Abstract: Sweet potato starch was sequentially treated with α-amylase (80 mg), β-amylase (10.6 mg) and different amounts (50, 100, 150, 200, 250 and 300 μL) of transglucosidase. The chain length distribution, branch density, crystal structure and rheological properties of sweet potato starch before and after enzymatic hydrolysis were analyzed by high performance anion-exchange chromatography equipped with pulsed amperometric detection (HPAEC-PAD), proton nuclear magnetic resonance (1H NMR) spectroscopy, X-ray diffraction and rheometry. The results showed that after the enzymatic treatment, the short-chain ratio of sweet potato starch increased significantly (P < 0.05) and the average chain length decreased significantly (P < 0.05), and when 300 μL of transglucosidase was added, the branch density of sweet potato starch was as high as 35%, significantly higher than that of the control sample. The crystal structure was transformed from C type to C + V type after the enzymatic modification. The relative crystallinity of the modified starches was lower than that of the natural starch, which proportionally increased from 10.8% to 14.8% with the amount of transglucosidase added. The solubility and transmittance were significantly higher than those of the control, and they increased with the addition of transglucosidase, reaching the highest value of 66.15% and 73.91%, respectively. The apparent viscosity, storage modulus and loss modulus were significantly lower than those of the control group (P < 0.05), and they all decreased with the addition of transglucosidase. These results provide a theoretical and practical basis for modifying the physicochemical properties of sweet potato starch by a multi-enzymatic method at the level of branch chain structure.

Key words: multi-enzymatic method, sweet potato starch, branch density, physicochemical properties, modification