## 4 Discussion  
During research, dried and fresh samples of sugar beet leaves were subjected to extraction assisted by high power ultrasound in order to determine and optimize the yield of total proteins and specialized plant metabolites (polyphenols). Ultrasonic extraction was used due to the possibility of achieving relatively low treatment temperatures (up to 40 \({}^{\circ}\)C with cooling), while the selected extraction solvent, deionized water, represents a cheap, ecologically, and technologically acceptable \"green solvent\" [38; 39; 40]. In general, regardless of the type of sample (dry/fresh), higher yields of total proteins were recorded in ultrasonically treated samples with a couple of exceptions. Different increases and decreases in protein yield within sonicated samples can be attributed to amplitude selection. With the increase of ultrasonic power, that is, the amplitude of the ultrasound, the implosion of the cavitation bubbles was stronger and a higher extraction yield was sampled. On the other hand, excessive ultrasound power can increase the number of bubbles in the solvent; thereby, reducing the effectiveness of the ultrasound energy transmitted to the medium, and consequently, reducing the protein yield [41]. Furthermore, by choosing optimal amplitude conditions, the unwanted degradation of extracted compounds, such as proteins, is avoided [42]. The protein yield in ultrasonically treated samples differed depending on the type of sample. Specifically, higher yields were observed in samples with fresh sugar beet leaves. Drying, as a preservation method, negatively effects protein stability; freezing (the preservation method used for fresh sugar beet leaves) is considered a more suitable method for preserving protein stability in leaves [43]. The thematically closest research to our own was conducted with alternative enzymatic extraction of sugar beet leaves, where, compared to thermal methods of isolation, an increase in protein yield (by 43.27%) was observed [44]. In relation to the protein yield, the yield of phenolic compounds was significantly lower. In general, lower phenolic values may be the result of poor solvent selection or the insolubility of all types of phenolic compounds in a particular solvent. Phenolic compounds form complexes with carbohydrates, proteins, and other plant components, so some high-molecular weight phenols and their complexes can be quite insoluble in the selected solvent [45]. Compared to thermally treated samples, higher yields of phenolic compounds were observed in ultrasonically treated samples. Furthermore, with a longer treatment time, a decrease in phenol yield was observed in thermally treated samples with dried sugar beet leaves. The obtained results are in accordance with the sensitivity of antioxidant compounds to elevated temperatures, where prolonged exposure to elevated temperatures leads to the breakdown of the mentioned compounds [46]. In contrast to dry samples, in fresh samples treated with the conventional extraction method (heat), a longer treatment time led to the yield increase of total phenols. As a result of prolonged exposure to elevated temperature, changes occur in the cellular structure of the plant, facilitating the diffusivity of bioactive components and increasing their solubility in the solvent [47]. Results similar to our research were recorded, where, as a result of ultrasound-assisted extraction and microwave extraction, increases in the yield of phenolic compounds were recorded (0.45-1.72 g(100 gd.m.)\({}^{-1}\)). The phenolic profile differed among the techniques, but the phenolic compound vitexin was the most abundant regardless of the applied technique [48]. So far, the lack of research on a similar issue makes the results obtained by ultrasonic extraction of proteins and polyphenols from sugar beet leaves incomparable with previously published data.