In vitro utilization of barley husk by human gut microbiota depends on husk composition and particle size

Barley husk is a cereal by-product rich in dietary fibers and phenolic acids, which can be used to design food ingredients having potential benefits for human health. In this study, we evaluated the effect of composition and physical structure manipulation of barley husk on fiber fermentability, release of phenolic acids as well as on fecal microbiota composition and activity. Samples of barley husk with two different levels of starch were milled, resulting each in four samples with D [4,3] particle sizes ranging from 20 to 1085 μm. The fractions were pre-digested with the INFOGEST protocol and fermented in vitro with a human fecal inoculum prepared by pooling fecal donations from five healthy adult volunteers. No significant effect of barley husk particle size was observed on short chain fatty acid (SCFA) production except for butyrate, which was significantly higher after fermentation of the coarsest particles at 48 h. Moreover, the barley husk samples richest in starch produced higher levels of SCFAs. Reduction of particle size increased phenolic acid bioaccessibility, especially for ferulic and coumaric acid during in vitro digestion and their release during in vitro fermentation. Gut microbiota composition determination with 16S rRNA gene amplicon sequencing, followed by PERMANOVA revealed that the starch content, rather than particle size, of barley husk drove microbial variation at 48 h. Our data illustrate that starch content, rather than particle size, is the main driver of phenolic acid bioaccessibility, microbial composition and SCFA production.