Impact of Passive Solar Drying and Storage on Secondary Plant Metabolites and Nitrate Contents of Abyssinian Mustard, Amaranth, and Pumpkin Leaves

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.