Phytochemical Analysis and Antioxidant Activity of Brebes Red Onion (Allium cepa L.) Peel Using Deep Eutectic Solvent Based on Green Chemistry

Abstract: Brebes Regency is the largest red onion (Allium cepa L.) production centre in Indonesia, generating large quantities of red onion peel waste that remains underutilised. Red onion peel contains bioactive compounds including flavonoids, phenolics, and tannins with potential as strong antioxidants. This study aimed to analyze the phytochemical content and antioxidant activity of Brebes red onion peel extracted using a green-chemistry-based Deep Eutectic Solvent (DES) compared with conventional ethanol solvent. This laboratory experimental study used a posttest-only control group design and the Ultrasound-Assisted Extraction (UAE) method, with ChCl:glycerol DES (molar ratio 1:2) as the treatment group and 96% ethanol as the control. Qualitative phytochemical tests (flavonoids, phenols, saponins, alkaloids, tannins), Total Flavonoid Content (TFC) analysis using the AlCl3 method, Total Phenolic Content (TPC) analysis using the Folin-Ciocalteu method, and DPPH antioxidant assay were performed. The results showed that the DES extract tested positive for flavonoids, phenols, alkaloids, and tannins, whereas the ethanol extract tested positive only for flavonoids, phenols, and tannins. The TFC of the DES extract (57.94 mg QE/g) was 52.4% higher than that of ethanol (38.02 mg QE/g). The TPC of the DES extract (42.06 mg GAE/g) was 23.1% higher than that of ethanol (34.16 mg GAE/g). The antioxidant activity of the DES extract, with an IC50 of 70.38 ppm (strong category), was better than that of ethanol (IC50 = 76.56 ppm), with vitamin C (IC50 = 6.61 ppm) as the positive control (very strong category). ChCl:glycerol DES (1:2) proved more effective than 96% ethanol in extracting bioactive compounds from Brebes red onion peel, supporting the sustainable utilization of agricultural waste.