Kinetics, physicochemical properties, and antioxidant activities of Angelica keiskei processed under four drying conditions

The herb Japanese angelica (Angelica keiskei) has been consumed for centuries as a healthy vegetable. The drying kinetics, microstructure, color, and polyphenol components, as well as antioxidant capacity of stems and leaves of A. keiskei were investigated and compared, after undergoing natural drying (ND), convective drying (CD), freeze-drying (FD), and vacuum oven drying (VOD). The freeze-dried products showed the lowest color differences compared with their fresh counterpart, and also provided the highest retention of the main polyphenol components (i.e., quercetin, luteolin, and chlorogenic acid) and antioxidant activity (DPPH and FRAP), followed by ND. The CD and VOD processes at 60 °C in which heat was applied caused the additional loss of color, total phenolic content (TPC), total flavonoid content (TFC), and antioxidant activity. In addition, the leaves exhibited stronger antioxidant activity and contained higher phenolic content than stems; for example, the quercetin content in fresh stems and leaves was 89.82 vs 638.20 mg/kg dry weight, respectively. Our results implied that FD could be a superior drying technique for A. keiskei, however, high cost of FD procedures may limit its application. Natural drying thus could be an alternative method when finance becomes the main concern.