## 1 Introduction
The use of plant by-products to produce high value compounds represents an interesting solution from an ecological point of view, but also an excellent opportunity to produce functional food products of high nutritional value, especially in the form of sustainable sources of protein, in order to meet growing demand, and for the health effects on the body. In addition to protein, the leaves contain numerous high-value compounds (dietary fiber, minerals, and specialized plant metabolites) that potentially expand the possibilities of using the leaves and increase their use in various sectors of the food industry and biotechnology. The most commonly considered application of the leaves is in biotechnology, where it can potentially be used as a feedstock in the production of bioethanol [17]. Sugar beet leaves have a balanced composition of amino acids, such as essential amino acids leucine (9.19 \(\pm\) 0.26%), valine (6.15 \(\pm\) 0.13%), phenylalanine (5.90 \(\pm\) 0.14%), lysine (6.50 \(\pm\) 0.38%), threonine (5.05 \(\pm\) 0.10%), isoleucine (4.95 \(\pm\) 0.09%), and methionine (2.08\(\pm\) 0.09%), which additionally contribute to the nutritional value and quality of sugar beet leaves [18]. The use of phenols (phenolic acids and flavonoids), obtained from plant sources, as a cross-linking agent during the production of stabilized gelatin gels and gelatin pectin coacervates was also investigated [19]. Gels cross-linked with phenolic compounds show high mechanical strength and thermal stability, and reduced swelling. Such properties enable their application in the form of new food ingredients. Phenolic compounds of plant origin can be used as functional ingredients of food products [20] for the purpose of increasing the antioxidant capacity of processed food, but also for the prevention of diseases caused by oxidative stress, which occurs as a result of an accelerated lifestyle and a polluted environment [21; 22; 23]. In addition to various phenolic compounds, plant extracts may contain other bioactive compounds which may have a similar, different, or complementary biological capacity compared to phenol. During consumption, the action of individual bioactive components can be modified by synergistic, additive, or antagonistic interactions between them, which can consequently change their physiological effect [24]. Not only in laboratory conditions, but also in the food industry, several conventional methods are used for extraction, mainly solid-liquid extractions, such as Soxhlet extraction or maceration. In general, any extraction method requires mechanical destruction of the cell to enable the release of the desired components from the complex cellular structures, which may consequently damage or reduce the quality of the extracted products [25]. In addition, the disadvantages of using conventional methods include a large volume of toxic organic waste that evaporates during extraction, long term extraction, high energy input, low selectivity, lower quality and yield of the final product, and risk to the environment [26; 27]. To avoid the mentioned shortcomings, alternative techniques have appeared in accordance with the concept of \"green extraction\", which result in a higher yield, shorter extraction time, and lower costs and impact on the environment [28; 29; 30]. These extraction methods are also known as non-thermal because the temperature during the process is relatively low and does not affect the stability of the extracted compounds. In some research, they are also referred to as mild methods [20], and depending on the type of extraction, can be used independently or as a pretreatment [31; 32; 33]. Among the new extraction methods, the use of ultrasound is becoming more and more prominent in the food industry due to its relatively simple use, lower financial investment, and the possibility of processing food without the addition of additives.