## 3 Results and Discussion  
### Determination of Resveratrol Content  
In order to isolate resveratrol from the _Polygonum cuspidatum_ herb, the UASE technique was applied, which is quite popular among currently used assisted extraction techniques, especially in the food and pharmaceutical industries [16]. The extraction is facilitated by the phenomenon of cavitation. It occurs during the propagation of ultrasound, i.e., a series of compression and dilution waves, in liquid systems. As a result, cavitation bubbles are induced in the liquid [17]. Within a few hundred microseconds, the bubbles grow in a pulsating manner to about 150 \(\upmu\)m, and after reaching a critical point, they collapse and release a large amount of energy. This causes the generation of extreme temperatures (5000 \({}^{\circ}\)C) and pressures (2000 atm) and mechanical waves propagating at a speed of 150 m/s [18]. During extraction, these extreme conditions (high temperature and pressure) are responsible for the destruction of cell walls, which facilitates the release of bioactive components and increases mass transport per unit of time [19]. It should be added that in this technique, the extraction efficiency depends on the frequency of ultrasound, the intensity of cavitation bubbles, the power of the ultrasound generator, as well as the temperature and time of exposure of the extracted material to ultrasound, and the type of solvent used for extraction. With this in mind, this study investigated the effect of changing these experimental parameters on the extraction efficiency of resveratrol from _Polygonum cuspidatum_. The results of this series of studies are presented in Figure 1.  
Figure 1 compares the content of resveratrol in extracts of Japanese knotweed obtained under different conditions of the UASE process. Three extractants were used for extraction: methanol (MeOH), methanol/water (MeOH/H\({}_{2}\)O), and water (H\({}_{2}\)O). Figure 1a collects data for extracts obtained at temperatures of 25 \({}^{\circ}\)C and 50 \({}^{\circ}\)C during 10 min of extraction carried out at a frequency of 37 kHz. In turn, Figure 1b presents the results for extracts obtained under conditions analogous to the previous ones, with the difference that the extraction time was extended to 20 min. Figure 1c,d shows the resveratrol content in extracts obtained similarly as before at temperatures of 25 \({}^{\circ}\)C and 50 \({}^{\circ}\)C during 10 and 20 min of extraction, but at a frequency of 80 kHz. For the purposes of statistical analysis, Figure 1a was compared with Figures 1b and 1c with Figure 1d. Lowercase letters (a-h) and uppercase letters (A-B) in Figure 1a,b and Figure 1c,d, respectively, indicate a lack of statistical significance. As can be seen, the extraction temperature affects the efficiency of resveratrol isolation. The data in Figure 1a show that the resveratrol content in the extracts tested is higher at higher temperatures. This effect is visible in the case of each extraction solvent used. These data are confirmed by the literature. The use of UASE for the isolation of resveratrol from _Polygonum cuspidatum_ is examined in [20]. The authors of the cited paper obtained extracts with a relatively high content of resveratrol when the extraction was carried out at a higher temperature (45 \({}^{\circ}\)C) and at an ultrasound frequency of 37 kHz. The situation changes when the extraction time is extended to 20 min. The data in Figure 1b indicate that extending the extraction time causes an increase in the amount of resveratrol at 25 \({}^{\circ}\)C (see data for 25 \({}^{\circ}\)C in Figure 1a,b) and a decrease in its content at 50 \({}^{\circ}\)C with time, clearly visible for the methanol and water extract (see data for 50 \({}^{\circ}\)C in Figure 1a,b, blue and gray bars). Thus, it seems that extending the extraction time favors the reduction of resveratrol amounts at higher extraction temperatures, which may be related to the transformation and/or degradation of this compound during extraction.