## 1 Introduction  
The agri-food industry constitutes a pivotal role in global food production, catering to the increasing demand for nutritious and safe food products [1; 2; 3; 4]. However, this industry also generates substantial amounts of by-products during processing, which, if not properly managed, can pose environmental challenges. By-products such as pomace from fruits, bran from grains and oilseed meals are often considered waste, leading to concerns regarding waste management, resource utilization and environmental sustainability [5; 6]. In recent years, there has been a growing interest in exploring the potential of agri-food by-products as valuable resources for various applications, including their utilization as food ingredients [7; 8]. This paradigm shift is driven by several factors, including the need to reduce waste generation, enhance resource efficiency and promote sustainable practices within the food industry [9; 10]. These by-products often contain valuable nutrients, bioactive compounds and functional components that can enhance the nutritional profile and functionality of extruded products [4; 11]. However, their characteristics, including particle size, moisture content, fiber content and composition, can also influence process parameters such as barrel temperature, energy consumption and pressure [12; 13]. Thus, a meticulous examination of the characteristics of by-products and their interplay with processing variables, including the extrusion-cooking technique, is paramount to guaranteeing the excellence of the final products [14].  
Extrusion-cooking processing is a huge potential technology for the valorisation of agri-food by-products due to its versatility, efficiency and ability to modify the functional and nutritional properties of raw materials [15; 16]. In the context of food processing, the extrusion-cooking process refers to a continuous process that involves forcing a raw material mixture through a die under controlled conditions of temperature, pressure and shear [17; 18]. This process results in the transformation of the raw material into a product with a desired shape, texture and structure. Such action can reduce water consumption and minimize the energy consumption of the process [19].  
Food pellets incorporating vegetable and fruit pomace present substantial potential for implementation, particularly with respect to sustainability and innovation within the food industry. The integration of such pomace facilitates efficient management of production waste, thereby mitigating waste generation and minimizing environmental impact. Additionally, these ingredients enhance the nutritional profile of the pellets, offering consumers healthier and more wholesome snacking alternatives. Technological advancements, such as optimizing the extrusion-cooking process, may establish new benchmarks in pellet production, thereby conferring a competitive edge in the market. Furthermore, the introduction of these products aligns with the increasing consumer demand for innovative and environmentally sustainable food solutions, rendering them an appealing option for both producers and consumers. Therefore, our research was designed to investigate the effect of the addition of varying content of selected pomace (apple, chokeberry, pumpkin, flaxseed and nigella seed pomace), as well as the variation of screw speed and plasticizing system on the extrusion-cooking process and the physical properties of the created pellets.