Valorization of Citrus limon residues for the recovery of antioxidants: Evaluation and optimization of microwave and ultrasound application to solvent extraction

Ultrasound assisted extraction (UAE) and microwave-assisted extraction (MAE) were optimized (by response surface methodology, RSM) and compared for the recovery of total phenolic compounds (TPC expressed as gallic acid equivalents (GAE)) from Citrus limon peels. The optimized result for MAE was 48% ethanol as extraction solvent, 28:1 mL/g of solvent: solid ratio, 123 s and 400 W for irradiation time and power. The optimized result for UAE was 63.93% ethanol as extraction solvent, 40 mL/g of liquid/solid ratio, 15.05 min of holding time and 77.79% for amplitude. Maximum predicted TPC recoveries under the optimized conditions for MAE and UAE were 15.74 and 15.08 mg GAE/g respectively, which were close to the experimental values of 15.78 ± 0.8 and 15.22 ± 0.88 mg GAE/g, indicating suitability of the employed model and the success of RSM in optimizing the extraction conditions. The antioxidant activity determined by the DPPH and reducing power tests confirmed the suitability of MAE for the preparation of antioxidant-rich plant extracts.