Comparative Evaluation of Ethanol and Malic Acid–Glycerol Deep Eutectic Solvent for the Extraction of Antioxidant Phytochemicals from Casuarina equisetifolia Stem Bark

Abstract

The growing interest in sustainable extraction technologies has promoted the application of deep eutectic solvents (DESs) as environmentally friendly alternatives to conventional organic solvents for recovering bioactive compounds from plant materials. However, comparative studies between ethanol and DES for extracting antioxidant compounds from Casuarina equisetifolia stem bark remain limited. This study evaluated the antioxidant potential of C. equisetifolia stem bark extracts obtained using a malic acid–glycerol DES and 96% ethanol. The DES was synthesized at a glycerol-to-malic acid molar ratio of 2:1 and characterized by Fourier transform infrared (FTIR) spectroscopy. Stem bark extraction was performed using ultrasound-assisted extraction (UAE), followed by qualitative phytochemical screening, determination of total phenolic content (TPC) using the Folin–Ciocalteu method, and antioxidant activity evaluation using the DPPH radical-scavenging assay. FTIR analysis confirmed the successful formation of the DES through characteristic shifts in the O–H and C=O stretching bands, indicating hydrogen-bond interactions between glycerol and malic acid. Qualitative phytochemical screening revealed the presence of phenolics, flavonoids, terpenoids, and saponins in both extracts. The DES extract exhibited a higher total phenolic content (50.3 mg GAE g^-1 extract) than the ethanol extract (42.2 mg GAE g^-1 extract). Consistently, the DES extract demonstrated stronger antioxidant activity with an IC₅₀ value of 42.2 ppm, compared with 54.6 ppm for the ethanol extract. The enhanced extraction performance of the DES is attributed to its hydrogen-bonding network, which facilitates the recovery of phenolic antioxidants. These findings demonstrate that the malic acid–glycerol DES is an effective green extraction solvent for producing antioxidant-rich extracts from C. equisetifolia stem bark and supports its potential application in the development of sustainable cosmetic ingredients.