Structural basis for the low digestibility of starches recrystallized from side chains of amylopectin modified by amylosucrase to different chain lengths

The structures and physicochemical properties of modified starch recrystallized from amylosucrase-modified amylopectin side chains were investigated during in vitro digestion. The digestion of the modified starches followed the first-order kinetics and revealed the existence of rapid and slow digestion phases. The B-type crystalline structure of the modified starches remained unchanged, but the degree of crystallinity remarkably increased during the initial 20-min digestion, followed by insignificant changes after 120-min digestion. Enzymatic digestion for 20 min increased the thermal stability of the starches, but a slight decrease in the gelatinization temperatures was observed after 120-min digestion. Starch digesta showed irregular shapes, and no obvious damage was observed. These observations suggest that the digestion of the modified starches started from the loosely packed outer shell consisting mainly of amorphous materials, followed by even digestion of the interior amorphous and crystalline regions of the starch particles to result in slow digestion properties.