## 2 Results and Discussion
### Biocatalyst Reusability Studies  
The reusability of immobilized lipase after six consecutive batch esterification reactions was investigated, as shown in Figure 5. The reaction times for each reaction system were the ones required to achieve maximum acid conversion percentage at equilibrium and are summarized in Table 3. According to results, a strong decrease of the original activity of the biocatalyst using highly polar alcohols (methanol and propanol) was observed, resulting in full inhibition of its original activity at 6 th batch esterification. This can be explained by a possible accumulation of water molecules on the biocatalyst surface that improves the partition/diffusion of highly polar alcohols; however, this may cause inhibitory effects on the catalytic performance of lipases in organic media [54; 55]. On the other hand, the creation of a hydrophilic layer can drastically reduce the diffusion of octanoic acid in the internal biocatalyst microenvironment due to its high hydrophobicity (log P value of 3.05) [51]. In fact, esterification reactions performed with hexanol and benzyl alcohol also exhibited progressive decrease of catalytic activity due to possible water accumulation inside the pores of the biocatalyst and thus an acid conversion percentage between 10% and 12% after six esterification reactions can be observed. Interestingly, successive esterification reactions using fused oil and its majority component (commercial isoamyl alcohol) demonstrated better preservation of the original catalytic activity of the biocatalyst, resulting in acid conversion percentage of 39% (for isoamyl alcohol) and 45% (for fused oil) at 6 th batch reaction (Figure 5). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Alcohol Type**} & \multirow{2}{*}{**Log P**} & \multirow{2}{*}{\begin{tabular}{c} **Biocatalyst** \\ **Form** \\ \end{tabular} } & \multicolumn{4}{c}{**Parameters**} \\ \cline{3-6}  & & & \begin{tabular}{c} **Co**\({}^{\text{a}}\) \\ **(mol**-**1**)**\({}^{\text{a}}\) \\ \end{tabular} & \begin{tabular}{c} **Y**\({}^{\text{b}}\) \\ **(\%)** \\ \end{tabular} & \
\begin{tabular}{c} **t\({}_{\text{e}}\)**\({}^{\text{c}}\) \\ **(min)** \\ \end{tabular} &
\begin{tabular}{c} **p**\({}^{\text{d}}\) \\ **(\(\mu\)mol**-**min**\({}^{\text{-1}}\)**-**m**g**\({}_{\text{prot}}\)**-**1**)** \\ \end{tabular} \\ \hline \multirow{3}{*}{Methanol} & \multirow{3}{*}{\(-0.77\)} & Soluble & 4.84 & 67.7 & 150 & 8.2 \\  & & Immobilized & 4.84 & 62.5 & 90 & 12.6 \\ \multirow{2}{*}{Propanol} & \multirow{2}{*}{0.28} & Soluble & 4.17 & 65.8 & 75 & 13.2 \\  & & Immobilized & 4.17 & 77.5 & 60 & 19.4 \\ \multirow{2}{*}{Fusel oil} & \multirow{2}{*}{Not found} & Soluble & 3.81 & 65.1 & 90 & 10.5 \\  & & Immobilized & 3.81 & 64.6 & 90 & 10.3 \\ \multirow{2}{*}{Isoamyl alcohol} & \multirow{2}{*}{1.27} & Soluble & 3.74 & 69.0 & 150 & 6.1 \\  & & Immobilized & 3.74 & 82.3 & 60 & 18.2 \\ \multirow{2}{*}{Hexanol} & \multirow{2}{*}{2.03} & Soluble & 3.70 & 31.0 & 30 & 12.8 \\  & & Immobilized & 3.70 & 85.8 & 75 & 14.2 \\ \multirow{2}{*}{ Benzyl alcohol} & \multirow{2}{*}{1.10} & Soluble & 3.65 & 72.6 & 60 & 16.5 \\  & & Immobilized & 3.65 & 75.4 & 60 & 17.2 \\ \hline \hline \end{tabular}  
* \({}^{\text{a}}\) initial acid concentration; \({}^{\text{b}}\) acid conversion percentage at equilibrium; \({}^{\text{c}}\) reaction time at equilibrium; \({}^{\text{d}}\) productivity.  
\end{table} 
Table 3: Estimation of productivity for the production of several octanoate esters catalyzed by soluble and immobilized lipase Eversa® Transform 2.0. These values correspond to a loss of the original activity of the biocatalyst of around 28% (from 62.5% in the first batch to 45% in the sixth batch) and 53% (from 82.5% in the first batch to 39% in the sixth batch) using fused oil and commercial isoamyl alcohol, respectively.