Properties of Water Extracts of Rambutan Peel Polyphenols during in Vitro Simulated Gastrointestinal Digestion and Colonic Fermentation and Their Effects on Gut Microbiota

The rambutan peel was rich in polyphenols, in order to further investigate the technology of rambutan peel polyphenol production and its product development, this study prepared rambutan peel polyphenol water extract (RPE) by high pressure steaming method. The total phenol content and antioxidant activity of RPE were determined, and its solubility was characterized. The changes in polyphenol composition, content and bioactivity of RPE in in vitro simulated gastrointestinal digestion were investigated, and the effects of digestion and fermentation treatments on the inhibitory activities of α-glucosidase and pancreatic lipase by RPE were evaluated, and the biotransformation of gastrointestinal-digested RPE in colonic fermentation and its effects on intestinal flora were analyzed. The results showed that the total phenolic content of RPE was 36.05 g gallic acid equivalent (GAE)/100 g RPE, the IC₅₀ values for scavenging ABTS⁺ and DPPH radicals were 5.22 and 5.96 μg/mL, and the EC₅₀ value for the total reducing power was 19.53 μg/mL. A total of 36 phenolic compounds were identified in RPE, mainly including coriandrin (37.26 mg/g RPE), gallic acid (31.4 mg/g RPE), ellagic acid (13.72 mg/g RPE), and geranlin (13.17 mg/g RPE). The contents of polyphenols reduced by 27.30% after gastrointestinal digestion. The antioxidant activities of digested RPE decreased, but still higher than that of V_C. The inhibition activity of digested RPE on α-glucosidase decreased, but the inhibition activity against pancreatic lipase improved. The phenolic content, antioxidant activity and enzyme inhibitory activity of gastrointestinal-digested RPE further decreased during colonic fermentation. But the gastrointestinal-digested RPE promoted the proliferation of beneficial bacteria such as Bacteroides and Megamonas, inhibited the growth of harmful bacteria such as Clostridium_sensu_stricto_1, Escherichia/Shigella. The contents of acetic acid, propionic acid and butyric acid increased in the colonic fermentation of gastrointestinal-digested RPE. Urolithin products were detected in the colonic fermentation system. The contents of urolithin A, urolithin M5, and urolithin D increased with fermentation time. This study provided a theoretical basis for the development of high-value active products based on RPE.