## 3 Result and Discussion
### Analysis of Response Surface  
#### 3.2.1 1,1-Diphenyl-2-picrylhydrazyl (DPPH) Scavenging Activity  
Figure 1a revealed a rapid increase in the scavenging activity with an increase in temperature and time from 45-55 \({}^{\circ}\)C and 80-100 min, respectively. The elevation of temperature induces a vigorous interaction between the red pitaya peels and the solvent, which aids in softening of the plant tissues. The weakened cell wall integrity subsequently enhances the release of the tightly bound antioxidant compounds [19]. Longer durations of extraction also contributes to a significant decrease of antioxidant compounds since oxidation tend to polymerize into insoluble compounds [20]. The scavenging activity also showed an increase with increasing ethanol concentration and temperature from 70-80% (\(v/v\)) and 45-55 \({}^{\circ}\)C respectively as shown in Figure 1b. Solvent polarity plays a vital role in the extraction of antioxidant compounds as certain phytochemicals exhibit limited solubility in water because of their hydrophobic characteristics, which depend on the functional groups attached to them. In Figure 1c, the maximum scavenging activity was achieved with 80-100 min and ethanol concentration of 70-80% (\(v/v\)). The scavenging activity started to decrease with the longer extraction time and this might have caused by oxidation and/or destruction of the antioxidant compounds by endogenous enzymes [21].  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Source**} & \multicolumn{3}{c}{**DPPH**} & \multicolumn{3}{c}{**FRAP**} & \multicolumn{3}{c}{**BCB**} \\ \cline{2-10}  & **Mean** & \multirow{2}{*}{**F**-**Value**} & \multirow{2}{*}{**\(p\)**-**Value**} & **Mean** & \multirow{2}{*}{**Square**} & \multirow{2}{*}{**F**-**Value**} & **Mean** & \multirow{2}{*}{**Square**} & \multirow{2}{*}{**F**-**Value**} & \multirow{2}{*}{**\(p\)**-**Value**} \\  & **Square** & & & & & & & & & \\ \hline Model & 50.16 & 1474.51 & \(<\)0.0001 & 0.46 & 1486.99 & \(<\)0.0001 & 65.39 & 1916.64 & \(<\)0.0001 \\ A–Temperature & 14.28 & 419.69 & \(<\)0.0001 & 0.14 & 453.82 & \(<\)0.0001 & 34.20 & 1002.56 & \(<\)0.0001 \\ B–Time & 20.63 & 606.46 & \(<\)0.0001 & 8.403 \(\times\) 10\({}^{-3}\) & 27.40 & 0.0004 & 31.43 & 921.35 & \(<\)0.0001 \\ C–Ethanol & 30.00 & 881.89 & \(<\)0.0001 & 0.048 & 155.09 & \(<\)0.0001 & 67.61 & 1981.67 & \(<\)0.0001 \\ concentration & 34.07 & 1001.60 & \(<\)0.0001 & 0.021 & 66.84 & \(<\)0.0001 & 23.56 & 690.58 & \(<\)0.0001 \\ AC & 65.95 & 1938.75 & \(<\)0.0001 & 0.15 & 492.19 & \(<\)0.0001 & 24.29 & 711.87 & \(<\)0.0001 \\ BC & 0.99 & 29.01 & 0.0003 & 8.001 \(\times\) 10\({}^{-3}\) & 26.08 & 0.0005 & 33.34 & 977.15 & \(<\)0.0001 \\ A\({}^{2}\) & 30.96 & 910.14 & \(<\)0.0001 & 1.45 & 4733.10 & \(<\)0.0001 & 173.51 & 5085.76 & \(<\)0.0001 \\ B\({}^{2}\) & 197.69 & 5811.29 & \(<\)0.0001 & 1.26 & 4097.01 & \(<\)0.0001 & 152.63 & 4473.69 & \(<\)0.0001 \\ C\({}^{2}\) & 101.62 & 2987.23 & \(<\)0.0001 & 1.76 & 5723.86 & \(<\)0.0001 & 121.45 & 3559.65 & \(<\)0.0001 \\ Lack of fit & 0.029 & 0.76 & 0.6137 * & 1.468 \(\times\) 10\({}^{-3}\) & 0.31 & 0.8849 * & 0.028 & 0.70 & 0.6468 * \\ \hline \hline \end{tabular}  
* \(p\) \(>\) 0.05, not significant.  
\end{table}
Table 3: The regression coefficients of the predicted quadratic polynomial models for the responses of the DPPH, FRAP, and BCB assays.  
Figure 1: _Cont._