Fragmentation of waxy rice starch granules by enzymatic hydrolysis

Small starch particles were prepared by hydrolyzing waxy rice starch using α-amylase and then ultrasonicating in ethanol. Differential scanning calorimetry (DSC) revealed that a mild hydrolysis for 3 hr increased the melting enthalpy of the starch, which might indicate that the hydrolysis was selective in the amorphous regions. Later, at 6-24 hr, the hydrolysis rate was reduced, with gradual decreases in DSC melting enthalpy, indicating that the crystalline regions were eroded simultaneously. X-ray diffraction patterns revealed the same trend as the DSC results. Average diameter of starch granules or particles was decreased dramatically in both volume- and number-based measurements (5.94→1.64 pm, and 0.45-→ 0.18 μm, respectively) during the early stage of rapid hydrolysis (up to 3 hr). Native waxy rice starch exhibited a particle size distribution with a major peak at 5.6 pm. After hydrolysis for 3 hr, the volume distribution of starch granules changed to two major size peaks at 0.5 and 3.6 μm. The starch fragment of 0.5 pm was assumed to consist of crystalline blocklets. With excessive hydrolysis (24 hr) or ultrasonication, however, starch particle diameter was increased, indicating that the particles might be swollen or aggregated into clusters.