Changes in composition and structure of wheat bran resulting from the action of human faecal bacteria in vitro.

Cell-wall material of wheat bran was incubated with human faecal bacteria for 24-72 h and the resulting structural changes were studied by methylation analysis. Of the carbohydrate content, approximately 39% was degraded after 24 h, increasing to only 44% after 72 h. Arabinoxylans and mixed-linkage beta-D-glucans from the aleurone layer were degraded preferentially. After treatment of the bran with alkali, the extent of degradation was increased three-fold as a result of saponification of ester cross-links which facilitated increased degradation of the polymers from both the aleurone and outer, lignified, layers. There was evidence that ester linkages between the glucuronosyl residues, attached to O-2 of the (1----4)-linked xylosyl residues, and phenolic groups of lignin were also saponified. The treatment with alkali also rendered the cellulose more susceptible to bacterial attack. The alkali-soluble acidic arabinoxylan fractions of the bran were degraded readily by bacterial action, but the xyloglucans cross-linked to arabinoxylans by phenolics were relatively resistant.