Microwave-assisted aqueous two-phase extraction of l-Citrulline from watermelon rind: optimization, antioxidant potential, and microstructural insight

Abstract

Watermelon is a rich natural source of Citrulline, an amino acid with potential antioxidant and vasodilation effects in the human body. An efficient Microwave-assisted aqueous two-phase extraction (MA-ATPE) technique was developed to extract and enrich L-Citrulline from watermelon rind in a one-step procedure. The aqueous two-phase system (ATPS) consisted of the ionic liquid 1-Butyl-3-methylimidazolium bromide [C4mim] [Br], and the salt dipotassium hydrogen phosphate (K₂HPO₄) was used as the extracting solvent. MA-ATPE conditions, such as the compositions of ATPS, particle size, liquid-solid ratio, irradiation power, and extraction time were investigated. The results indicated that the optimal conditions were as follows: salt concentration 35%, ionic liquid concentration 1.6 M, particle sizes 60 mesh, irradiation power 300 W, and extraction time 10 min. Under the optimized conditions, the extraction yields of L-Citrulline reached 87.74%. According to the DPPH results, at a concentration of 2 mg/mL, the DPPH radical-scavenging activity of the MA-ATPE extract from watermelon rind (0.388 mg/ml) was greater than that of the MAE (Microwave-assisted extraction) extract (0.456 mg/ml) and the conventional extract (0.468 mg/ml). DLS and TEM were used together to examine the microstructure of the top phase and explore the potential extraction mechanism. The findings revealed that the aggregation and embrace phenomenon were the primary driving forces for the separation. Compared with the conventional method and regular MAE, MA-ATPE exhibited a higher extraction yield and a shorter extraction time. In conclusion, MA-ATPE is an efficient, rapid, eco-friendly, and economical technique, representing an excellent alternative for the extraction of bioactive compounds.

Graphical Abstract