## 3 Results and Discussion
### Biocatalyst Selection Based on Economic Evaluations and Sustainability Indicators  
It is widely described that, for a biocatalytic process to be economically viable, it is essential that the biocatalyst can be reusable in consecutive reaction cycles, as it is the factor that most influences the final cost of the process ([PERSON], 2018). In this present work, the synthesis of NPGDL has been performed in five successive batches reusing both immobilized lipases (Novozym(r) 40086 and Novozym(r) 435) under study. For this purpose, the procedure described in Section 2.3 has been applied and the results obtained are shown in Figure 7. As can be seen, both biocatalysts preserve a high percentage of their initial activity (84.5% Novozym(r) 40086 and 99.2% Novozym(r) 435) after five successive uses. These figures again highlight the poorer behavior of Rml lipase, as already discussed in previous sections. Figure 6: Evolution of NPGDL synthesis under best reaction conditions with Novozym® 40086 (**a**) and Novozym® 435 (**b**): LA, A NPG,  NPGML, and  NRGDL. The biocatalyst productivity of the process using both lipases increases significantly with the reuse of lipase. Thus, for the immobilized derivative Novozym(r) 40086, it increases from 12 kg NPGDL/kg biocatalyst with single-use to 56 kg NPGDL/kg biocatalyst when used five times. A similar increase is observed for the case of Novozym(r) 435 (from 25 kg NPGDL/kg biocatalyst for one use to 122 kg NPGDL/kg biocatalyst for five uses). In this scenario, the productivity values were approximately double when Novozym(r) 40086 was used since, as mentioned above, the optimal enzyme concentration to achieve similar conversions is half when Novozym(r) 435 is used. The development of a preliminary economic study is an indispensable tool that allows the final choice of the biocatalyst to be used in the synthesis of NPGDL, as well as establishing the possibility of a future industrial implementation of this process. Therefore, in this paper, this study has been undertaken on the following basis:  
1. Direct operating costs have been calculated for the production of 1 kg of NPGDL;
2. Unit prices for both substrates have been obtained from suppliers who provide them in bulk, with a minimum purchase of 1 kg;
3. The immobilized lipases have been donated by Novozymes, Espana S.A., and their prices were specified in personal communication;
4. The energy costs have been calculated by measuring the energy consumption of the thermostatic bath and overhead mixer (Section 2.4) and considering an energy price of 0.2288 EUR/kW h (the average value of the second half-year of 2022 in Spain);
5. Five consecutive uses of the biocatalysts have been considered in the calculations. Table 1 shows the costs of each item and, in the last row, the total direct production costs for each process. As can be seen, the production of NPGDL using Novozym(r) 40086 is the most cost-effective process due to its lower price and the fact that the synthesis takes place at a moderate temperature although it requires relatively high operating times. It is important to note, that in a future industrial application of the biocatalytic process, the energy costs would be lower than those shown in Table 1 since the energy consumption of laboratory equipment connected to a domestic electrical network has been considered and the prices do not correspond to those of the industrial market. It has been described that the energy cost of a medium-sized industry can be as much as half that of a residential system [37]. Finally, to establish whether the biocatalytic synthesis of the cosmetic ingredient NPGDL meets the criteria of green chemistry, some of the most commonly used sustainability indicators have been calculated and the results obtained are shown in Table 2.