## 3 Results and Discussion
### Extracts' Characterization by HPLC
#### 3.4.2 Polyphenolic Composition
As can be seen from the chromatogram of 25% EtOH and 100 us PEF extract at 320 nm (Figure S3 in Supplementary Materials), four main compounds were identified that belong to the group of phenyl-propanoids and flavones' derivatives. Peak 1 was identified as caffe acid and peak 4 as rosmarinic acid. The identification was based on the retention time (Figure S3 in Supplementary Materials) and absorption spectrum of the compounds and the corresponding reference substances. From our previous works, peaks 2 and 3 were identified as 6-hydroxy luteolin 7-\(O\)-glucose [29] and luteolin 7-\(O\)-glucuronide [44], respectively. The amounts of the identified compounds, achieved in the extracts of reference samples processed with 25% EtOH, were 0.76 mg g\({}^{-1}\) dw for 6-hydroxy luteolin 7-\(O\)-glucose, 1.04 mg g\({}^{-1}\) dw for luteolin 7-\(O\)-glucuronide, 0.17 mg g\({}^{-1}\) dw for caffe acid, and 0.37 mg g\({}^{-1}\) dw for rosmarinic acid. Quantification of the main identified components of SOLL was carried out in PEF extracts processed with the optimum EtOH-H\({}_{2}\)O ratio (25% EtOH) for both 100 us and 10 us pulse duration. The corresponding results prove the influence of PEF treatment (Table 3, Figure 3). As indicated by the estimation of Y\({}_{\text{TP}}\), HPLC total area and oxidation temperature, the best performance in the extraction of identified phenolics was achieved by the pulses of 100 us. Specifically, extraction was enhanced for all examined compounds rising to a 403.12% increase for the 100 us pulses and up to 354.61% for the 10 us pulses. This remarkable and significant (\(p<0.05\)) increase was shown for rosmarinic acid, achieving 1.85 mg g\({}^{-1}\) dw in the optimum sample/extract (25% EtOH, 100 us). It is well known that beyond the solvents and the extraction method chosen, the seasonality and the locality of a plant rule also the type and the levels of the components detected in plant extracts [5]. However, it is worth mentioning at this point that the above quantity of rosmarinic acid (1.85 mg g\({}^{-1}\) dw achieved in the optimum extract) is significantly higher than the amount (0.45 mg g\({}^{-1}\) dw) achieved by [PERSON] et al. [45] using water ultrasound extraction for 30 min at high temperature (80 \({}^{\circ}\)C) and a higher liquid to solid ratio (10:1 mL g\({}^{-1}\)). For caffe acid and 6-hydroxy-luteolin-7-O-glucose, the significant (\(p<0.05\)) increases for 100 us pulses, reached 41.76% and 49.78%, respectively. An almost double increase, 93.49% (significant at \(p<0.05\)), was reached for luteolin 7-O-glucuron. It is obvious that both pulse durations succeeded in a permeabilization effect on the membranes and affected the extraction percentage of intracellular compounds from the cells. The big variance in percentage increases between the main compounds, for the same extraction conditions (Table 3, Figure 4), indicates the potential selectivity of this extraction method, a very important fact considering that the achievement of selective extraction is usually a tedious, time- and energy-consuming procedure. The main factors that possibly assist this selectivity are the molecular size and structure, the differences in cell membrane disintegration (such as pore size), the solubility of the extracted components, and the solvent's polarity [31; 46]. The differentiation of PEF processing parameters, as supported by the literature, also seem to support extraction selectivity. For 6-hydroxy luteolin 7-O-glucoside, in our study, the change in the pulse duration from 10 to 100 us was followed by a higher increase in the levels of the metabolite, in comparison to the increase obtained for the three other identified metabolites (Table 3, Figure 4). A similar effect was observed during another study performed by our team regarding enhanced polyphenol extraction from olive leaves using PEF [27]. As in the current study, the effect of different duration pulses, 10 and 100 us, was tested. The amounts of obtained metabolites varied depending on the applied pulse duration.