## 3 Discussion
### Theoretical Prediction of Solubility of Wild Thyme Compounds in Green Solvents
#### 3.1.3 Principal Component Analysis (PCA) of Theoretical Prediction Results  
The PCA analysis was conducted in order to correlate the results from HSPs and COSMO-RS software. The biplot of PCA assessment is given in Figure 1. A total of 52.37% of variance was clarified in the observed data set. The biplot showed that analysed compounds have spread over all quadrants, while HSPs and COSMO-RS parameters were placed in first, second and third quadrants. A grouping of selected compounds from the same group (i.e., oxygenated monoterpenes, sesquiterpenes, flavonons, etc.) is notable, where all compounds from the same group were grouped in the same quadrant or at least, at the same side of \(x\) or \(y\) axis, except monoterpenes that were placed in first, second and fourth quadrants. Additionally, grouping of solvents was also notable. For example, short chain alcohols (ethanol, methanol, 1-butanol and isopropanol) were placed all in the third quadrant according to HSPs and in second and third quadrants according to COSMO-RS results. All monoterpenes, regarding the analysed method, were placed in the first quadrant of the PCA biplot. It is clear that neither of solvents showed completely different results with two theoretical prediction models, i.e., no solvents were placed in inversely proportional quadrants (I-III or II-IV). Furthermore, most solvents were placed in the same quadrant with HSPs and COSMO-RS prediction including \(n\)-hexane (1), methyl acetate (3), ethyl oleate (4), ethanol (5), methanol (8), limonene (9), \(\alpha\)-pinene (10), cymene (11), \(\beta\)-myrcene (12), and CPME (13). For that reason, it can be concluded that similar data are given with HSPs and COSMO-RS results, although some differences could be noted, especially with higher probability of solution given with COSMO-RS data (Table 2). Experimental Assessment of Extraction of Wild Thyme Bioactive Compounds by Means of High Voltage Electrical Discharge (HVED)  
3.2.1 Physical Properties of Wild Thyme Etracts Obtained by HVED Compared to Conventional Extraction  
The aim of the experimental analysis was to extract BACs from wild thyme by HVED extraction. During the HVED treatment, a gas is flowing through the needle (electrode) and it is being ionized forming a cold plasma. The type of gas used in the treatment since different gas ionize at different voltages and different radical species are being formed during discharge. Therefore, different results could be obtained with different gases [31]. For that reason, it was difficult to obtain electrical discharges with nitrogen under 20 kV so 20 and 25 kV were chosen for nitrogen treatments, while lower voltages of 15 and 20 kV were chosen for argon treatments. Results of physical parameters, including pH, conductivity, temperature before and after the treatment and power used during HVED treatment, are given in Table 3. pH of wild thyme extracts ranged from 5.12 \(\pm\) 0.18 to 6.31 \(\pm\) 0.15 meaning that all extracts were slightly acidic, and no significant changes in pH were noted between HVED extracts and extracts obtained by CE. Electrical conductivity was in range from 89.0 \(\pm\) 6.3 \(\upmu\)S/cm (extract TA11) to 646.0 \(\pm\) 30.1 \(\upmu\)S/cm (extract TA2). Statistical analysis of influence of main effect (treatment time, voltage, ethanol content and treatment type) to results of physical parameters is shown in Table 6. Statistically significant influence (\(p\leq\) 0.05) to pH and conductivity had only ethanol content. With higher ethanol content, pH increased, while conductivity decreased. Regarding the temperature of extracts, it was shown that the maximum measured temperature after the HVED treatment was 37.8 \(\pm\) 1.3\({}^{\circ}\)C. For statistical analysis, a temperature difference calculated as a difference after and before HVED treatment, was taken in account for statistics. Maximum temperature difference was 12.2 \({}^{\circ}\)C for sample TA2 treated for 9 min with argon at 15 kV and water as a solvent, where also the highest conductivity was noted.