Optimization of Flavonoid Extraction and Tyrosinase Inhibitory Activity from <i>Rhodomyrtus tomentosa</i> Leaves Using NADES-Based Ultrasound-Assisted Extraction

Flavonoid compounds have attracted considerable interest due to their promising potential applications cosmetic products, particularly as skin-lightening agents, while efficient and sustainable extraction remains a major challenge. This study aimed to optimize the extraction of flavonoid compounds and their tyrosinase inhibitory activity from Rhodomyrtus tomentosa leaves using ultrasound-assisted extraction (UAE) using natural deep eutectic solvents (NADESs). Initially, several choline chloride-based NADESs were evaluated for their extraction efficiency to identify the most effective solvent, which was subsequently used in the next phase involving the optimization of extraction conditions. The extraction variables, including extraction time, water addition to the NADESs, and the solid-to-liquid ratio were optimized using Response Surface Methodology (RSM) with a Box-Behnken design. Both total flavonoid content and tyrosinase inhibitory activity were determined spectrophotometrically. Choline chloride-propylene glycol (1:1) was identified as the optimal NADES compared to choline chloride-based NADESs combined with various other hydrogen bond donors. The developed models showed high R² value (0.9944 for flavonoids; 0.9998 for tyrosinase inhibition) with non-significant lack-of-fit values, indicating the excellent predictive performance of the response model. The optimal extraction conditions were obtained at an extraction time of 60 min, 20% water addition, and a solid-to-liquid ratio of 0.05 g/mL, resulting in a total flavonoid content of 14.3729 mg QE/ g dried leaves and tyrosinase inhibition of 83.56%. These findings highlight the potential of NADES-UAE as an efficient and environmentally friendly platform for producing bioactive extracts of R. tomentosa suitable for skin-lightening applications.