Dynamic gastrointestinal digestion of grape pomace extracts: Bioaccessible phenolic metabolites and impact on human gut microbiota

Grape pomace is a winery by-product rich in polyphenols and dietary fibre, two food constituents in whose bioaccessibility gut microbiota is implicated. To overcome the limitations of in vivo studies concerning difficult access to proximal regions of the colon, this paper reports, for the first time, the in vitro colonic digestion of grape pomace extracts (GPEs) using a dynamic gastrointestinal digestion model. Experiments consisted in the inoculation of the simgi (R) model with faecal microbiota from healthy volunteers, and further feeding of the system with a single (acute feeding of 700 mg) and continuous (chronic feeding of 700 mg/day; 14 days) dose of GPE. Two independent experiments with faecal microbiota from two volunteers (#1 and #2) were carried out. Results were determined in terms of microbial functionality [phenolic metabolites, short-chain fatty acids (SCFAs) and ammonium ion] and composition (plate counting and qPCR) in the different simgi (R) compartments (ascending, transverse and descending colon). Different benzoic, phenylacetic and phenylpropionic acids were found as the main bioaccessible phenolic metabolites released from GPE. As an indicator of microbial fermentative activity, a significant increase (p < 0.05) in the concentration of SCFAs (acetic, propionic and butyric acids) was observed after chronic feeding (19-54% increase for volunteer #1, and 83-150% increase for volunteer #2). In relation to microbial proteolytic activity, increasing trends in ammonium production (although not significant, p > 0.05) were observed during chronic feeding for both experiments. Also, during the chronic feeding, the main bacteria groups increased (p < 0.05) in respect to the steady state, with the highest increments being for Lactobacillus and Bacteroides groups. Additionally, a bacteria strain capable of metabolizing (-)-epicatechin gallate, a phenolic compound characteristic of grapes and wine, was isolated from the simgi (R) stable microbiota and was identified as Raoultella ornithinolytica or Raoultella planticola.