Simultaneous ultrasound-microwave assisted deep eutectic solvent extraction of essential oil from calamansi (Citrus × microcarpa) peel: optimization, volatile profiling, and antioxidant activities

Calamansi (Citrus × microcarpa) peel, a byproduct of juice processing, is often discarded, leading to resource waste and environmental issues. This study developed an eco-friendly and efficient method for extracting essential oil (EO) from calamansi peel using deep eutectic solvent (DES) combined with simultaneous ultrasound-microwave-assisted steam distillation (DES-UM-SD). Through screening ten DES formulations and eight extraction methods, choline chloride-lactic acid (ChCl-LA) was identified as the optimal solvent, and DES-UM-SD was selected as the most effective method. The optimal extraction conditions were determined as follows: DES water content of 55.4 %, solid-liquid ratio of 1:4, ultrasonic power of 432 W for 30 min, and microwave treatment at 52 °C and 300 W for 30 min. Under these conditions, the EO yield reached 1.76 % ± 0.02 %, a 120 % increase over conventional steam distillation (0.80 % ± 0.02 %). Scanning electron microscopy (SEM) revealed different effects of ultrasound, microwave and DES on the peel's microstructure. A total of 139 volatile compounds were identified based on gas chromatography-mass spectrometry (GC-MS) analysis, with D-limonene as the most abundant. The DES-UM-SD-extracted EO exhibited the highest antioxidant activity in DPPH and FRAP assays and moderate ABTS scavenging capacity. Correlation analysis indicated that DES altered the profile of minor volatile compounds, influencing antioxidant properties. This work provides a sustainable strategy for high-value utilization of calamansi peel byproducts.