Food Structure


Intrinsic characteristics of plant cell walls limiting susceptibility of structural carbohydrates to microbial attack in the ruminant's gastrointestinal tract are lignification of the cell wall, covalent bonding of phenolic acids to cell wall polysaccharides, the crystalline structure of cellulose and limited fibrolytic enzyme accessible space. The exact mechanism by which or degree to which each of these characteristics affect rate and/or extent of cell wall polysaccharide hydrolysis by gastrointestinal tract microbes is not well understood. Lignification and limited enzyme accessible space probably affect the extent of cell wall degradation by preventing contact between microbial enzymes and cell wall polysaccharides. Phenolic acids may limit cell wall carbohydrate degradation by steric hindrance of the fihrolytic enzyme, which could affect both rate and extent of degradation, and by their potentially toxic effects on microbes. Crystalline cellulose, occurring in secondary cell walls, may be degraded at a slower rate than amorphous cellulose. Further research is needed to gain a better understanding of the mechanisms by which these characteristics limit structural polysaccharide degradation by gastrointestinal tract microbes and to determine to what degree each contributes to limiting digestibility of cell wall carbohydrates by ruminants .

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