The effect of acid hydrolysis on morphological and crystalline properties of Rhizoma Dioscorea starch

Starch isolated from Rhizoma Dioscorea (Jichengerhao-JCEH) was subjected to acid hydrolysis for 2, 4, 8, 16 and 32days, respectively. The acid-thinned starch was evaluated for starch yield, morphological, crystalline and particle size distribution properties by scanning electron microscope (SEM), wide-angle X-ray powder diffractometer (WAXRD) and light scattering particle size analyzer (LSPSA). The starch yield reduced from 100% for unmodified starch to 25.5% for starch subjected to 32days of acid hydrolysis. The SEM results revealed that the hydrogen ion preferentially attacked the inner amorphous regions of the starch granules followed by the outer crystalline areas. That is why starch granules did not show any corrosions or fissures at the first stage of acid hydrolysis. The average particle size of starch decreased from 23.4 μm for unmodified starch to 19.9 μm for acid-thinned starch for 16days. Wide-angle X-ray diffraction patterns indicated that the crystalline type of starch changed from C-type to A-type after 16 days of acid hydrolysis. This result demonstrated that B-type starch present in C-type starch was firstly degraded in the process of acid hydrolysis. The degree of crystallinity of starches increased gradually following the acid thinning.