Degradation and digestion of wheat and legume starch in broiler chickens as affected by pelleting and extrusion
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Starch is a quantitatively most important source of energy in broiler chicken diets. Diets may contain up to 50% starch on a dry matter basis. Despite high starch contents in the diet, broiler chickens can utilize a variety of starch sources very efficiently. Dietary starch is the major energy source for broiler chickens, so any knowledge about its digestive behavior can be important. Pancreatic α-amylase is the major responsible enzyme for starch digestion in birds. Susceptibility of starch to α-amylase may differ from source to source. According to previous studies, the rate and extend of starch digestion in legumes is lower than cereals. To test this hypothesis, digestibility trial with 200 broiler chickens was studied. Starch digestion was determined using the slaughter technique, which involves the removal of the small intestine from the recently killed chicken, with manual collection of the contents. Site and extent of starch digestion of pelleted and extruded wheat (WS) and air classified faba bean starch (FBS) fraction based diet examined in 30 day-broiler chickens. The digesta samples were collected from the upper and lower parts of both jejunum and ileum. In addition to starch digestibility analysis, light microscopy imaging was performed to evaluate the rate of starch digestion of the broiler chickens fed with pelleted and extruded diets. Scanning electron microscopy (SEM) imaging was also performed to compare starch morphology in pelleted diets and digesta samples taken from upper jejunum and ileum. The effect of starch digestion rate on the nitrogen digestion was also evaluated. Results of the light microscopy imaging study showed that starch from the pelleted faba bean starch fraction (FBS-P) based-diet was digested more slowly, whereas starch from pelleted wheat (WS) based-diet was digested most rapidly. On the other hand, starch from the extruded diets was digested and absorb mainly in the upper part of the jejunum. SEM images also showed that starch in digesta samples taken from the upper jejunum and ileum of the broiler chickens fed with faba bean starch fraction (FBS) based-diet remained undigested compared to wheat based-diet. Furthermore, extend and rate of the starch digestion were discussed for both cereal and legume based diets. The literature study on different starch sources and factors affecting starch digestion were extensively reviewed and presented in this thesis.