Characterization and comparative study of starches from seven purple sweet potatoes

Sweet potato starch has great edible and economic value and has been widely studied. Starches from different varieties have significantly different properties. Previous studies are focused on white, yellow and orange sweet potato. In this study, starches were isolated from purple sweet potato variety Ningzi 1 and six advanced breeding lines with different genotypes, their structural and functional properties were investigated and compared. Seven starches all had round, polygonal, oval and semi-oval shapes with central hila, but showed different granule sizes. The starch content in dry root tubers varied from 40.1 to 55.1%. The apparent amylose content ranged from 24.6 to 31.0%. The seven sweet potatoes all had CA-type starch. The swelling power and water solubility at 95 °C ranged from 22.5 to 29.0 g/g and 13.1 to 16.9%, respectively. The starches displayed a two peak gelatinization thermogram, the first peak temperature ranged from 64.1 to 69.0 °C and the second peak temperature ranged from 73.5 to 80.8 °C. The peak viscosity, hot viscosity, breakdown viscosity and setback viscosity varied from 2535 to 3750 mPa s, 1517 to 2450 mPa s, 1018 to 1622 mPa s, and 474 to 1036 mPa s, respectively. The seven starches had different hydrolysis degrees to porcine pancreatic α-amylase and Aspergillus niger amyloglucosidase. All the starches had high resistance to the in vitro digestion. This study could provide important information for the utilization of purple advanced breeding lines in breeding and food and nonfood industries.