## 3 Results and Discussion
#### 3.1.2 Impact of Different Extraction Factors on the Total Phenolic Content (TPC)  
According to the experimental results obtained from fifteen trials and the mathematical results predicted by this model and reported in Table 2, the highest total phenolic content (TPC) of _C. monocantha_ extracts was 39.78 \(\upmu\)g GAE/mg DE. This value was obtained in trial number 8 with a 1.5:10 g/mL solid-to-liquid ratio, an extraction temperature of 40 \({}^{\circ}\)C, and an extraction time of 30 min. On the other hand, the lowest total phenolic content (TPC) of _C. monocantha_ extracts was 19.89 \(\upmu\)g GAE/mg DE, which was obtained in trial number 10 with a 1:10 g/mL solid-to-liquid ratio, 45 \({}^{\circ}\)C extraction temperature, and 10 min extraction time. Furthermore, based on the analysis of variance (ANOVA) reported in Table 3, the high values of the determination coefficient (\(R^{2}\)), the adjusted determination coefficient (\(R^{2}_{(adj)}\)), and the low value of the variation coefficient C.V.% generated by the TPC model were 0.9892, 0.9698, and 2.53, respectively, proving the relevance of this model. Similarly, the high value of the TPC model's lack-of-fit probability was greater than 0.05 (\(p>0.05\)), which was 0.4057, demonstrating that there was no significant difference detected, and, therefore, the experimental results had an excellent fit to the regression equation of this model. On the other hand, the data provided by the statistical analysis mentioned in Table 3 demonstrated that the different effects of three independent variables, either the linear effect (of extraction time (\(X_{k}\))), the interaction effects (of temperature-extraction time (\(X_{jk}\)), and solid-to-liquid ratio-extraction time (\(X_{ik}\))), or the quadratic effects (of the square of solid-to-liquid ratio (\(X_{ii}\))) positively and significantly influenced the total content of phenolic compounds from _C. monacantha_. However, the interactive effect of the solid-to-liquid ratio-extraction temperature (\(X_{ij}\)) and the quadratic effect of the square extraction temperature (\(X_{jj}\)) negatively and significantly affected the total content of phenolic compounds from _C. monacantha_. Once the various effects of insignificant factors were removed from the second-order polynomial equation for the TPC response (\(p>0.05\)), the final equation for this response was expressed in Equation (6) as follows:  
\[Y_{TPC}=-112.6500-4.48100X_{k}+11.07500X_{ii}-0.130450X_{jj}-0.855000X_{ij}+0. 501500X_{ik}+0.102350X_{jk} \tag{6}\]  
These results were visualized by the different three-dimensional response surface TPC shown in Figure 2. Figure 2c shows that there was a proportional linear correlation between total phenolic content and extraction time from 10 min to 30 min. These results agree with [PERSON] et al. [83], [PERSON] et al. [75], and [PERSON] et al. [68], who declared that the TPC increases when the extraction time increases from 6.36 min to 73.64 min for _Myricaria cauliflora_, from 5 to 15 min for Ciriguela residue flour (_Spondias purpurea_ L), and from 20 to 30 min for _Abelmoschus esculentus_, respectively. Since the contact time between the solid matter and the solvent at certain thresholds promotes increased fragmentation of the cell wall and solid vacuoles, which improves the penetration of the solvent and the diffusion of bioactive substances [84]. Likewise, Figure 2d confirmed a quadratic correlation between the phenolic content of _C. monacantha_ and the extraction temperature, where the phenolic content increases with increasing extraction temperature from 35.00 \({}^{\circ}\)C to 40.07 \({}^{\circ}\)C. Above this temperature, increasing the extraction temperature is accompanied by a progressive decrease in total phenolic content (TPC). These observations are similar to those of [PERSON] et al. [85], [PERSON] et al. [86], and [PERSON] et al.