## 3 Results and Discussion
### Optimisation of Volatiles Extraction from Pineapple By-Products by HS-SPME
#### 3.2.2 Effect of Independent Variables on Volatiles Extraction  
As can be seen in Table 4, the interaction AC was the most significant factor that affected the extraction of volatiles, followed by the NaCl concentration (C), the quadratic term (C)\({}^{2}\), extraction time (B) and, finally, the quadratic term A\({}^{2}\). Regarding the effect of independent variables, the extraction temperature negatively affected the volatile extraction. This means that when temperature increased, the extraction of target volatile compounds decreased. This fact could be explained because high temperatures may increase the degradation of some of the compounds [36] and the unwanted generation of artifacts [33]. Additionally, a previous study reported similar behavior when HS-SPME was used to extract volatile compounds from tropical fruits, suggesting that low temperature favors the extraction of the more volatile compounds [37]. In this study, this effect was not significant. Regarding the significant and positive effect of the extraction time, in previous works, it has been confirmed that longer times led to a greater retention of the substances until fiber saturation [37]. Regarding the negative and significant effect of NaCl concentration, it has been previously reported that the use of highly concentrated solutions can be responsible for inadequate repeatability values, probably due to adsorption of the compounds. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Source** & **Sum of Squares** & **Mean Square** & **F-Value** & \(p\)**-Value** \\ \hline A: Temperature (\({}^{\circ}\)C) & \(2.64\times 10^{16}\) & \(1\) & \(2.64\times 10^{16}\) & 6.62 & 0.0618 \\ B: Time (min) & \(4.15\times 10^{16}\) & \(1\) & \(4.15\times 10^{16}\) & 10.40 & 0.0321 * \\ C: NaCl (M) & \(6.04\times 10^{16}\) & \(1\) & \(6.04\times 10^{16}\) & 15.13 & 0.0177 * \\ AA & \(3.95\times 10^{16}\) & \(1\) & \(3.95\times 10^{16}\) & 9.89 & 0.0347 * \\ AB & \(8.50\times 10^{11}\) & \(1\) & \(8.50\times 10^{11}\) & 0.00 & 0.9891 \\ AC & \(9.03\times 10^{16}\) & \(1\) & \(9.03\times 10^{16}\) & 22.63 & 0.0089 ** \\ BB & \(2.28\times 10^{15}\) & \(1\) & \(2.28\times 10^{15}\) & 0.57 & 0.4916 \\ BC & \(8.27\times 10^{15}\) & \(1\) & \(8.27\times 10^{15}\) & 2.07 & 0.2236 \\ CC & \(5.96\times 10^{16}\) & \(1\) & \(5.96\times 10^{16}\) & 14.92 & 0.0181 * \\ Lack of Fit & \(7.40\times 10^{16}\) & \(3\) & \(2.47\times 10^{16}\) & 6.17 & 0.0555 \\ Pure Error & \(1.60\times 10^{16}\) & \(4\) & \(3.99\times 10^{15}\) & - & - \\ Cor. Total & \(4.22\times 10^{27}\) & \(16\) & - & - & - \\ \hline \hline \end{tabular}  
* Significant, \(p<0.05\). ** Very significant, \(p<0.01\).  
\end{table} 
Table 4: Analysis of variance (ANOVA) for the quadratic model of volatiles extraction. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Run** & **Temperature (\({}^{\circ}\)C)** & **Time (min)** & **NaCl (M)** & \multicolumn{2}{c}{**Absolute Sum of Areas**} \\ \cline{3-5}
1 & 50 & 35 & 2 & \(2.58\times 10^{8}\) & \(2.52\times 10^{8}\) \\
2 & 50 & 60 & 4 & \(3.37\times 10^{8}\) & \(2.87\times 10^{8}\) \\
3 & 50 & 35 & 2 & \(2.88\times 10^{8}\) & \(2.52\times 10^{8}\) \\
4 & 30 & 35 & 4 & \(3.37\times 10^{8}\  
tion effects at the surface of crystals that can occur in the solution [38]. On the other hand, it is confirmed that the model presents a curvature, due to the significant contribution of the effect of the quadratic terms A\({}^{2}\) and C\({}^{2}\). After treating the data, response surface figures were obtained to study the interactions between variables to obtain the optimal region and thus verify which independent variable values produced the best response. Three-dimensional contour plots showing the correlation between the factors to the extraction of target volatiles are shown in Figure 2.  
Figure 2a illustrates the correlation between temperature and NaCl on the sum of areas of volatiles when the time was kept constant at 60 min.