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

## TL;DR

This study compares solar drying methods on vegetable nutrients, finding that passive solar drying preserves metabolites and keeps nitrate levels safe.

## Contribution

The novel contribution is demonstrating that passive solar drying preserves secondary metabolites and meets nitrate safety standards in leafy vegetables.

## Key findings

- Passive solar drying preserved carotenoids in amaranth and pumpkin leaves but reduced them in Abyssinian mustard.
- Pumpkin leaves showed higher phenolic acid content after solar drying compared to controls.
- Nitrate levels in dried vegetables were below EU safety limits, regardless of drying method.

## 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.

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.