical cluster analysis (HCA). This study provides valuable new insights into the phenolics in grains of hulless barley varieties and their antioxidant potentials.