## 3 Results and Discussion
### Influence of Process Variables on Physical Properties of Extruded Snacks  
Extrusion-cooking is a process whereby ingredients are subjected to high temperatures, pressures and mechanical shear, resulting in the formation of granules with a specific structures, textures and physical properties. The type of ingredients present in extruded food granules can significantly influence their physical properties. One of the most important physical characteristics of expanded products is the expansion rate. During the course of the research, the authors observed that increasing the proportion of vegetable pomace in the blend resulted in a reduction in the expansion rate. In the conducted research, pellets with added fruit pomace exhibited a relatively high expansion index. The highest value for mixtures enriched with fruit pomace was achieved with a mixture containing 10% addition of aronia pomace processed at 40 rpm (3.17), while the lowest was observed with 30% addition of chokeberry pomace processed at 80 rpm (2.37). In the case of pellets with the addition of nigella seed pomace, the highest value was obtained for pellets based on a mixture with 10% seed pomace processed at 40 rpm (3.11), while the lowest value was observed for a mixture with 30% nigella seed pomace processed at 60 rpm (1.65). Similarly, for the mixture with added flaxseed pomace, the highest value was also achieved for a mixture with 10% ingredient processed at 40 rpm (2.97), and the lowest value for a mixture with 30% ingredient at 60 rpm (1.74) (Figure 5). Similar correlations were presented by [PERSON] and [PERSON] [25], where increasing the percentage of fresh vegetables in the mixture led to a reduction in the expansion ratio. Similarly, [PERSON] et al. [27] observed a reduction in expansion rate when plant ingredients were present. The authors ascribed this phenomenon to the high fibre and lycopene content of the plants, which diminished the expansion capacity. A study by [PERSON] et al. [28] utilising a single-screw extruder to process a raw material mixture enriched with 5, 10 and 20% by-product ingredients demonstrated that the incorporation of fruit pomace (chokeberry and cherry) into extruded maize snacks also resulted in a reduction in expansion rate. The higher fibre and polyphenol content of these ingredients resulted in a reduction in expandability, which in turn affected the texture and structure of the final products. Figure 4: Response plots of specific mechanical energy demand during the extrusion-cooking process of snack pellets as a function of the level of addition of selected plant pomace and the processing variables used: (**a**) apple pomace; (**b**) chokeberry pomace; (**c**) pumpkin pomace; (**d**) flaxseed pomace; (**e**) nigella seed pomace. In the next stage of the experiment, the effect of screw speed and the type and percentage of pomace content on the bulk density of snack granules enriched with selected by-products of the agro-food industry was studied. The results showed that all three factors had a significant effect on the value of the characteristic studied. The highest bulk density value was obtained for the mixture with the addition of 20% nigella seed pomace processed at 40 rpm (392.11 kg/m\({}^{3}\)). Conversely, the lowest value of the studied characteristic was found in snack pellets based on a mixture with the addition of 30% chokeberry pomace processed at 60 rpm (262.47 kg/m\({}^{3}\)). The results of the analysis of snack pellets indicate that the bulk density value decreases with the increasing percentage of pomace addition (Figure 6). This is primarily due to the increased expansion and porosity of the products (Figure 7). This dependence applies to extrudates based on all formulation mixtures studied in the conducted research  
[PERSON] and [PERSON] [29] also observed that the addition of fresh vegetable pulp decreases bulk density. They also conducted their research using a mixture based on potato products processed using a single-screw extruder. Similar conclusions were presented by [PERSON] et al.