The effects of three drying methods on phenolic and carotenoid content and antioxidant activity (AOA) of Tainong 66 cultivar sweet potato (
Ipomoea batatas
Lam.) were investigated. Roots of 12 batches of freshly harvested tuberous root were washed in tap water, allowed to dry, peeled and cut into small pieces of 0.5 to 1 cm of thickness. Three samples of 500±2 g of each batch were dried at 25°C in a low-temperature dryer adjustable within the temperature range of 0 and 50°C, the second and third sets of three in hot air in an oven at 50 and 75°C. The last set of three served as control. The samples (dried and fresh) were then extracted and analyzed for total phenolics, total flavonoids, total carotenoids, β-carotene, and AOA. The AOA was assessed using 2,2-dipheny-1-picrylhydradzyl (DPPH) and anti-oxidative potency in linoleic acid system model (AOP) assays. The color parameters were also measured using a Color Quest II - Sphere Colorimeter (Hunter Lab, Reston, VA, USA) using the L, a, b scale. The results showed significant decrease as a result of drying in the color parameters and the contents of total phenolics, total flavonoids, total carotenoids and β-carotene. DPPH free radical scavenging activity (FRSA) and AOP also decreased significantly. Large variations of decrease were observed among the different treatments in terms of color change, phenolic and carotenoid content, and AOA during the drying process. On average, higher drying temperature corresponded to higher extent of losses of phenolics and AOA. The losses of DPPH FRSA were chiefly due to losses of phenolics during the drying process. Those results suggested that the thermal process is prejudicial to the Tainong 66 cultivar sweet potato in terms of phenolic and carotenoid content and AOA. When drying is to be done, low temperature drying (LTD) is more suitable to preserve the AOA of Tainong 66 cultivar sweet potato.
