Structure characterization of C-type starch granule by acid hydrolysis

The structural changes of C-type Dioscorea rhizoma starch are evaluated by scanning electron microscope (SEM), X-ray powder diffraction (XRD) and cross polarization/magic-angle spinning (CP/MAS) 13C nuclear magic resonance (NMR) during acid hydrolysis. SEM shows that the amorphous areas are mainly located in the core part of C-type starch granules or distribute alternately in the crystalline regions. XRD analysis reveals that the B-type polymorphs present in the C-type starch granules are preferentially degraded or degraded faster than A-type polymorphs. NMR spectra confirm that the amorphous regions in the starch granules are firstly hydrolyzed and could be hydrolyzed completely as long as the hydrolysis time is enough. After 40 days of hydrolysis, the acid-modified starch shows typical A-type characteristics whether for the X-ray diffraction pattern or for the 13C CP/MAS NMR spectra. The finding suggests that the B-type polymorphs consisting of the C-type starch granules are more unstable than the A-type polymorphs. This is not in agreement with the fact that B-type starches (potato starch) are more resistant to acid and enzyme than A-type cereal starches.