Effect of different drying methods on nutritional and functional properties of leafy vegetables

Abstract:

Hypothesis: if vitamin C degrades primarily through heat and oxygen exposure rather than time alone, then a drying method combining lower temperature with faster moisture removal should outperform both traditional sun drying and high-temperature oven drying on nutrient retention, a prediction this research set out to test directly on amaranth and jute mallow leafy vegetables. Four drying methods, open sun drying, shade drying, oven drying (60 °C), and solar tunnel drying, were compared for vitamin C retention, chlorophyll content, antioxidant activity, and rehydration ratio. Solar tunnel drying preserved vitamin C most effectively at 68% retention relative to fresh leaf content, outperforming sun drying (54%), shade drying (61%), and oven drying (41%), broadly confirming the research's initial hypothesis, since solar tunnel drying combines a moderate temperature profile with meaningfully faster drying time than shade drying achieves at a similarly gentle temperature. Chlorophyll retention followed a similar ranking, with oven drying causing the most severe chlorophyll degradation despite its shortest total drying duration, confirming that temperature, not drying time alone, drives much of the pigment loss recorded across treatments. Antioxidant activity, measured through DPPH radical scavenging assay, showed solar tunnel drying retaining the highest activity among the four methods, while rehydration ratio, relevant to how the dried product performs when reconstituted for cooking, favored shade drying slightly over solar tunnel drying despite the latter's superior nutrient retention profile. These findings support solar tunnel drying as a genuinely superior method for nutrient-focused leafy vegetable preservation, offering most of shade drying's gentle temperature benefit while achieving considerably faster processing time, a combination with direct relevance for rural processing operations balancing nutrient quality against practical processing throughput.