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

Abstract

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 ( M w ) of 254×10 6 g/mol, and the M w 's of amylopectin and amylose in normal corn starch were 243×10 6 and 3.13×10 6 g/mol, respectively. However, some amylose chains in normal corn starch tended to aggregate by stirring. Those in high amylose corn starch were 197×10 6 and 1.49×10 6 g/mol, respectively. Therefore, as the amylose level increased in corn starch, the M w 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.