Structural changes in corn starches during alkaline dissolution by vortexing

Corn starches of different amylose contents (waxy, normal, and high amylose) were dissolved in a strong alkaline solution (1 M NaOH) by vigorous vortexing at room temperature, and analyzed by a medium-pressure size exclusion chromatography (SEC), using 50 mM NaOH as eluent, connected to refractive index and multi-angle laser light scattering detectors. The vortexing increased the solubility of the corn starches, measured as the recovery from filtration (5.0 μm pore size). After 20 min vortexing, the solubility of waxy corn starch reached 89.4%. For normal and high amylose corn starches, however, vortexing for less than 5 min yielded a solubility of more than 90%. It indicates that amylopectin is more difficult to dissolve in alkaline solution than amylose. The size exclusion chromatographic pattern changed with vortexing time, indicating that excess vortexing caused molecular degradation. In the dissolution condition with minimum degradation and good solubility (15 min vortexing), waxy corn starch exhibited an average molecular weight (Mw) and radius of gyration (Rg) as 185×106 g/mol and 214 nm, respectively. The Mw (Rg) values of amylopectin and amylose in normal corn starch, following vortexing for the optimal time of 5 min were 164×106 (255 nm) and 3.3×106 g/mol (154 nm), respectively. The Mw (Rg) of amylopectin and amylose in high amylose corn starch were slightly smaller than those in normal corn starch: 113×106 (175 nm) and 2.7×106 g/mol (99 nm), respectively.