Study on the Stability and Antioxidant Activity of Phenolics in Green Tea, Citrus Peel and Soybean before and after in Vitro Digestion

In this study, three kinds of foods (green tea, citrus peel and soybean) rich in three representative phenolic compounds (flavanols, flavanones and isoflavones) were used as raw materials to simulate the in vitro digestion process in human oral and gastrointestinal tract. High performance liquid chromatography diode array detector/electrospray ionization quadrupole time of flight mass spectrometry detector (HPLC-DAD/ESI-Q-TOF-MS) was used to detect the variety and content of phenolic compounds before and after the in vitro digestion. The total phenol content (TPC), total flavonoids content (TFC) and antioxidant activity (DPPH, ABTS, FRAP, ORAC) in different digestion stages were determined. The results showed that four phenolic compounds (epigallocatechin, (+)-catechin, epigallocatechin gallate, epigallocatechin gallate) were detected in green tea extract, four phenolic compounds (daidzin, genistin, daidzein and genistein) were detected in soybean extract, and two phenolic compounds (naringin, hesperidin) were detected in citrus peel extract. Among the three food extracts, phenolic compounds in the green tea extract were the most unstable after in vitro digestion, except (+) -catechin, the other three phenolic compounds were almost completely degraded, the loss rate was more than 95%. TPC of green tea, citrus peel and soybean extracts was significantly increased in gastric digestion stage (P<0.05), and significantly decreased in intestinal digestion stage (P<0.05). TFC of green tea extract was significantly increased in oral and gastric digestion stage (P<0.05), and significantly decreased in intestinal digestion stage (P<0.05). TFC and TPC of citrus peel and soybean extract had the same change trend. The four antioxidant activities of green tea extract increased first and then decreased after gastrointestinal digestion. Antioxidant activities of ABTS, FRAP of soybean extract were significantly decreased in oral phase (P<0.05), antioxidant activities of DPPH, ORAC were significantly increased in oral and gastric digestion phase (P<0.05), and significantly decreased in intestinal digestion phase (P<0.05). The ORAC antioxidant activity of citrus peel extract was significantly increased in intestinal digestion stage (P<0.05), and the antioxidant activity of ABTS and FRAP showed a trend consistent with the change of total phenol content in in vitro digestion stage.