Structural features of soluble cereal arabinoxylan fibers associated with a slow rate of in vitro fermentation by human fecal microbiota

•Alkali-solubilized arabinoxylans had a range of in vitro fecal fermentation rates.•All arabinoxylans were highly branched and substituted.•Complex wheat and corn arabinoxylans had slow initial but complete fermentation.•Fast fermenting rice and sorghum arabinoxylans had simple branched structures.•Terminal xylose and complex trisaccharide branches related to slow fermentation.