Molecular weight, chain length distribution and long-term retrogradation of cassava starch modified by amylomaltase

Cassava starch was modified with amylomaltase (AMase) at different enzyme/substrate ratios (E/S) and reaction times to explore the structural changes and the retrogradation properties. AMase-treated starch (ATS) showed an increase in weight-average molecular weight (Mw) at a short reaction time, and the action mode of AMase revealed two trends separated by the E/S. At high E/S (≥2 U/g), Mw increased initially and then decreased with time, while during the 48-h treatment with low E/S (≤1 U/g), Mw increased slightly with time. AMase treatment led to the reorganization of chain distributions in amylose and amylopectin. Amylose was hydrolyzed, its content and degree of polymerization (DP) were reduced, and the hydrolyzed segments were transferred to amylopectin, thereby extending the chain length and broadening the distribution. Some DP 9–25 amylopectin chains were elongated to DP > 25, resulting in the mass increase in DP 26–75 at high E/S and in DP 35–55 at low E/S; the short chains (A or B1 chains) of amylopectin were extended to long chains (B2, B3, and B4 chains). AMase treatment resulted in a severe long-term retrogradation at high E/S along with long reaction time. The long-to-short chain ratio in amylopectin (L/S) was positively correlated with long-term retrogradation. (E/S)2t was proposed as a parameter to determine whether the domination of AMase action mode was disproportionation or hydrolysis, with a critical value of 49.2. A new insight into the structural characteristics and long-term retrogradation of ATS was revealed through L/S, E/S, and reaction time.