Drying methods affect bioactive compound contents and antioxidant capacity of Bletilla striata (Thunb.) Reichb.f. flower

Abstract

Bletilla striata (Thunb.) Reichb.f. is a perennial plant widely used as food, medicine and ornamentation. To determine the optimal processing method and promote the application of B. striata flowers, this work investigated the influences of diverse drying treatments on properties, structures, bioactive compound contents and antioxidant capacity of B. striata flowers. The drying methods applied in the study included natural drying, microwave drying, hot air drying, infrared drying, vacuum drying and freeze drying. Microwave drying was the fastest way for completion of drying, followed by infrared drying. Freeze drying had a relatively good color protection effect on the B. striata flowers, and the degree of browning was low. Compared with natural drying, the content of polysaccharide decreased following freeze drying, while the retention of total phenols and total anthocyanins increased significantly. Natural drying required a long time, and the degree of browning was high. Under the condition of natural drying, anthocyanins were almost completely degraded after long drying time and the degree of browning was high, but the retention of total flavonoids and total polysaccharides were higher compared with freeze drying. The methanolic extracts of flowers treated by natural drying and freeze drying both showed high antioxidant capacity. The reduction ability of natural drying group was slightly worse than that of infrared drying and hot air drying. The diammonium 2,2′-azino-bis-(3-ethylbenzothiazoline-6-sulphonate) (ABTS), 1,1-diphenyl-2-picrylhydrazyl (DPPH), and OH radical scavenging ability were higher following natural drying. As a consequence, freeze drying was the best way for protecting most characters, contents and activities of B. striata flowers. In consideration of production costs, we recommend infrared drying as a more practical choice for industrial production.