Carbohydrate chemistry and rumen digestion

Carbohydrates are an essential component of the diet of all animals.The carbohydrate component of ruminant diets can be divided into two groups, the first consisting of the so-called available carbohydrate which is available not only to the ruminant but to all animals.This group can also be considered as the intracellular carbohydrates.The second group consists of the unavailable or partially available carbohydrates.This group can be considered as non-available to monogastric animals but can be degraded and utilized to a large extent by ruminants.These carbohydrates are the structural polysaccharides of the plant cell wall and are commonly referred to by nutritionists as 'fibre'.The simple sugars found in the cell contents of most plants are glucose, fructose and sucrose.These sugars do not need to be degraded in the rumen before being metabolized by the rumen micro-organisms.Also considered to be in the cell contents are the storage polysaccharides of the plants.They are the energy store of the plants and require to be rapidly mobilized and metabolized.Animals can degrade these polysaccharides with relative ease too.The most widespread storage polysaccharide in the plant kingdom is starch which can account for 70% of the dry matter of some cereal grains and up to 3070 of the dry matter of fruits, roots and tubers.All starches contain two components, amylose and amylopectin, and the properties of starches depend on the proportion of each component.Amylose is basically a linear molecule of repeating ~+qa-~-glucopyranosyl residues with chain lengths of 1000-2000 sugar residues.Amylopectin is a much larger branched molecule of up to 200000 sugar residues.These are arranged such that short chains of 20-25 ~+qa-~-glucopyranosyl residues are linked at the reducing end by a 1+6a linkage to another of these short chains.Different models have been proposed but the Whelan model is currently accepted.Starch is degraded by the action of a variety of amylases.T h e enzymes are present in the saliva of many animals and are also secreted from the pancreas.Starchdegrading enzymes are