How does temperature govern mechanisms of starch changes during extrusion?

Abstract

Potato and pea starches were processed on a twin-screw extruder under various moisture and thermomechanical conditions, chosen to keep material temperature T_e close to starch melting temperature, T_m, whilst avoiding die expansion. Extruded rods were analysed by asymmetrical flow field flow fractionation coupled with light scattering, X-ray diffraction, DSC, and light microscopy with image analysis. Molar mass of extruded materials decreased more for potato than for pea starch, when specific mechanical energy SME increased, likely because of larger amylopectin sensitivity to shear. No crystallinity was detected when ΔT = (T_m-T_e) ≤ 0. Residual gelatinization enthalpy ΔH_g decreased with ΔT. As illustrated by larger ΔT values for ΔH_g = 0, decreasing moisture favored melting, likely by increasing solid friction. The fraction of granular remnants of potato starch was inversely correlated to SME. These results could be explained by considering starch melting during extrusion as a suspension of solid particles embedded in a continuous amorphous matrix.

Keywords: Crystal; Granule; Pea; Potato; Specific mechanical energy; Suspension.