Structural changes of corn starches by heating and stirring in DMSO measured by SEC-MALLS-RI system

The molecular structure of corn starches of different amylose contents (waxy, normal, and high amylose) was analyzed in 90% dimethyl sulfoxide (DMSO) solution by a medium-pressure size-exclusion chromatography connected to refractive index (RI) and multi-angle laser light scattering (MALLS) detectors. The starches were purified as amorphous powders using DMSO and ethanol, and then dissolved in aqueous DMSO (90%) using different boiling and stirring times. For starch dissolution, boiling was required, and subsequent stirring at room temperature increased the starch solubility, measured by filtering (5.0 μm pore size). After 1 h boiling followed by 24 h stirring at room temperature, waxy and normal corn starches were completely soluble in the DMSO solution. For high amylose corn starch, however, 8 h stirring after 1 h boiling yielded complete dissolution, indicating that amylopectin is more difficult to dissolve than amylose. However, excessive boiling (2 h or longer), autoclaving (121 °C, 15 min), or even excessive mechanical stirring caused starch chain degradation. For minimum chain degradation with good solubility, three corn starches were dissolved by stirring for 8 h after boiling for 1 h. Under the condition, waxy corn starch showed a weight-average molecular weight (Mw) of 254×106 g/mol, and the Mwʹs of amylopectin and amylose in normal corn starch were 243×106 and 3.13×106 g/mol, respectively. However, some amylose chains in normal corn starch tended to aggregate by stirring. Those in high amylose corn starch were 197×106 and 1.49×106 g/mol, respectively. Therefore, as the amylose level increased in corn starch, the Mw of the amylopectin decreased. The amylose contents in normal and high amylose corn starches, measured by chromatography under the selected dissolution condition, were 28.2 and 74.5%, respectively.