**4. Conclusions**

The evaluation of the active properties, that is, antimicrobial and antioxidant properties, of OEO, RE, and GTE showed that OEO was the active substance that presented the highest antimicrobial activity against *S. aureus* and *E. coli*. This effect was mainly attributed to the effectiveness of its most representative active compounds, that is, carvacrol and thymol, showing identical MIC and MBC values of 0.312 and 0.625 μL/mL, respectively. The antioxidant activity of OEO was also higher than that for the RE and GTE. In particular, the percentage of inhibition of the DPPH was 91.96%. Thereafter, these active substances were incorporated at 10 wt.% into fruit waste derived PHBV by electrospinning. To this end, the solution properties of the PHBV containing OEO, RE, and GTE were first evaluated to determine the optimal conditions to obtain homogenous fibers. The diameters of the fibers were relatively similar, with a mean size of approximately ~0.8 μm, being uniform and smooth, without any superficial and structural defects. It was observed that the addition of the OEO and the NEs did not alter the fiber formation during electrospinning or the morphology of the electrospun ultrathin PHBV fibers. A good compatibility and, then, high solubility of the OEO and NEs with the PHBV matrix was considered.

In order to obtain an interesting active continuous layer to be applied in the design of biopackaging, the electrospun mats of the PHBV fibers containing the active substances were subjected to a thermal post-treatment at 125 ◦C. Continuous PHBV films of ~80 μm, with a smooth surface were obtained, though the presence of the active substances induced a slight porosity in their cross-section. The optical properties of the PHBV films were slightly impaired by the addition of the active substances, particularly GTE, reducing their transparency from 3.13 up to 16.42 through blocking the passage of UV-Vis light and increasing their opacity, which was caused by the scattering of light. In any case, all the PHBV films were contact transparent. All active substances also decreased the values of the *Tonset* by 54 ◦C for OEO, 3 ◦C for RE, and 2.2 ◦C for GTE, and the thermally decomposed mass at *Tdeg* decreased to the 60–70% range. However, all the active PHBV films were stable up to 200 ◦C. Referring to the hydrophobicity of the films, the addition of active substances decreased the superficial tension of the PHBV films with respect to the control, but the angles for each of the films studied were still within the hydrophobic range. The PHBV films containing OEO, RE, and GTE showed antimicrobial activity against strains of *S. aureus* and *E. coli* in both the here-studied open and closed systems, where the bacterial reduction improved over time due to the release and accumulation of the active compounds on the film surface. The antimicrobial activity was higher in the case of the closed system due to the presence of volatiles stored in the headspace. The films containing OEO presented the highest reduction values against the two bacterial strains (*R* ≥ 3), while the films containing RE and GTE showed lower reduction values (1 ≤ *R* < 3), which agreed with the MIC and MBC values of the pure active substances. The antioxidant activity of the fibers and films was much lower than that of the

neat active substances, which was related to entrapment and loss during electrospinning and film processing, and which was also reduced with the passage of days due to the continuous release of the active compounds.

The here-developed electrospun PHBV layers with OEO, RE, and GTE are potential candidates for use in the design of sustainable active multilayer biopackaging. The antimicrobial and antioxidant performance of these materials is advantageous to prolonging the shelf life of foods, delaying the proliferation of microorganisms, and the enzymatic oxidation of foodstuffs.

**Author Contributions:** K.J.F.-L. performed all the experiments, measurements, data analysis, and wrote the manuscript draft. S.T.-G. proposed, planned, guided the execution of the research work, and wrote and edited the manuscript. A.A.V. provided the active substances and co-wrote the manuscript. M.A.M. Reis synthesized and characterized the biopolymer. J.M.L. supervised the research and performed the project administration.

**Funding:** This research was supported by the Ministry of Science, Innovation, and Universities (MICIU) program number AGL2015-63855-C2-1-R and by the EU H2020 project YPACK (reference number 773872).

**Acknowledgments:** K.J.F.-L. is a recipient of a Santiago Grisolía (GRISOLIAP/2017/101) gran<sup>t</sup> of the Generalitat Valenciana (GVA) and S.T.-G. is on a Juan de la Cierva - Incorporación contract (IJCI-2016-29675) from MICIU.

**Conflicts of Interest:** The authors declare no conflict of interest.
