Soluble lignin: a potential prebiotic modulating gut microbiota and short-chain fatty acid production in in vitro fermentation

Lignin (Lg), a major dietary phenolic polymer, remains poorly characterized as a substrate for colonic fermentation. We compared an aqueous soluble lignin (SLg) with two less soluble fractions produced by thermal (Lg-200) or alkali-thermal treatment (Lg10–200) processing using an in vitro batch fermentation model. SLg produced modestly higher total short-chain fatty acids (SCFAs) than the other lignin fractions (from ~7.80 to ~8.72 mM), with relatively greater acetate (from ~3.01 to 4.24 mM) and propionate (from ~2.71 to ~3.50 mM), and a pH decrease from 6.60 to 6.15. It also increased microbial diversity and enrichment of taxa linked to carbohydrate fermentation ( e.g. , Ruminococcus torques , Megamonas ), alongside reduced pro-inflammatory Collinsella . Moreover, SLg significantly decreased the Firmicutes/Bacteroidota (F/B) ratio relative to the blank (from ~44.02 to ~30.59, p  < 0.05). These results suggest that soluble lignin may influence gut microbial composition and could be further explored as a potential functional ingredient.