Explain the Fermentability or Degradability?

As you are aware that colon contains over 400 known species of bacteria that exist in a symbiotic relationship with the host. All fibres are broken down to some extent by these microorganisms. Fermentation depends on the accessibility of the molecules to the microorganisms, which in turn depends on physical properties particularly solubility. Soluble fractions especially pectin, gums, mucilages and algal polysaccharides are very accessible and ferment rapidly. Insoluble fibre fraction ferment much more slowly. The first step in fermentation is the breakdown of polysaccharides, oligosaccharides and disaccharides to their monosaccharide subunits by hydrolytic enzymes of bacteria. Monosaccharides are further converted to various end products as seen in Figure.

Figure: Overview of carbohydrate fermentation on colon

According to the calculations by Cummings and Macfarlane, if approximately 20 g of fibre is fermented in the colon each day, 200 mM SCFA will be produced, of which 62% will be acetate, 25% propionate and 16% butyrate. Of these, butyrate is almost completely consumed by the colonic mucosa, while acetate and propionate enter the portal circulation. The mechanism by which SCFAs cross the colonic mucosa is thought to be a saturable process-passive diffusion of unionized acid into mucosal cells. In addition to these acids, other products of fibre fermentation are hydrogen, carbon dioxide and methane gases that are excreted as flatus or expired by the lungs.