## 2 Results and Discussion
### Influence of Contact Time on the Adsorption Process: Kinetic and Mass Transfer Studies
For reaction systems using methanol (Figure 4A), it is possible to observe that the immobilized biocatalyst gave the highest acid conversion percentage of 62.5% after the first 90 min of contact time, while soluble lipase required nearly 150 min of reaction to achieve an acid conversion of 67.7%. After replacing methanol with propanol, soluble lipase was more active than the immobilized biocatalyst during the first 30 min of reaction (Figure 4B); however, maximum acid conversion was also achieved using immobilized biocatalyst (77.5% after 60 min of reaction). Soluble lipase was also slightly more active than the heterogeneous biocatalyst up to 45 min of reaction using fused oil; however, similar acid conversion percentages (around of 65%) can be observed after 90 min of reaction catalyzed by both biocatalyst forms (Figure 4C). Using isoamyl alcohol, a maximum acid conversion of 82.3% was obtained after 60 min of reaction for immobilized lipase, whereas soluble lipase gave only 45% in this reaction time (Figure 4D). A maximum conversion percentage into isoamyl octanoate around of 70% can be observed at 150 min of reaction for soluble lipase. Comparing commercial isoamyl alcohol and fused oil systems, the immobilized lipase exhibited a highest preference towards isoamyl alcohol, while soluble lipase presented preference towards highly polar short-chain alcohols such as ethanol, propanol and isobutyl alcohol present in fused oil composition. This could be explained through high partition of the hydrophobic commercial isoamyl alcohol to the internal surface of the hydrophobic heterogeneous biocatalyst due to its highest hydrophobicity compared with highly polar short-chain alcohols (see Log P values in Table 3). In fact, high catalytic performance using immobilized lipase was also observed for the reaction system prepared with hexanol, the most hydrophobic alcohol tested in this study (Log P = 2.03--Table 3), due to its better diffusion and partitioning effect in the internal microenvironment of the heterogeneous biocatalyst. Thus, maximum acid conversion percentage around of 85% was achieved after 75 min of reaction, almost three-fold higher than those observed using soluble lipase, that exhibited its maximum acid conversion (only 31%) after the first 30 min of reaction, as shown in Figure 4E. A similar acid consumption profile for both homogeneous and heterogeneous biocatalysts using benzyl alcohol, an aromatic alcohol with Log P value lower than hexanol and very close to commercial isoamyl alcohol, was also observed (Figure 4F), as shown in Table 3. This indicates a possible improvement of its partition in the microenvironment of soluble lipase, yielding similar acid conversion (around of 75%) for both reaction systems after 60 min of reaction. These results suggest a better partition of hydrophobic starting materials to the microenvironment of the heterogeneous biocatalyst, thus providing the highest acid conversion values in shorter-reaction times. Exceptionally for fused oil and benzyl alcohol, both biocatalysts presented similar acid conversion percentage values. These results confirm the strong influence of the different alcohols on the catalytic properties of soluble and immobilized lipases. The experimental data obtained for acid conversion at equilibrium were also expressed in Table 3 to determine the productivity values for the different reaction systems evaluated in the present study using fixed initial protein concentration (20 mg of protein that corresponds to a protein concentration of 3.33 mg per gram of reaction mixture). According to these values, it is possible to note that the highest productivity values from 10.3 to 19.4 \(\mathrm{\SIUnitSymbolMicro m}^{-1}\cdot\mathrm{mg}_{\mathrm{prot}}{}^{-1}\) were obtained for immobilized lipases due to the excellent dispersion of lipase molecules on the support surface, which can be confirmed by SEM analysis (see Figure 2).