Structural changes of debranched corn starch by aqueous heating and stirring

Aqueous dispersions (2 mg/mL) of debranched corn starches of different amylose contents (waxy, normal, and high-amylose) were subjected to extensive autoclaving and boiling-stirring, and then the changes in starch chain profile were examined using medium-pressure, aqueous, size-exclusion column chromatography. As autoclaving time increased from 15 to 60 min, weight-average chain length (CL_w) of waxy, normal, and high-amylose corn starches determined using pullulan standards decreased from 46 to 41.2, from 122.1 to 96.3, and from 207.3 to 151.8, respectively. Number-average chain length (CL_n) measured by the Nelson-Somogyi method also decreased from 23.0 to 18.4, from 26.4 to 21.8, and from 66.5 to 41.5, respectively, indicating that thermal degradation of starch chains occurred. The CL_w/CL_n ratio for normal corn starch was higher than that for waxy corn starch, indicating an increase in polydispersity of the amylose fraction. Thermal degradation was also observed when the debranched starch was subjected to the boiling-stirring treatment (0-96 hr). During 96 hr, the CL_w and relative proportion of B≥2 chains of amylopectin released by debranching waxy corn starch increased, whereas those of B1 chains decreased. This change may indicate physical aggregation of B1 chains. But branches from normal and high-amylose corn starches showed increases in CL_w and the proportion of both B1 and B≥2 chains, along with substantial decreases in those of amylose chains. Therefore, thermal degradation of amylose was greater than that of amylopectin.