Starch, the major source of carbohydrates in commercial avian diets, is the most abundant storage polysaccharide in plants. Native starches differ in granule size, amylose to amylopectin ratio and amylose structure. Extrusion in simple terms is a process of cooking under pressure, moisture and elevated temperature. There are several effects of extrusion processing on protein, lipid and starch by changing physical, chemical and nutritional properties. Yet, starch plays a key role during extrusion, and undergoes several significant structural changes, which include starch gelatinisation, melting and fragmentation.

Related literature gives no consensus on the impact of extrusion processing; hence this review will give an overview on factors affecting gelatinisation degree of starch and the effects of extrusion processing on performance and nutrients digestibility in poultry.

We evaluated the influence of the main cereal of the diet and the degree of heat processing of rice on apparent ileal and total tract digestibility of nutrients and ileum morphology of pigs at 37 d of age. Control pigs were fed a complex diet that contained 500 g/kg cooked and flaked maize with a degree of starch gelatinisation (SG) of 840 g/kg. Experimental groups received the same complex diet in which maize was substituted (w/w) by rice with three different degrees of SG; 110, 520 and 760 g/kg that corresponded to raw rice and cooked rice processed under two different set of conditions. The digestibility of dietary components, except for nitrogen, was greater for the cooked rice – than for the raw rice – and the maize diet (P < 0·001). Pigs fed cooked rice had higher villus height to crypt depth ratio (P < 0·001) and greater percentage of zigzag-oriented villi and lesser percentage of tongue-oriented villi (P < 0·01) than pigs fed other diets. Compared with feeding maize, feeding rice improved dietary component digestibility and ileal morphology in piglets. Mild cooking of rice (SG = 520 g/kg) enhanced diet digestibility and ileum morphology. However, processing the rice to increase SG to 760 g/kg did not increase nutrient digestibility further and in fact impaired ileal morphology.