## 3 Results and Discussion
### Process Parameter Optimization Using Response Surface Methodology
Likewise, for antioxidant activity, the liquid/solid ratio (linear effect) emerged as the key factor boosting antioxidant activity in _L. japonica_ extracts with 60% NADES4 (30.908). The temperature (quadratic effect, 8.489) revealed a non-linear optimal range, whereas the interaction between incubation time and solvent ratio adversely affected yields (\(-\)8.391). The three-dimensional response surface and contour plots (Figure 8) elucidate the interactive effects of process variables on total phenolic content (TPC) in _L. japonica_ flower extracts using 60% NADES4. Temperature showed non-linear effects, with an intermediate temperature of approximately 60 \({}^{\circ}\)C yielding better results than extremes like 80 \({}^{\circ}\)C (Exp. 2: 59.51 mg/g), indicating that thermal degradation occurs at higher temperatures. However, the combination of moderate time (35-45 min) and high temperature (65-75 \({}^{\circ}\)C) suggests a synergistic effect that enhances phenolic solubility and recovery under these conditions. Figure 8 further validates the findings illustrated by the Pareto charts (Figure 7), highlighting that the solvent-to-extracted material ratio is vital for the effective extraction of phenolic compounds. High liquid-to-solid ratios (15 \(v/w\)) combined with moderate time and temperature enhanced the TPC. In contrast, low ratios (5-10 \(v/w\)) decreased TPC to 58-63 mg/g, even at elevated temperatures, indicating limited solvent accessibility. Additionally, shorter processing times (<20 min) at high ratios resulted in a TPC reduction to below 63 mg/g. Regarding the antioxidant activity (Figure 9), the peak AA (450-480 \(\upmu\)mol/g) was reached at temperatures between 70 and 80 \({}^{\circ}\)C for 10 to 30 min and solvent ratios of 12 to 16 \(v/w\). Lower ratios (5-8 \(v/w\)) or temperatures greater than 85 \({}^{\circ}\)C led to reduced AA levels (<220 \(\upmu\)mol/g), again because of limited solvent penetration or thermal degradation. Utilizing a high solvent volume (12-15 \(v/w\)) with increased heat (70-80 \({}^{\circ}\)C) maximized yields; however, prolonging the duration beyond 50 min at low ratios reduced efficiency. Figure 7: Pareto charts illustrating standardized effects for the Box–Behnken design, with responses being (**a**) total phenolic content (mg of chlorogenic acid/g d.m.) or (**b**) antioxidant activity (\(\upmu\)mol of Trolox/g d.m.) in the extracts from _L. japonica_ flowers extracted using 60% NADES4 (choline chloride–urea). Explanations: 1—incubation temperature, 2—incubation time, 3—liquid/solvent ratio, L—linear; Q—quadratic; and 1L-2Q—the interaction between a linear effect of incubation temperature and a quadratic effect of incubation time in the statistical model, etc. Figure 8: Three-dimensional (3D) response surface and contour plots for the interaction between (**a**) time and temperature, (**b**) liquid/solid ratio and temperature, and (**c**) liquid/solid ratio and time on the total phenolic content in the extracts from _L. japonica_ flowers extracted using 60% NADES4 (choline chloride–urea). Explanations: TPC—total phenolic content (mg of chlorogenic acid/g d.m.). In conclusion, the contour plots shown in Figure 10 indicate that the optimal parameters for maximizing total phenolic content and high antioxidant activity are a solvent-to-flower ratio of 12-15 \(v/w\), a temperature range of 55-75 \({}^{\circ}\)C, and a moderate incubation time of 10-30 min. Figure 9: Three-dimensional (3D) response surface and contour plots for the interaction between (**a**) time and temperature, (**b**) liquid/solid ratio and temperature, and (**c**) liquid/solid ratio and time on the antioxidant activity in the extracts from _L. japonica_ flowers extracted using 60% NADES4 (choline chloride–urea). Explanations: AA—antioxidant activity (\(\mu\)mol of Trolox/g d.m.). [PERSON] et al. [35] explored betaine-based DESs for extracting chlorogenic acid from _L. japonica._ They identified a betaine-glycerol mixture as the optimal DES for enhancing CGA extraction through the Box-Behnken design.