Study on structural changes of microwave heat-moisture treated resistant Canna edulis Ker starch during digestion in vitro

Microwave heat-moisture treated Canna edulis Ker starch has been applied to study structural changes in vitro using porcine α-amylase, pancreatin and amyloglucosidase. The structures at different digestion stages were characterized using scanning electron microscopy (SEM), polarized optical microscopy (POM), small angle X-ray scattering (SAXS), X-ray diffraction (XRD), Fourier transforms infrared (FT-IR) spectroscopy and solid state 13C nuclear magnetic resonance (NMR) spectroscopy. The increase in molecular order was observed with increasing digestion time and reflected in higher scattering intensity measured by SAXS, higher crystallinity observed by XRD, the increase of the double helix order obtained by solid state 13C NMR spectroscopy, and corresponding changes in the measurement of FT-IR, in favour of increasing resistant starch content. SAXS reveals a single peak around 0.6335 nm−1, suggesting that the enzyme may erode the special site of granule surface and catalyze whole hydrolysis reaction through pitted canals. In the process of digestion, amorphous region of starch granule is susceptible to the attack of enzyme. Moreover, the result also demonstrates that the resistance to enzymatic digestion may mainly depend on specific structure in the treated starches.