## 2 Results and Discussion
### Statistical Analysis
#### 2.3.2 Effect of Reaction Parameters  
In the analysis of the Pareto chart, shown in Figure 2, it visually tells us if an independent variable will produce or no effect on the dependent variable; it also shows if an interaction occurs between independent variables will result in a significant planning response. With the Pareto chart, it is possible to observe that the independent variables linear temperature (\({}^{\circ}\)C) and linear biocatalyst content (g) do not influence the response variable, which is the conversion into esters. In the chart, it is possible to observe positive and negative values; this implies that the independent variables that present influence under the response in a positive way, the higher the value, the higher the percentage in esters conversion. And for the variables that present a negative influence, the higher the negative value, the lower the percentage in esters conversion [74; 75]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Source of Variation** & **Sum of Squares** & **Degrees of Freedom** & **Mean Squares** & **Fcal** &
\begin{tabular}{c} **Probability** \\ **(P) \textgreater F** \\ \end{tabular} \\ \hline
**Regression** & 2568.10 & 14 & 183.44 & 58.98 & 0.0001 \\
**Residual error** & 37.32 & 12 & 3.11 & - & - \\
**Lack of fit** & 37.31 & 10 & 3.73 & 867.68 & 0.001152 \\
**Pure error** & 0.009 & 2 & 0.00 & - & - \\
**Total** & 2605.41 & 26 & 100.21 & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Analysis of variance (ANOVA) for the empirical model. The interaction between the independent variables and the dependent variable is shown by the three-dimensional response of the fatty acid ethyl esters (FAEE) yield in Figure 3, in which several response surfaces are created for the relationship between the effects of two parameters on the FAEE yield while the third invariable. Figure 3A shows the response surface for the analysis of the interaction between time and temperature. Longer reaction times increase the conversion into fatty acid ethyl esters [48]. In the present study, the time varied between 2 and 6 h, but there were no variations in the conversion values in the adopted time interval. This time range adopted in the present work differs from that reported in the literature by [PERSON] et al. (2017) [41], in which time ranging from 12 to 72 h was evaluated, using a 1:12 molar ratio (sweet basil seed oil: methanol), 5% w/w biocatalyst content (Novozym(r) 435) and a reaction temperature of 40 \({}^{\circ}\)C. The yield in ester gradually increased in the interval from 24 to 72 h. In 48 h of reaction, the conversion presented was 89% and at 72 h, the conversion was 92%. It was possible to observe in the present work, in the time of 2 h, using a ratio of 1:15 (free fatty acids/ethanol), with a load of 0.15 g of biocatalyst at a temperature of 30 \({}^{\circ}\)C, it was possible to obtain the conversion of 91.11%. Figure 3: Response surfaces for (**A**) temperature versus time; (**B**) temperature versus molar ratio (fatty acid: alcohol); (**C**) Temperature versus biocatalyst content; reaction medium: babassu free fatty acids 1:1-1:1:15 (free fatty acids/alcohol), biocatalyst mass of 5-15 g. Reactions were carried out for 2-6 h at 30–50 \({}^{\circ}\)C at 200 rpm. Further specifications are described in Section 3. Figure 2: Pareto chart indicating the effect of the estimated values for the studied variables in the enzymatic esterification of the babassu free fatty acids. (X\({}_{1}\)) temperature (\({}^{\circ}\)C), (X\({}_{2}\)) time (hours), (X\({}_{3}\)) molar ratio (FFAs: alcohol), (X\({}_{4}\)) biocatalyst content (% m/m). Figure 3B shows a response surface obtained by plotting the biodiesel conversion in relation to the molar ratio (fatty acids/ethanol) and temperature. It is possible to observe that the biodiesel conversion was reduced in a certain molar ratio (fatty acids/ethanol).