Abstract: The peels of 21 different fruit species were screened for their contents of total phenolics and total flavonoids and their antioxidant activity in terms of 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2’-azino-bis(3-ethylbenzothiazoline- 6-sulfonic acid (ABTS) cation radical scavenging activity. The selected fruit peels with higher contents of total phenolics and stronger antioxidant activity underwent in vitro simulated gastrointestinal digestion to evaluate the effects of different digestion times on the release of total phenolics and total flavonoids. The bioavailability was investigated by simulated intestinal absorption using a dialysis bag. In addition, the changes in phenolic composition were studied by ultra performance liquid chromatography-diode array detector/electrospray ionization-time of fight-mass spectrometry (UPLC-DAD/ESI-TOF-MS). The results showed that the contents of total phenolics and total flavonoids in pomegranate peel, (58.09 ± 0.46) mg/g and (47.50 ± 0.39 mg/g), respectively, and mangosteen peel (52.09 ± 1.52) mg/g and (36.07 ± 0.46) mg/g respectively were higher than those in the other fruit species. The antioxidant activity of pomegranate peel was stronger than that of mangosteen peel. The release of both total phenolics and total flavonoids from pomegranate peel gradually increased during in vitro simulated gastric digestion, but increased initially and decreased later during in vitro simulated intestinal digestion. The release of total phenolics and total flavonoids during the gastric digestion was significantly lower than during the intestinal digestion. The bioavailability of total phenolics was always higher than that of total flavonoids at each observation time. Five phenolic compounds (α-punicalag, β-punicalag, ellagic acid hexoside, ellagic acid deoxyhexoside, and ellagic acid) were detected from pomegranate peel extract by UPLC-DAD/ESI-TOF-MS. The loss rate of these phenolic compounds was lower during the gastric digestion than during the intestinal digestion, suggesting that phenolic compounds in pomegranate peel maintain a stable structure in acidic environments.

Key words: fruit peels, phenolic compounds, antioxidant activity, in vitro simulated digestion