Ionic Liquid-Based Ultrasonic-Assisted Extraction of L-Citrulline from Watermelon Rind

Purpose L-citrulline is a non-essential and non-protein amino acid with potent antioxidant properties. In this study, the new aqueous two-phase system (ATPS) was used with and without ultrasound for the first time to isolate and purify L-citrulline from watermelon rind simultaneously. Methods This system consists of an ionic liquid 1-Butyl-3-methylimidazolium bromide [C4mim] [Br] and dipotassium hydrogen phosphate (K 2 HPO 4 ) salt. Accordingly, the impact of increasing the salt concentration, the ionic liquid, the solid–liquid ratio, and the ultrasound power and time on the extraction efficiency of L-citrulline and sugar compounds were examined. The HPLC technique Was employed to determine the concentration of L-citrulline in the sample with the highest efficiency rate. Moreover, the DLS tests were applied to determine the particle size. Results As the concentrations of the ionic liquid, salt, and solid–liquid ratio were increased in the aqueous two-phase extraction system (ATPS) without applying the ultrasonic assistant, the L-citrulline extraction efficiency was increased initially and then decreased. Based on the results, applying the ultrasound to the sample with the highest amino acid efficiency rate, improved the extraction process and enhanced the amino acid extraction efficiency by 90.45%. The best extraction conditions obtained are related to the set, including the 1.5 M ionic liquid, 35% salt, the 1:40 solid–liquid ratio, the power of 100 W, and the ultrasonic time of 15 min. Conclusion In general, the ionic liquid is a perfect extracting substance, and the ILUAE approach is a simple, efficient, fast, green, and environmentally friendly method for extracting ingredients. Graphical Abstract