Abstract
The present study compared the effect of passive direct solar dryer (PDSD) and passive indirect solar dryer (PISD), each followed by a 30-day storage period, on the secondary plant metabolite and nitrate contents of amaranth, Abyssinian mustard, and pumpkin leaves. Total carotenoid contents were retained in amaranth (0.70-0.85 mg g(-1) dry weight [DW]) and pumpkin leaves (1.04-1.16 mg g(-1) DW) following PDSD and PISD. However, for Abyssinian mustard, a significant decrease of the total carotenoid contents (45% PDSD, 33% PISD) was observed compared to the control (oven-dried leaves) (0.98 mg g(-1) DW). Total flavonoid contents were maintained across the vegetables (2.83-78.20 mg g(-1) DW) irrespective of the drying treatment, except for amaranth leaves, where a significant decline (54% PISD) was observed compared to the control (5.83 mg g(-1) DW). Although no significant difference was found in total phenolic acid contents between the control and the variants in amaranth, a significantly higher content was observed in pumpkin leaves (25.12 mg g(-1) DW, PDSD) compared to the control (17.38 mg g(-1) DW). Exceptionally for pumpkin leaves, a significant decline of the total chlorophyll contents (18%, PDSD; 16%, PISD) was observed after a 30-day storage compared to the initial contents (8.37 mg g(-1) DW, PDSD; 6.49 mg g(-1) DW, PISD). The nitrate contents obtained in this study (80-460 mg kg(-1) fresh weight [FW], PDSD; 364-915 mg kg(-1) FW, PISD) were below the maximum allowable limits by the European Commission Regulation (2000-4500 mg kg(-1) FW), implying the safety of the dried vegetables with respect to nitrate levels. PRACTICAL APPLICATIONS: This study suggests the utilization of passive solar dryers as a cost-effective drying alternative for smallholder farmers and households in vegetable quality preservation and minimization of postharvest losses.