## 3 Results and Discussion
### Physicochemical Properties of Baked Meat, Hybrid, and Plant Burgers
The redness of the burgers with the highest content of plant-origin parts (M25_P75 and M0_P100) was similar to that of the meat burger (Table 5). The meat is pink and red after thermal treatment due to previous curing. Oxygened myoglobin (MbO2), in the presence of nitrites, is transformed into unstable shiny red nitrosomyoglobin (NOMb) and then, under the influence of heat treatment, into stable bright red nitrosomyochromogen [71]. In turn, the reddish color of the surface of hybrid and plant burgers in recipe variant II results from the presence of carrots, which contain a lot of \(\beta\)-carotene, which is responsible for their typical orange color. Therefore, carrots are characterized by a high value of the a* color parameter even after thermal treatment (on average in the range of 10-40, depending on the composition of the raw material, type, temperature, and time of initial/proper processing) [72; 73]. Furthermore, the incorporation of plant ingredients into the composition of pork burgers significantly increased the value of the color parameter b* (indicating their yellowness) compared to a 100% meat burger. Even with 25% content of plant parts, the average value of this parameter increased approximately two or three times, regardless of the recipe variant and the measured area (Table 5). However, a statistically significant higher value of the b* parameter was observed for hybrid burgers with the highest content (75 or 100%) of a plant mixture from recipe variant II (with lentils and carrots) compared to their counterparts from recipe variant I (based on a plant mixture with millet goats and red tomatoes). It can therefore be concluded that these differences were primarily the result of the chemical composition of the individual ingredients in the burgers. Both carrot roots (as previously mentioned) and lentil seeds are raw materials rich in carotenoids. However, it should be taken into account that the plant part of recipe variant II of the burgers is dominated by red lentils (about 45%--Table 1), the color parameter b* of which has much higher values (up to 5 times) than the a* color parameter, even after heat treatment [74]. In order to determine the overall differences in color between hybrid or plant burgers, compared to a meat burger, the total color difference \(\Delta\)E\({}_{1}\) was determined. Its average value exceeded 5.0 for each comparison set (Table 5), which means that the observer may have the impression of two different colors [30]. Moreover, the lower the ratio of meat to plant parts in the recipe, the higher the \(\Delta\)E\({}_{1}\) level. Similar relationships were observed when measuring the color of both the surface and cross-section of burgers. Nevertheless, the largest total color difference compared to the meat sample (\(\Delta\)E\({}_{1}\)\(\sim\)20.2) was recorded in the case of a cross-section of a plant burger based on lentils and carrots (recipe variant II). Due to the fact that baked meat products have a characteristic brownish surface, the Browning Index (BI) was determined (based on the basic color coordinates: L*, a*, and b*) of the tested meat, hybrid, and plant burgers, as shown in Figure 1.  
The higher the content of plant parts in a burger recipe, the greater the Browning Index of its surface after baking (Figure 1). However, the degree of these changes depended on the recipe variant. Plant burgers based on lentils and carrots (recipe variant II) were characterized by more than twice the BI value of a full-meat burger. In addition, each burger from recipe variant II showed a greater (\(p\leq 0.05\)) degree of surface browning than the samples based on recipe variant I (Figure 1). The brown surface of baked or roasted food products affects their final acceptance by the consumer. This is mainly the result of non-enzymatic transformations, including Maillard reactions (amine groups with carbohydrate groups), caramelization (degradation of sugars), oxidation of lipids, or structural modifications of proteins.