Shear degradation of molecular, crystalline, and granular structures of starch during extrusion

This work describes the first investigation of starch degradation during extrusion occurring at multiple structural levels and explains the effects of the thermal and mechanical energy of extrusion. Investigated samples comprised starches with a range of amylose contents and of glycerol/water plasticizer contents. Structural analysis was performed using size-exclusion chromatography, XRD and light microscopy. The (branch) chain length distribution did not show apparent changes upon either thermal or mechanical energy treatment. Statistical analysis showed that mechanical energy played a dominant role in reducing starch molecular size and degree of starch crystallinity, while thermal energy only partially gelatinized starch granules with negligible effect on molecular size. The rigid crystallites of amylopectin in starch granules are more susceptible to shear degradation than the flexible amorphous amylose. Previous studies did not draw quantitative conclusions as to the relative importance of these two types of energy in extrusion on starch structural degradation. This mechanistic understanding from multi-level characterization is helpful to design the processing of starch-based biopolymers with improved functional properties.