Combined microwave and enzymatic treatment improve the release of insoluble bound phenolic compounds from the grapefruit peel insoluble dietary fiber

Increasingly attention has been paid to the active substances in grapefruit peel. In this study, insoluble bound phenolic compounds (BP) were released from grapefruit peel insoluble dietary fiber (GP-IDF) by microwave extraction (M-BP), enzymatic hydrolysis (E-BP) and combined microwave and enzymatic treatment (ME-BP). The results showed that the yield of ME-BP was the highest (148.856 +/- 0.620 GAE mg/100 g), and conditions of microwave treatment (microwave power, temperature, time) were 600 W, 85 degrees C, and 37 min, while that of enzymatic treatment were 8% addition, 60 degrees C, and 2 h. Scanning electron microscopy displayed that GP-IDF became looser by ME. ME mainly resulted in the removal of lignin in IDF from X-ray diffraction, which was consistent with FT-IR. Twenty seven BP were identified by UPLC-ESI-QTOF-MS/MS. The high content of gallic acid (42.50 + 3.21 mu g/g DW), ferulic acid (18.46 +/- 1.65 mu g/g DW) and protocatechuic acid (6.16 + 0.72 mu g/g DW) was quantified by LC-ESI-QqQ-MS/MS. Furthermore, ME-BP showed stronger antioxidant capacities than that of others (ABTS, oxygen and DPPH free radical scavenging ability), and showed good inhibition of alpha-glucosidase. Our results demonstrated that combined microwave and enzymatic treatment was an effective treatment for improving the release of BP.