Exogenous Microbial Amylase in the Diets of Poultry: What do We Know?

Ingested starch is a polymer of glucose that consists of α-1,4 and/or α-1,6 bonds. When α-1,4 bonds occur between glucose monomers, the resulting polymer is called amylose, whereas when α-1,6 bonds occur the resulting polymer is referred to as amylopectin. Amylopectin, on the other hand, has a more chaotic macro-structure as the α-1,6 branching points disrupt the helical structure creating amorphous regions often referred to as “waxy.” The ratio between amylose and amylopectin in starch determines whether a starch may be categorized as “waxy” or “high amylose” and this varies between and within cereal grain types. Waxy corn may contain as much as 99% amylopectin, whereas high amylose corn starch may be more than 70% amylose. Furthermore, there is considerable heterogeneity in the size of the starch granules in seeds from various plants and in the nature of the crystalline micro- and macro-structure that favors, or not, digestion. Following initial hydrolysis by endogenous amylases, the process of starch digestion is completed by the brush-border enzymes maltase and iso-maltase, and the resulting glucose is absorbed via Na-dependent transport systems for eventual metabolism via the citric acid cycle. As most poultry diets contain 400–500 g/kg starch, successfully converting this ingested starch into glucose is critically important to energetic status and this may be particularly true in older animals with a much greater starch intake. Therefore, factors which delay, impede or interfere with the digestion of starch have a profound effect on the energy value of a given diet. Such factors include the nature of the starch crystals, the extent of starch “damage” during hydrothermal processing of feed or feed ingredients, factors that may inhibit the optimal functioning of endogenous amylases and issues around extraction of glucose from the lumen via Na-dependent transport systems. It was the purpose of this review to discuss such factors and to outline the usefulness of exogenous sources of amylase in monogastric nutrition with emphasis both on direct digestible energy effects.