Morphology and structural characterization of high-amylose rice starch residues hydrolyzed by porcine pancreatic α-amylase

High-amylose starches are attracting considerable attention because of their potential health benefits and industrial uses. Enzyme hydrolysis of starch is involved in many biological and industrial processes. In this paper, starches were isolated from mature grains of high-amylose transgenic rice line (TRS) and its wild type rice cultivar Te-qing (TQ). The morphological and structural changes of starch residues following porcine pancreatic α-amylase (PPA) hydrolysis were characterized by SEM, DSC, XRD, 13C CP/MAS NMR, and ATR-FTIR. TQ starch was hydrolyzed faster than TRS starch. PPA pitted the starch granule surface first, then penetrated into the interior and hydrolyzed the granule from the inside out. The peripheral region of subgranule and the surrounding band of TRS starch were more resistant to PPA hydrolysis than the external region of TQ starch. Both amorphous and crystalline (the long- and short-range ordered) structures were simultaneously hydrolyzed in TQ starch. A-type polymorph of TRS C-type starch was hydrolyzed faster than B-type polymorph. The long-range ordered structure was hydrolyzed faster than the amorphous structure, whereas the short-range ordered structure was hydrolyzed slower than the amorphous structure in TRS starch in the initial hydrolysis stage. The above results indicated that B-type crystallinity and short-range ordered structure in the peripheral region of subgranule and the surrounding band of TRS starch increased the resistance of TRS starch to PPA hydrolysis.