## 3 Results and Discussion  
### Extraction Yield of Licorice Extracts and Quantification of Bioactive Compounds  
Extraction yields are reported in Table 1, together with the deviations obtained in duplicate experiments. Taking into account the solvents used, extraction yield increased in the following order: ethanol, methanol:water (50:50) and ethanol:water (50:50). Based on the dielectric constants of the solvents (ethanol 24.3, methanol 32.7 and water 78.4, at 25 \({}^{\circ}\)C), it can be established that the addition of 50% water to the organic solvents and thus, the increase of solvent polarity, resulted in a significant increase of extraction yield (\(p<0.05\)). Nevertheless, extraction temperature did not affect yield significantly (\(p>0.05\)), at least in the range of temperatures explored (25 and 50 \({}^{\circ}\)C).  
While the addition of 50% water to methanol produced nearly a 2-fold increase of extraction yield, the addition of 50% water to ethanol resulted in a 6.5-fold yield increase. Then, the combination of a medium polar solvent (ethanol) with a high polar solvent (water) was the most suitable alternative to obtain high yields in the extraction of licorice root. These results are in accordance to other reported extraction studies, such as the work of [PERSON] et al. [39] concerning the extraction of grape stems, [PERSON] et al. [40] in the extraction of marjoram with different ethanol:water mixtures, or [PERSON] et al. [41] in the UAE extraction of pomegranate peels, reporting an increase in the extraction yield with an increase in solvent polarity. Moreover, it was found that the combination of solvents is more efficient for extraction of phenolic compounds than a single solvent [42].  
The main well-known bioactive compounds of licorice (glycyrrhizin, glabridin, liquiritigenin, isoliquiritigenin and liquiritin) were identified and quantified by HPLC analysis and the results obtained are reported in Table 2. Values in the range of 79.0 to 157.9 mg of these bioactive compounds were determined per gram of the different extracts obtained. Except in the case of ethanolic extracts, the higher concentrations were determined for glycyrrhizin, followed by glabridin, and in considerably lower concentrations, liquiritigenin, isoliquiritigenin and liquiritin.  
In the case of ethanolic extracts, the concentration of glabridin was higher than glycyrrhizin, while significantly lower concentrations were observed for the rest of the components. Glabridin, an isoflavan, and glycyrrhizin, a triterpene glycoside, are the most abundant and distinctive compounds of licorice roots with several recognized pharmacological properties [43; 44]. While glabridin is nearly insoluble in water, the glycosylated sugars present in glycyrrhizin provides some polarity to this compound and thus is better extracted using water. Figure 1 shows the concentration of these compounds in the extracts (mg/g) obtained at 25 \({}^{\circ}\)C as a function of the dielectric constant of the solvent used. In the case of water:organic solvent mixtures, the dielectric constant was estimated as the weighted average of the pure solvent dielectric constants. As it can be observed in Figure 1, the extraction of glycyrrhizin increased considerably as the solvent dielectric constant increased, while the opposite effect is observed for the non-polar compound glabridin. Similar results were observed in the case of licorice root UAE extracts obtained at 50 \({}^{\circ}\)C.