From Food Waste to Functional Ingredients: Comparative Analysis of the Physicochemical Components and Antioxidant Capacity of Vegetable Peels

Vegetable processing and household consumption generate substantial amounts of peel waste, which may serve as valuable sources of bioactive compounds. In this study, the pH, titratable acidity (TA), total phenolic content (TPC), total flavonoid content (TFC), and antioxidant capacities determined by FRAP, CUPRAC, and DPPH assays were comparatively evaluated in peels obtained from 12 different vegetable wastes. The pH values ranged from 3.99 to 6.36, while titratable acidity varied between 1.19 and 6.02 g/100 g. Total phenolic content ranged from 3.32 to 27.96 mg GAE/g DW, whereas total flavonoid content ranged from 0.12 to 32.19 mg QE/g DW. Antioxidant capacities varied from 2.26 to 29.28 mg TE/g DW for FRAP, 0.30 to 3.35 mg TE/g DW for CUPRAC, and 0.53 to 12.99 mg TE/g DW for DPPH. Red onion peel exhibited the highest total phenolic content and FRAPS value, whereas eggplant peel showed the highest total flavonoid content and CUPRAC value. Red cabbage peel demonstrated the highest DPPH radical scavenging activity. Overall, vegetable peels with purple pigmentation tended to exhibit higher phenolic contents and antioxidant capacities than non-pigmented samples. These findings highlight the potential of vegetable peels as natural antioxidant sources and functional food ingredients, contributing to the sustainable valorization of food waste within circular economy frameworks. The utilization of antioxidant-rich vegetable peels in functional foods is recommended; however, the absence of individual phenolic compound characterization represents a limitation of the present study.