## 1 Introduction  
In recent years, changes in environmental conditions and a decrease in the availability of natural resources have resulted in greater pressure on the economy and industrial food production. For this reason, the concept of a circular economy, as part of sustainable development, represents a way to overcome the existing production model, which would successfully increase the use of available resources, and thereby reduce industrial waste [1]. Sustainable development has three main goals: economic efficiency, social justice, and environmental sustainability, which requires significant changes in the way of thinking and the use of resources. By changing the way of thinking, each individual should promote equality and respect for individual rights, recycle waste, and use renewable energy sources, for the purpose of reducing waste and preventing water shortages for current and future generations. According to the European Green Deal, strategies that include sustainable development in the food industry sector are the \"Green Plan\" and \"From field to table\", which encourage the reduction of greenhouse gas emissions, the use of renewable energy sources, and different ecological processing methods [2; 3; 4; 5]. Accordingly, there is an increasing interest in the conversion of processing by-products [6; 7; 8; 9; 10] into commercially valuable products using \"green\" methods, whereby ultrasound-assisted extraction stands out as a suitable technique for extracting high-value compounds from such sources. The main effect of ultrasound is based on the growth and implosion of air bubbles, better known as the phenomenon of acoustic cavitation, which occurs as a result of physical forces [11]. The implosion of the bubbles destroys the plant cell walls, and consequently facilitates the penetration of the solvent into the plant cell. The aforementioned contributes to speeding up the extraction process [12]. Furthermore, cavitation causes hydration and swelling of plant tissues, which simplifies the release of polyphenols and increases their yield [13]. From an ecological and economic point of view, \"green extraction\" techniques represent one of the most important steps in obtaining bioactive compounds from side streams [14]. Based on the exceptional nutritional composition and availability in the form of agricultural waste, finding an application for the high value components of the leaves has been encouraged through the extraction of proteins and specialized plant metabolites for use in human nutrition and the food industry [15]. The extraction of protein from specialized streams (by-products) of processing represents a major shift in reducing the adverse effects of the food industry on the environment, but also a potential economic benefit, considering the valorization of the protein fraction obtained from such a source. After harvesting and processing in the sugar industry, sugar beet leaves behind a number of by-products that can be used for other purposes. One of the main by-products of sugar beet is the smooth, dark green leaves, separated from the roots during harvesting, which are mainly used as livestock feed or left in the fields. They make up 20-34% of the plant, and depending on the variety, harvest time, and growing season, their share in relation to the root can vary [16]. According to official data available on the FAOSTAT website, Europe is by far the largest producer of sugar beet (69% of world total production). On the territory of Europe, in the last 20 years, the biggest jump in sugar beet production was recorded in 2017 (219,867,250 tons). The last official data for sugar beet production in Croatia is from 2020, when annual production was 774,330 tons. Statistical analysis on the exploitation of sugar beet leaves in Croatia are not given, but according to information available from local producers, leaves are most often destroyed. Only in rare cases are they used as fertilizer or feed.