In vitro colon fermentation behaviors of Ca2+ cross-linked guluronic acid block from sodium alginate

The degradation of sodium alginate by human gut microbiota was found to be retarded via calcium cross-linking in our previous study. We hypothesized that the guluronic acid block (GB) on the alginate molecule might be the key structural region affecting alginate degradation by the gut microbiota when cross-linked with calcium. This study aims to prove this hypothesis by studying the structural features of the cross-linked GB on its in vitro fecal fermentation behaviors concerning the aspects of total carbohydrate contents, monosaccharide contents, short-chain fatty acids production, calcium state variations, and structural variations. Herein, GB isolated from sodium alginate was cross-linked under ranges of molar ratios of [Ca²⁺]/[-COOH] that further restricted the degradation by gut microbiota similar to the cross-linked alginates. First, total carbohydrate contents, short-chain fatty acids production, monosaccharides contents, and calcium state analyses confirmed that the degradation of GB by gut microbiota was restricted by calcium cross-linking. Furthermore, the tracking analysis of structural variations during in vitro fermentation revealed that the "granules" structure could further restrict degradation by the gut microbiota, leaving more cross-linked GB fragments surviving in comparison to the "networks" structure. In addition, Bacteroides xylanisolvens showed a significant positive correlation to the "cross-linking porosity (R = 0.825, p < 0.001), which supported our previous findings on fermentation behaviors of cross-linked alginate. Together, guluronic acid blocks are the key structural regions that retard the degradation of sodium alginate by the gut microbiota when cross-linked with calcium.