Evacuated tube collector based solar drying of pre-treated okra (Abelmoschus esculentus): Analysis of thermal performance, bioactive compounds, antioxidant activity, and functional properties

Okra (Abelmoschus esculentus) is a good source of micronutrients and a highly perishable vegetable crop. The okra pods were dried in an evacuated tube collector-based novel solar drying system, i.e. an evacuated tube solar dryer with heat pipe (ETSD_HP), and evacuated tube solar dryer without heat pipe (ETSD_WHP) and sun drying. The okra was pre-treated at different temperatures prior to drying, and the thermal profiling, moisture diffusivity, and retention of quality attributes were investigated and compared. The highest moisture diffusivity was found in the sample pre-treated at 80 °C and dried in ETSD_HP. The results show that Total Phenolic Content (TPC), 2,2-diphenylpicrylhydrazyl (DPPH), and 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) content increased across all drying techniques, while the Total Flavonoid Content (TFC) and Ferric Reducing Antioxidant Power (FRAP) content were decreased. The bioactive compounds, namely TPC and TFC retention, were maximum for okra dried in ETSD_HP. The maximum retention of chlorophyll content (76.70 to 93.20%), vitamin C (73.84 to 93.73%), and minimum color changes (7.70 to 13.48) were observed for the all the sample dried in ETSD_WHP. The pre-treatment temperature of 80 °C was found to be appropriate for the maximum retention of quality attributes of okra. FTIR analysis identified functional groups of active compounds in dried okra for all the drying methods. Therefore, both the ETSD_HP and ETSD_WHP techniques are appropriate to retain the bioactive compounds and functional properties of okra as compared to Sun drying. ETSD_HP is recommended for increased moisture diffusion with maximum retention of bioactive compounds.