Characterization of Starch by Size-Exclusion Chromatography: The Limitations Imposed by Shear Scission

Shear degradation is examined in size-exclusion chromatography (SEC, or GPC) of native starch in an eluent system (dimethylsulfoxide and LiBr) in which the starch is completely dissolved. Changes in apparent size distribution with flow rate suggested extensive shear scission of the amylopectin region. For smaller sizes, largely amylose, there was no significant scission for lower flow rates. Quantification by analogy to shear breakup of dispersed droplets gives a scaling law for conditions for shear scission of highly branched polymers. This shows both that it is impossible to obtain reliable size distributions for the amylopectin component of starch using current SEC technology and also that the amylose region is not significantly polluted by degraded amylopectin for lower flow rates. Hence, the complete size distribution of starch can only be obtained with SEC for smaller sizes (largely amylose), plus a size-separation technique with very low shear, such as field-flow fractionation, for the amylopectin region.