Changes in Polyphenolic Content and Radical-Scavenging Activity of Sweetpotato (Ipomoea batatas L.) during Storage at Optimal and Low Temperatures

Polyphenolic content and radical-scavenging activities (RSA) of four sweetpotato (Ipomoea batatas L.) cultivars were characterized after storage at optimal (15 °C) or low temperature (5 °C) for 0, 13, 26, and 37 days. The polyphenolic content increased during storage in three cultivars but not in ‘Murasakimasari’. The change in 1,1-diphenyl-2-picrylhydrazyl radical-scavenging activity (DPPH-RSA) correlated very well with polyphenolic content. The increases in polyphenolics and the RSA in ‘Benimasari’ were significantly greater during storage at 5 °C than at 15 °C. The main polyphenolic components in all cultivars were chlorogenic acid (ChA) and 3,5-di-O-caffeoylquinic acid (3,5-diCQA). ChA level increased more at 5 °C than at 15 °C, whereas that of 3,5-diCQA was greater at 15 °C. Caffeoylquinic acids and RSA in ‘Murasakimasari’, which contains a large amount of anthocyanin in flesh tissue, were extremely high at the beginning of storage and remained nearly constant or decreased over time. A non-caffeoylquinic acid component that increased during storage, especially in ‘J-Red’ at 15 °C, was purified by successive chromatographic steps. The isolate was identified as caffeoyl sucrose [CSu, 6-O-caffeoyl-(β-d-fructofuranosyl-(2→1))-α-d-glucopyranoside] by fast atom bombardment−mass spectroscopy (FAB-MS), infrared spectroscopy (IR), and nuclear magnetic resonance spectroscopy (NMR). These results suggest that storage under cultivar-dependent, controlled temperature is one approach for increasing desirable physiologic function associated with RSA of polyphenolic compounds in sweetpotato roots.