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Most insoluble dietary fibers are known to be relatively poorly fermented by the human gut microbiota. Here, the potential of microwave (MW) treatment to enhance the susceptibility of insoluble fruit polysaccharides to fermentation by the human gut microbiota was evaluated. Insoluble fruits dietary fibers before (xylan A, xylan T, and arabinan) and after MW (xylan A-MW, xylan T-MW, and arabinan-MW) treatment were fermented using an in vitro fermentation model. Gas production, shifts in pH, and short chain fatty acids (SCFAs) production showed an increase in fermentability of all tested dietary fibers, with an average 4-fold increase in SCFAs production after microwaving with total SCFAs ranging from 17.1 mM in the arabinan-MW to 40.4 mM in the xylan T-MW. While arabinan-MW and xylan T-MW promoted all three SCFAs proportionally (acetate:propionate:butyrate), xylan A-MW led to a marked and slow increase in butyrate reaching 28.1% of total SCFAs at 24 h. Rearrangements in three-dimensional structure that potentially facilitate bacterial accessibility to the dietary fiber were observed by scanning electron microscopy in xylan A-MW, forming coin-like particles with ~1.1 µm diameter. 16S rRNA gene sequencing indicated that microbiota shifts were related to both treatment (native versus MW) and dietary fiber type with many butyrogenic species being promoted by xylan A-MW. Overall, MW treatment enhanced insoluble dietary fiber fermentability promoting increased SCFAs production and bacterial shifts which are related to health benefits.

During processing of refined cereal grain foods, cell wall fibers present in whole grains are diminished and are often replaced with dissimilar soluble fibers. Contrary to a view that soluble fibers are generally preferable for health and that the removed insoluble bran cell wall fibers of whole grains provide bulk for laxation, a body of evidence now shows that these insoluble fibers support key mucosal-associated commensal Clostridial bacteria important for gut and whole-body health. Commensal Clostridia possess cellulosomes, a specialized extracellular apparatus with enzymes, that can degrade the physical form of the insoluble fiber matrices. They show a preference to matrix fibers, and when the same insoluble bran fiber polymers are made soluble, Clostridia are outcompeted by other bacteria. Clostridial group members are major producers of butyrate, an important fermentation metabolite impacting human health. Butyrate suppresses localized and systemic inflammation, maintains and promotes good epithelial barrier function, and provides energy to the colonocytes, among other associated benefits. This article proposes that an important aspect of whole grain fiber consumption is to support these beneficial gut bacteria, and not just simply providing a higher amount of dietary fiber. This view differs from the regulatory perspective where all fibers, regardless of structure, are labeled as “dietary fiber”, and where the addition of a soluble fiber to a refined cereal product is the same as the removed insoluble matrix fiber.

The purpose of this study was to compare the fermentative properties of enzymatically extracted barley bran dietary fibers using a batch in vitro fecal fermentation method. Fibers with different β-glucan contents and molecular weights (Mw) were utilized. Fecal fermentation profiles were evaluated based on pH values, produced gas, total short chain fatty acid (SCFA), ratio and amounts of SCFA including acetate, propionate and butyrate during 6, 12 and 24 h of fermentation. The samples were compared with fructooligosaccharide (FOS) and blank for mentioned properties. Fiber extracted by α-amylase which had the lowest β-glucan purity (~55%) and the highest Mw (163 kDa) indicated the lowest gas production, the lowest amount of total SCFA production and the highest pH after 24 h of fermentation. On the other hand, Fiber extracted by α-amylase, protease and glucoamylase which had moderate β-glucan purity (~68%) and relatively low Mw (26 kDa), showed the greatest gas production, the highest total SCFA amount, the highest amounts of acetate, propionate and butyrate and the lowest pH, at the end of the fermentation period. This study helps to provide a new approach to obtain details regarding the fermentability of the enzymatically extracted dietary fibers.