Optimized microwave pretreatment and ultrasound-assisted deep eutectic solvent extraction for efficient valorization of Citrus paradisi peel bioactives

This study presents a sustainable approach for recovering bioactive compounds from grapefruit (Citrus paradisi) peel using a glycerol-glucose-based deep eutectic solvent as a green extraction medium. Spectroscopic characterization confirmed the formation of stable hydrogen-bonded eutectic networks within the solvent systems. Response surface methodology (RSM) based on a Box-Behnken design was employed to systematically investigate the effects of solvent concentration, solvent-to-solid ratio, and extraction time on phytochemical recovery and antioxidant activity. The extraction process was successfully optimized using RSM, with the optimum conditions identified as 74% water-content Aq-DES, 20 min sonication time, and a solvent-to-solid ratio of 35 mL/g. Under these conditions, the optimized extract exhibited total phenolic and flavonoid contents of 86.33 mg gallic acid equivalents (GAE)/g DW and 5.93 mg rutin equivalents (RE)/g DW, respectively, along with strong antioxidant activities, achieving 87.40% scavenging of 2,2-diphenyl-1-picrylhydrazyl (DPPH) radicals and 55.09% scavenging of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radicals. The developed RSM models demonstrated strong statistical adequacy and reliable predictive performance. Furthermore, the optimized extracts exhibited notable antibacterial activity against Staphylococcus aureus and Salmonella typhi, producing inhibition zones of 21 and 20 mm, respectively, in the undiluted extract. Overall, the findings demonstrate that glycerol-glucose deep eutectic solvent based microwave pretreated ultrasound-assisted extraction provides an efficient, rapid, and environmental friendly strategy for valorizing grapefruit peel waste into high-value antioxidant and antimicrobial products, with promising potential for applications in the food and pharmaceutical industries.