Enzymatic arabinose depletion of wheat arabinoxylan regulates in vitro fermentation profiles and potential microbial degraders

Arabinoxylan (AX) is a major dietary fibre in wheat/rye consisting of a xylan backbone with one or two single arabinose units attached to selected backbone xylose units. Whether these substitution patterns affect AX-degrading gut microbes, and to what extent, is incompletely understood. Herein an in vitro fermentation study is reported where a porcine faecal inoculum (as a model for human large intestine fermentation) was used to investigate the fermentability of wheat AX before and after hydrolysis by enzymes that selectively hydrolyse mono- or di-substituted arabinose residues, or both. Results showed that lower arabinose-to-xylose (A:X) ratios and lower di-substitution levels contributed to slower substrate disappearance, but higher microbial diversity evenness (alpha diversity). AX structure also affected microbial beta diversity, where genera regarded as potential AX degraders differed between the enzymatically-modified AX samples. Interestingly, higher Prevotella relative abundance was associated with higher A:X ratios and faster substrate fermentability, while higher relative abundances of Lachnospiraceae XPB1014 group, Lachnospiraceae MK4A136 group and Lachnospiraceae UCG-009 were linked to more slowly fermentable AX. Gas kinetics and end-product analyses revealed: i) gas production rates but not total gas production depended on both the A:X ratio and substitution patterns; ii) faster fermentation rates led to higher propionate proportions but lower acetate and butyrate proportions; iii) ammonia concentration resulting from different substrates reflected fermentation kinetics. Correlation analyses revealed the potential link between microbial AX degraders and fermentation outcomes specific to AX fine structures. Overall, this study reveals that gut microbial composition and fermentation outcomes can be altered by modifying the fine structure of arabinoxylan.