Comparison of Phenolic Compounds and Antioxidant Properties of Sweet Potato (Ipomoea batatas (L.) Lam) according to Variety and Moist Heat Cooking

Sweet potato [Ipomoea batatas (L.) Lam] is usually cooked by steam treatment. Various cooking processes using heat could certainly induce changes in physical characteristics and chemical composition. Therefore, this study was performed to investigate the total polyphenol contents (TPCs), phenolic composition, antioxidant activities [1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity (RSA), total radical trapping antioxidant potential (TRAP), oxygen radical absorbance capacity (ORAC), and cellular antioxidant capacity (CAC)], and antigenotoxic activities by comet assay of three sweet potato varieties (general Ipomoea batatas: GIB, yellow Ipomoea batatas: YIB, and purple Ipomoea batatas: PIB) subjected to ethanol and hot water extractions in order to identify changes in biological activities under fresh and steamed conditions. The highest TPC was observed for PIB under fresh and steamed conditions. After steaming, TPCs of YIB [0.5 vs 0.6 mg/100 g garlic acid equivalents (GAE)] and PIB (1.4 vs 6.9 mg/100 g GAE) were elevated in ethanol extract, and TPC of GIB hot water extract was significantly changed. The extracts showed great variation in phenolic composition, including phenolic acids (chlorogenic acid, caffeic acid, and vanillic acid etc.) and flavonoids (morin, quercetin, and quercitrin etc.) in their high-performance liquid chromatography profiles. Under steamed condition, chlorogenic acid, vanillic acid, and morin compounds were reduced compared to fresh sweet potatoes. Especially, p-coumaric acid, ferulic acid, and resveratrol compounds were detected in PIB and decreased in steamed condition. In the DPPH RSA result, PIB showed the highest level among all extracts. At the same time, the DPPH RSA value under steamed conditions significantly increased. TRAP activity, ORAC, and CAC values of PIB were the highest among the three kinds of fresh sweet potatoes. However, under steamed conditions, TRAP activity of PIB significantly increased. In the comet assay, PIB under fresh and steamed conditions showed the highest inhibitory capacity against H2O2-induced DNA damage, followed by YIBGIB. There were no differences between fresh and steamed conditions. This study shows that antioxidant and antigenotoxic capacities were affected by sweet potato varieties and steamed conditions.