Determination of the Antioxidant Activity of Ethanolic Lemon and Grapefruit Peel Extracts Using the DPPH Assay

Abstract

Aims: The aim of this study was to quantitatively evaluate and compare the antioxidant activity of ethanolic extracts of lemon and grapefruit peels, as well as their mixture, using the DPPH spectrophotometric method, and to assess their free-radical-scavenging activity at different concentrations. Study Design: A laboratory-based quantitative experimental study was designed to evaluate the antioxidant activity of the samples. Methodology: Ethanolic extracts of lemon peel (Citrus limon), grapefruit peel (Citrus paradisi), and their mixture were prepared. Antioxidant activity was determined using the DPPH (2,2-diphenyl-1-picrylhydrazyl) spectrophotometric method by measuring absorbance at 517 nm. Different extract concentrations (approximately 10–50 μL, according to the experimental design) were tested. The relationship between concentration and absorbance was analysed using linear regression. Results: All tested extracts exhibited concentration-dependent antioxidant activity. Lemon peel extract had the highest absorbance values (approximately 0.20 at 20 μL, 0.33 at 30 μL, and 0.53 at 50 μL), indicating the strongest antioxidant activity among the samples. Grapefruit extract had lower absorbance values (approximately 0.09 at 10 μL, 0.11 at 20 μL, and 0.24 at 50 μL), whereas the mixture exhibited intermediate values (approximately 0.11 at 10 μL, 0.19 at 30 µg/mL, and 0.26 at 40 μL). Strong linear correlations between concentration and absorbance were observed, with coefficients of determination (R²) of 0.9979 for lemon, 0.9852 for grapefruit, and 0.9804 for the mixture, confirming the reliability of the method. Conclusion: All tested extracts exhibited significant antioxidant activity, which increased with increasing concentration. Lemon peel extract showed the highest antioxidant activity, followed by the extract mixture, whereas grapefruit peel extract demonstrated the lowest activity. The results confirm the reliability of the DPPH method for assessing the antioxidant capacity of plant extracts and indicate the potential application of citrus peels as a natural source of antioxidants in the food, pharmaceutical, and cosmetic industries.