## 4 Discussion  
From the experimental results shown previously, there is evidence for a difference between the yield of EO extraction using only the traditional method, i.e., HD, and the extraction with PEF pre-treatment and HD afterwards. The eucalyptus EOs extracted after the PEF pre-treatment presented more evidence of this positive effect. The EO yield of a 30 min extraction had an average increase of 17% compared to the traditional method, and the 60 min extraction seems to provide the maximum EO to be extracted from this plant material. From this, it is also possible to  
Figure 6: Results for TPC analysis of eucalyptus with (PEF) and without PEF application (Control) and different distillation times, phenols (mg/g eucalyptus). Vertical lines refer to the STDev. * Indicates statistical significance (\(p\)-value < 0.05). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **TPC** & \(\Delta\)**TPC** \\  & **(mg Polyphenols/g Eucalyptus)** & **(\%)** \\ \hline C.E\_30 & 0.15 \(\pm\) 0.03 * & \\ P.E\_30 & 0.19 \(\pm\) 0.04 * & +27 \\ C.E\_60 & 0.25 \(\pm\) 0.06 * & \\ P.E\_60 & 0.28 \(\pm\) 0.06 * & +12 \\ \hline \hline \end{tabular}  
* Significant at \(\leq\)0.05 \(p\) level.  
\end{table} 
Table 5: Summarized results for TPC analysis of eucalyptus EO extracted with and without PEF application. extrapolate that the maximum EO from eucalyptus could be extracted while lowering the extraction time, due to the effect of the PEF pre-treatment. From the rosemary EO extractions, similar results were obtained. The lack of more repetitions, due to the reduced amount of plant materials available, in order to get a higher statistic confidence, do not allow to take a more definitive conclusion. For this reason, only the eucalyptus EO extracted both with and without PEF application was analysed for their TPC. In this case, statistical significance was achieved, and the experimental results showed that with PEF a higher TPC is extracted for both HD times. Furthermore, these results seem to show indirectly that, the yield of EO with PEF is higher than the traditional method. The impact of PEF in the extraction of EO from thyme was more evident than with eucalyptus and rosemary, for the same PEF protocol and HD conditions. This may indicate a different cell disintegration mechanism, depending on the plant material but, also, the fact that the material conditions were different, as eucalyptus and rosemary were used a couple of weeks after harvesting (fresh material) and thyme was completely dry. Hence, it is possible to say that the application of a PEF pre-treatment to eucalyptus, rosemary, and thyme leaves could result in an overall higher extraction yield of EO or the possibility of a lower extraction time to obtain the same EO quantity from these plants, when using the HD technique, resulting in an important reduction of energy consumption. From the literature, different studies, with different plant materials, and different PEF conditions suggest that the application of PEF is seen as positive to increase the EO extraction yield. The EO extraction method applied in all studies was HD. In Table 6 are summarized the different PEF conditions and yield increase conditions for different plants described in the literature. Considering the extraction of EO from white roses done by [7], the plant material was treated with PEF, with \(W_{s}\) both 10 kJ/kg and 20 kJ/kg. The EO was extracted through HD with a Clevenger apparatus, with \(t_{dist}\) between 30 min and 2:30 h (standard extraction time). The results were compared with EOs extracted from untreated plants. The extraction yield was maximized for \(W_{s}=\) 10 kJ/kg and \(t_{dist}=\) 1:30 h. The extracted EOs were characterized by GC-MS, and the results converged to the conclusion that for extractions of at least 1:30 h, there were not significant changes to the EO quality.