Molecular characterization of corn starch using an aqueous HPSEC-MALLS-RI system under various dissolution and analytical conditions

Molecular characteristics based on absolute weight-average molecular weight (M-w) and z-average radius of gyration (R-g) of normal corn starch were an alyzed by high-performance size-exclusion chromatography (HPSEC) attached t o multiangle laser-light scattering (MALLS) and refractive index (RI) detec tors under different starch dissolution and analytical conditions. Autoclav ing (121 degrees C, 20 min) or microwave heating (35 sec) provided better H PSEC recovery and higher M-w for starch molecules than simple dissolution i n hot water. The M-w for the autoclaved corn amylopectin and amylose fracti ons separated with a TSK G5,000 column at 60 degrees C were 201 x 10(6) and 3.3 x 106, respectively. The specific volume for gyration (SVg) calculated from M-w and R-g could be used for the comparison of molecular compactness which was inversely related to the degree of branching. The SVg values of amylopectin and amylose fractions in the chromatogram (TSK G5,000, autoclav ed for 20 min) were 0.092 and 0.529, respectively. But a portion (20-30%) o f large amylopectin molecules did not pass the injection membrane filter (3 .0 mu m) and the SEC column, resulting in incomplete recovery. The unfilter ed portion varied according to the dissolution treatment. Homogenization (7 ,000 rpm, 5 or 10 min) of the starch solution improved the recovery of the amylopectin fraction, but significantly increased the M-w of the amylose fr action (17 x 10(6)). Sonication for 5 min degraded starch molecules. For ac curate analysis of a native starch using an aqueous SEC, the starch should be fully dissolved with proper treatment such as autoclaving or microwaving , and the column should be improved for full recovery of large amylopectin molecules.