Starch Digestion- Factors limiting Strach digestion

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Starch, a glucan, is the natural glucose storage polysaccharide of plants. It occurs as microscopic birefringent granules, build up in concentric layers, which stain blue or red with iodine. Starch is most abundant in seeds, which may contain up to 700 g/kg and in fruits, tubers and roots, which may contain 300 g/kg. The two types of starch are an unbranched structure consisting of long chains of a-D-glucose units linked through carbon 1,4. The other is branched and consist of short chains of a-1,4-linked glucose units joined with other chains to form a large molecule. The linear component is amylose and the branched one is amylopectin, which contain an appreciable number of a-1,6-glucisidic linkages, responsible for branching. There are over 1,000 glucose units in amylopectin, and that branching occurs about one over twenty five units. Most starches of cereal grains and potatoes contain between 15-30% amylose and 85-70% amylopectin. The enzyme, b-amylase, of malted barley completely degrades amylose to maltose. There a few a-1,3 linkages in amylose which are degraded by a specific plant enzyme called Z enzyme. b-amylose does not completely degrade amylopectin, rather, it stops acting after approximately 50% of the amylopectin is converted to maltose. Amylopectin consists of chain lengths of 25-30 glucose units with some 50-70 branches connected through a-1,6 glucosidic linkages. Only the amylose fraction of starch reacts with iodine to yield a blue colour.

Starch granules are insoluble in cold water, but when a suspension in water is heated to about 10°C, the granules swell and eventually gelatinise. On gelatinization, potato starch granules swell greatly and then burst open; cereal starches swell but tend not to burst. The digestibility of starches depends on the degree of crystallisation or character of the outermost layer of the starch granule. To increase the digestibility of starches, their granular structure must be disrupted by gelatinization through cooking, by dextrinization (degradation to lower molecular weight units) through enzymatic or acid hydrolysis or by ball-milling for two or more hours.

Factors limiting starch digestion in ruminants:

Incomplete intestinal starch digestion has been reported in many studies. Based largely on these studies, the following factors suggested to be responsible for the incomplete digestion of starch in ruminants.            

Pancreatic amylase. An inadequate supply of pancreatic amylase has been suggested a factor limiting small intestinal starch digestion. Thus, at high levels of starch intake the small intestine unhydrolyzed because of limited supply of pancreatic amylose. Amylase is also secreted from the small intestinal mucosa. Starch hydrolysis is most rapid when amylase is associated with intestinal mucosa, because association of such acids causes the transfer of released maltose to maltase, which results in increased absorption of released glucose. In addition to total starch intake, percentage of starch in the diet also affects amylase production. The pancreatic amylase activity was noted to be elevated in pancreatic tissue of steers fed higher levels of corn.           

Intestinal pH level: The optimal pH for bovine pancreatic amylase activity is very important. The optimal pH for pancreatic amylase near neutrality (6.8). Decreases or increases in pH by 0.5 unit result in 20% reduced amylase activity. A decrease in faecal pH with increased faecal starch content could result from increased large intestinal microbial fermentation of the starch that escape digestion in the small intestine. Intestinal pH also depends on diet composition. The pH of digesta throughout small intestine is usually lower with concentrate that with roughage diet. Therefore , digestion of starch in the small intestine is pH dependent besides amylase concentration.

Glucose Absorption: It is known that glucose is readily absorbed from the small intestine, which limit the starch digestion. The digesta reaching small intestine is quite low in soluble carbohydrates due to extensive fermentation in the rumen, which had low soluble carbohydrates then to decrease the digestibility of starch in the small intestine.

Starch Content In Grains:  Many of the processing effects and species differences in starch digestibility can be explained by particle size and starch exposure to alterations due to grain processing and mastication. The rate of grain starch digestion in the rumen varies inversely with particle size of the grain. Furthermore, processing of corn either by high moisture grinding and ensiling or steam flaking result in more total in vivo starch digestibility than does dry rolling or treating high moisture shelled corn. Younger cattle chew this feed more thoroughly than older, so they can utilise grains more efficiently than others.

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