Tri-frequency ultrasound-assisted extraction of phenolic compounds from Citrus maxima/grandis peels: Process optimization and cultivar-dependent phytochemical profiling.

Pomelo (Citrus maxima/Citrus grandis) peels habitually discarded as agronomic waste, holds a valuable reservoir of bioactive secondary metabolites. To promote sustainable waste valorization, this study investigated tri-frequency ultrasound-assisted extraction (TFUAE) for the recovery of phenolic compounds from pomelo peels and optimize extraction conditions using response surface methodology based on a Box-Behnken design. Extraction time (20-60 min), temperature (20-60 °C) and solid-to-liquid ratio (1:20-1:60 g/mL) were evaluated using total phenolic content (TPC) and total flavonoid content (TFC) as response variables. The optimized conditions were 40 °C, 40 min with a solid-to-liquid ratio of 1:40 g/mL. Under these conditions, water TFUAE produced the highest TPC (339.81 mg GAE/g DW) and TFC (384.82 mg QE/g DW). Compared with conventional solvent extraction, TFUAE enhanced TPC and TFC yields by 4.24- and 4.22-fold, respectively and signify superior extraction efficiency over mono- and dual-frequency ultrasound systems. Antioxidant activity assessed by DPPH, ABTS and FRAP assays also differed significantly among cultivars and extraction solvents. HPLC-VWD analysis identified 13 major phenolic compounds with substantial variation among cultivars and FTIR analysis further confirmed the presence of characteristic functional groups associated with phenolic constituents.