Phytochemical Characterization of Cucumis callosus towards Potential Valorization: Effect of Maturity Stage and Drying Temperature on Bioactivity of Polyphenols Extracted from Fruit Waste
Cucumis callosus is an underutilized desert fruit with significant nutritional and therapeutic promise. Despite its availability in arid and semi-arid regions, C. callosus remains largely untapped regarding its phytochemical and nutraceutical potential. In this study, three anatomical parts of the fruit- peel, seed, and pulp were investigated across different physiological maturity stages: tender, mature, and old, with particular emphasis on the peel and seeds, which are typically treated as agro-food waste or domestic kitchen waste. Each sample was subjected to various drying methods, including 35°C, 40°C, and 45°C, as well as shade- and sun-drying, to assess the influence of processing conditions on phytochemical retention. Antioxidant activity was evaluated using the DPPH radical scavenging assay, along with quantification of total phenolic content (TPC) and total flavonoid content (TFC), followed by the identification of key metabolites by LC-MS analysis of fruit peel extracts. Principal component analysis of fruit peels revealed a correlation between TPC, TFC, and antioxidant activity. The results revealed marked variations in antioxidant capacity across maturity stages and in response to drying temperatures, highlighting the impact of post-harvest treatment procedures on the bioactive profile of Cucumis callosus. Drying at 35 °C resulted in the highest phytochemical retention, with tender peels exhibiting the maximum TPC- 9.33 ± 0.07 mg GAE/g and TFC 130.78 ± 2.15 mg QE/g, followed by tender seeds TPC: 1.57 ± 0.07 mg GAE/g; TFC- 16.05 ± 0.25 mg QE/g and tender pulp TPC- 1.20 ± 0.04 mg GAE/g; TFC- 12.56 ± 0.29 mg QE/g of the extract. LC–MS analysis of samples dried at 35 °C revealed the presence of bioactive metabolites, including chlorogenic acid, rutin, and ellagic acid in tender peels; rutin, and kaempferol-3-O-neohesperidoside in mature peels; and caffeic acid, vitexin, and methyl ellagic acid in old peels. The findings reveal that fruit components, often treated as waste, possess considerable antioxidant potential, reinforcing sustainable use of the resources and alignment with responsible production and consumption practices (SDG 12). This study also provides foundational data for the potential valorization of this desert fruit as a source of natural antioxidants, supporting its integration into functional food systems or nutraceutical formulations.