## 3 Results and Discussion  
### Effect of the DESs and the PEF Usage on the TPC of the Extracts  
In order to maximize the extraction yield of the polyphenols, the use of DESs was evaluated. The selection of DESs was based on previous studies [27; 28; 29; 30]. The selected DESs achieved the maximum extraction recoveries, compared to other types of DESs. The results from the extracts prepared using different solvents are presented in Figure 1. When water was used as an extraction solvent the TPC was found to be 108.2 \(\pm\) 0.7 mg GAE/g dw. All four tested DESs resulted in extracts with an increased TPC. More specifically, DES-4 caused a 52% increase in the TPC, compared to the extract prepared with water, DES-1 and DES-3 caused a -64% increase, while the maximum increase was achieved when DES-2 was used (75%). In many published reports, it is mentioned that hydroethanolic solutions are superior, compared to water, for polyphenol extraction, with 60% (\(v/v\)) ethanol-water mixtures, exhibiting a much better performance [32; 33; 34]. To this end, an extraction with this solvent was carried out, so as to compare the DES-based extracts with the hydroethanolic extract, in terms of the TPC, serving as a positive control. As can be seen in Figure 1, the hydroethanolic solution achieved a 47% increase in the TPC, compared to plain water. DES-4 achieved nearly the same extraction yield (no statistically significant differences were recorded), whereas DES-1, DES-3, and DES-2 resulted in extracts with an 11-19% increased TPC, compared to the hydroethanolic solution. It is evident that DES-4 can be used as an alternative option to extract polyphenols from _Cistus creticus_, yielding extracts with a comparable TPC with hydroethanolic extracts, whereas the other three DESs increase notably the extracted polyphenols. In order for the extraction yield to increase, the extraction medium needs to match as closely as possible, the polarity of the target compounds [35]. Since all tested DESs resulted in extracts with an increased TPC, the polarities of the tested DESs may be more favorable to extract polyphenols, compared to water [36]. According to a previous study, the main mechanism that governs the increased efficiencies of DESs for polyphenol extraction is the formation of an extended network of hydrogen bonds that not only helps solubilize the increased amounts of polyphenols, but also stabilize them [37]. In a previous study, it was reported that the TPC of _Cistus nonspeliensis_ and _Cistus saluifolius_ was nearly 55 mg GAE/g dw [38]. Moreover, it was reported that the TPC from the Syrian and Bulgarian _Cistus creticus_ was 70 and 115 mg GAE/g dw, respectively [7; 39]. In the previous cases, the polyphenols were extracted using hydroethanolic solutions. The slightly increased TPC in our case, when the ethanol-water mixture was used, can be attributed to the environmental conditions [11]. The increased TPC, when DESs were employed, was also found in previous studies. For instance, in the study of [PERSON] et al. [36], it was described that lactic acid: glycine: water (3:1:3) was the most efficient DES, compared to other lactic acid-based DESs for the extraction of polyphenols from dittany, fennel, and sage, while this was not the case for mint (all tested DESs achieved similar extraction yields). This may be due to the physicochemical characteristics of the polyphenols that can be extracted from each plant, which can be affected by the properties of the DESs. The increased extraction yield from the glycine-based DESs was also reported in previous studies [40; 41]. To date, there are many reports that examine the use of different DESs for the extraction of polyphenols from plant materials [40; 41]. Furthermore, there are a few reports that examine the combination of DESs with other extraction techniques (such as microwave-assisted extraction and ultrasound-assisted extraction) [42; 43; 44; 45].