Digestive Stability of Tannin-Enriched Fraction of Rubus chingii Hu Fruits and Its Regulatory Effect on the Intestinal Microflora

Tannin is the major active ingredient of the dried unripe fruits of Chinese raspberry (Rubus chingii Hu), Fupenzi in Chinese, and has the functions of lowering blood sugar, reducing blood lipid, resisting inflammation, delaying aging and preventing cardiovascular and cerebrovascular diseases. In this study, a tannin-enriched fraction was prepared from Fupenzi, and the changes in its tannin content, antioxidant activity and α-glucosidase inhibitory activity before and after in vitro simulated digestion were investigated. Its regulatory effect on the intestinal microflora was analyzed by in vitro anaerobic fermentation. Finally, ultra-high performance liquid chromatography-quadrupole time of flight-tandem mass spectrometry (UPLC-ESI-QTOF-MS/MS) was used to compare the changes in the contents of tannin and its metabolite after in vitro digestion and fermentation. The results showed that the content of hydrolyzed tannin increased first and then decreased during both gastric and intestinal digestion. Gastrointestinal digestion increased 1,1-dipheny1-2-picryl-hydrazyl (DPPH) radical scavenging capacity of the tannin-enriched fraction, with half-maximal inhibitory concentration (IC₅₀) values ranging from 29.15 to 39.65 µg/mL, but had no significant effect on 2,2’-azino-bis(3-ethyl-benzothiazoline-6-sulfonic acid) (ABTS) cation radical scavenging capacity (P > 0.05). In addition, after simulated intestinal digestion, the α-glucosidase inhibitory activity increased by 7.16–7.86 times. In vitro colonic fermentation experiments showed that tannin-enriched fraction significantly increased the relative abundance of beneficial bacteria such as Coprococcus, Anaerostipes, Ruminococcus and Faecalibaterium. Simulated gastrointestinal digestion reduced its effect in improving beneficial bacteria such as bifidobacteria. UPLC-ESI-QTOF-MS/MS analysis showed that the content of tannin metabolites decreased first and then increased during gastrointestinal digestion; the contents of urolithin A and urolithin B decreased significantly after colon fermentation, and neither urolithin C nor urolithin D was detected. Therefore, this study provides a theoretical basis for the establishment of stable tannin delivery system to improve its bioavailability.