Combined Effect of Microwave and Steam Cooking on Phytochemical Compounds and Antioxidant Activity of Purple Sweet Potatoes

This study evaluates the effects of a microwave and steam cooking combination on the sugar composition, phytochemical compounds (total phenolics, total flavonoids, total anthocyanins and phenolic acids and anthocyanin compositions) and antioxidant capacity of purple sweet potatoes (PSPs). The combination of microwave and steam cooking could significantly decrease the cooking time. The results show an obvious increase in the quantity of total phenolics, flavonoids, and anthocyanins in PSPs after cooking. The microwave and steam cooking combination induced the greatest increase for the total flavonoids and total phenolics, with values of 22.2% and 26%, respectively. Among the phenolic acids in the PSP extract, chlorogenic acid was the major constituent, and phenolic acid concentrations increased after cooking, except for caffeic acid. An HPLC-MS analysis of anthocyanin showed that the cyanidin-3-(6"-caffeoyl-6"'-feruloylsophoroside)-5-glucoside and peonidin-3-(6"-caffeoy1-6"'-feruloylsophoroside)-5-glucoside contents in PSPs cooked with steaming for 10 min followed by microwaving for 3 min (S-10 - M-3) were highest among the cyanidin and peonidin compounds. The PSPs exhibited a greater antioxidant capacity after cooking, and the S-10 - M-3 cooking method produced the greatest antioxidant capacity. Therefore, an appropriate combination of microwave and steam cooking can better retain phytochemical compounds in PSPs and increase antioxidant capacity compared with only steaming or microwaving.