## 3 Results and Discussion
### Biocatalyst Comparisons under Best Reaction Conditions  
The results previously discussed have made patent the differences between the performances of the biocatalysts under the same reaction conditions. Even though, when optimized values are used, the lower temperature required by Novozym(r) 40086 and its lower cost may compensate for the fact that Novozym(r) 435 seems more effective from a technical point of view [32]. Figure 5 compares the reaction course under the best reaction conditions for Novozym(r) 40086 (7.5% \(w/w\) and 60 \({}^{\circ}\)C) and Novozym(r) 435 (3.75% \(w/w\) and 80 \({}^{\circ}\)C), where it can be observed that Novozym(r) 435 leads to a conversion of 90.03% after 5 h, while the 242 value for Novozym(r) 40086 is 72.28% at the same time. A similar final conversion (91.03%) will not be attained by Novozym(r) 40086 until three hours later, which may be excessively longer to compensate for the savings of operating 20 \({}^{\circ}\)C below than with Novozym(r) 435. Under these conditions, the calculated NPGDL productivities are 0.105 kg/L h for the case of using Novozym(r) 40086 and 0.169 kg/L h if the lipase is Novozym(r) 435. These productivities are similar to those declared in other studies of ester production in solvent-free systems. Thus, some authors reported productivities of 0.033 kg/L h in the production of panthenyl triacetate [42], 0.15 kg/L h for neopentyl glycol dicaprilate/dicaprate [43], and a value of 0.217 kg/L h panthenyl monolaurate [44]. In order to monitor the evolution of all the chemical species involved (substrates, intermediates, and final products) under the best reaction conditions, the GC analysis of the samples withdrawn over time was performed. The results are shown in Figure 6, from which it can be confirmed that high yields were achieved with Novozym(r) 40086 (a) and Novozym(r) 435 (b) during the esterification process. As NPGML is an intermediate product of the reaction, its concentration increases over time, reaching a maximum value after \(\sim\)1.5 h of reaction for both biocatalysts and then decreasing as the esterification of the remaining hydroxyl group occurs. By observing the NPGDL curve, it can be noticed that the final product is obtained since the beginning of the reaction and that the esterification of the first hydroxyl group happens faster than the second, as, logically, the diester formation depends on the amount on intermediate available. In addition, by comparing Figure 6, it can be noticed that the reaction rate of these secondary steps is higher with Novozym(r) 435 than with Novozym(r) 40086, which explains why the former reaction system requires less reaction time. Regardless of that, when both reactions are allowed to evolve, final products are composed mainly of NPGDL with a small amount of LA unreacted, even though both substrates are initially provided in stoichiometric amounts. This is probably due to the evaporation of NPG [45], as, in fact, no traces of the alcohol or the intermediate product (NPGML) were found in the final product samples. In both cases, high yields and products of purity above 95% are obtained, comparable with many cosmetic ingredients. Figure 5: Comparison of NPGDL synthesis under best reaction conditions: Novozym® 40086 (7.5%, 60 \({}^{\circ}\)C) and \(\blacklozenge\) Novozym® 435 (3.75%, 80 \({}^{\circ}\)C). While considering that temperature plays a decisive role in Rml biocatalyst activity, the results obtained once this parameter has been optimized suggest that an enzyme's preference for certain substrates is still a factor to consider in evaluating their performance. As stated in Section 3.1, from the available literature it can be concluded that both enzymes are not affected by an LA carbon chain length, so, in view of the foregoing, CalB activity appears to be more specific to the branched polyol NPG than Rml's, or even to the monoester formed after the first step of the process, the NPGML.