## 4 Discussion  
The optimal extraction parameters of the tested plant material in terms of the antioxidant activity and TPC were 7.5 g raw material/100 mL of solvent, 47% \(v/v\) ethanol as a solvent, and a processing time of 10 min. The presented studies used indirect ultrasound-assisted extraction performed using an ultrasonic bath with a frequency of 40 kHz. [PERSON] et al. (2014) optimised the aqueous ultrasonic extraction of grape pomace using the RSM in terms of antioxidant capacity as well as total phenolic and flavonoid content. The acoustic frequency (40, 80, 120 kHz), ultrasonic power density (50, 100, 150 W/L), and extraction time (5, 15, 25 min) were used as independent variables. They set the optimal process conditions at 40 kHz, 150 W/L, and 25 min, respectively ([PERSON] and [PERSON], 2013). [PERSON] et al. (2013) optimised the parameters of UAE for unsolic acid from _Ocinum sanctum_. Similar to our studies, they analysed the extraction time and solid-to-solvent ratio as well as other parameters such as solvent selection, process temperature, and ultrasound power and frequency. They obtained the maximum extraction efficiency of unsolic acid at 12 min, a solid/solvent ratio of 1:30, 45 \({}^{\circ}\)C, and 25 kHz. However, by analysing the effect of the ultrasound frequency (25 and 40 kHz), they showed higher extraction efficiency when using the frequency of 40 than 25 kHz ([PERSON] and [PERSON], 2013). Lower ultrasound frequencies in the range of 18-40 kHz seem to be effective in extracting biologically active compounds from plant materials, including those with antioxidant potential, by disrupting cell walls, resulting in increased solvent penetration and mass transfer rates ([PERSON] and [PERSON], 2013). Therefore, in our own research, an ultrasound frequency of 40 kHz was used in the process of extraction of biologically active compounds from the herb _S. officinalis_, and in order to increase the extraction efficiency, powdered plant raw material was used. In our research, we used powdered _S. officinalis_ herb (<0.25 mm). [PERSON] and [PERSON] (2020) showed that, as the particle size of the plant material decreases, the extraction efficiency increases, resulting in a higher content of biologically active compounds extracted from the plant material. Furthermore, they observed that, in the case of the extraction of powdered material, most of the extraction was obtained directly after the contact of the raw material with the solvent. They suggest that, when grinding into a fine powder, bioactive compounds are released; therefore, a short-term contact of the raw material with a solvent is sufficient to recover most of the active substances ([PERSON] and [PERSON], 2013). [PERSON] et al. (2016) analysed the effect of grinding and sieving of plant raw materials, _Hypericum perforatum_ and _Acthillea millefolium_, on antioxidant activity and total polyphenol content. In their studies, they used raw materials with particle sizes of 20 to 500 \(\upmu\)m. They observed the highest antioxidant activity for the 100-180 \(\upmu\)m fraction, while the 180-315 \(\upmu\)m fraction contained the most polyphenols. Based on the obtained results, they concluded that the optimal raw material fractions in terms of the tested properties were those with a diameter of 100-315 \(\upmu\)m ([PERSON] and [PERSON], 2013). These researchers confirmed  
Figure 3: An example of the UV-Vis spectrum of an optimised extract obtained from _S. officinalis_. their observations in subsequent studies on _Hieracium pilosella_. The raw material fractions 180-315 \(\upmu\)m and 315-500 \(\upmu\)m exhibited the highest antioxidant activity and content of the selected phenolic compounds [27]. [PERSON] et al. (2019) showed the highest antioxidant activity and total polyphenol content in granulometric classes of 0-180 \(\upmu\)m for _Boscia senegalensis_, 180-212 \(\upmu\)m for _Dichrostachys glomerata_, and 212-315 \(\upmu\)m for _Hibiscus sabbaraffifa_.