Starch structure and digestibility enzyme-substrate relationship

Digestion of starch is effected by hydrolysing enzymes in a complex process which depends on many factors; these include the botanical origin of starch, whether the starch is amorphous or crystalline, the source of enzymes, substrate and enzyme concentration, temperature and time, as well as the presence of other substances in the multicomponent matrix in which starch occurs naturally, e.g. cereal grains. Native starch is digested (i.e. hydrolysed) slowly compared with processed (gelatinised) starch whose crystallinity has been lost and where the accessibility of substrate to enzymes is greater and not restricted by alpha-glucan associations such as double helices (especially in crystallites) or amylose-lipid complexes (in cereal starches). The restriction of starch digestion (primarily in the human digestive system) due to forms which are resistant to hydrolysis has led to the concept of dietary 'resistant-starch'. Different forms of resistance can be identified which hinder hydrolysis. With regard to digestibility, whether in the human or animal digestive tract, it is important to understand the mechanisms of enzymatic hydrolysis, and the consequence of incomplete digestion i.e. the potential loss of glucose as a valuable source of energy. This review deals with starch hydrolysis by specific amylases in model (in vitro) and more broadly in in vivo systems. The optimisation of starch hydrolysis and digestion is discussed in the light of modern knowledge.