## 3 Results and Discussion
### Effect of HPP and Storage Time on Lipid Oxidation of Beef
[34], it was shown that subjecting beef to pressures in the range of 400 to 600 MPa combined with storage at 0 \({}^{\circ}\)C increased the TBARS index values, but did not exceed the critical value of 2 mg MDA/kg product [35]. This level was not exceeded in our studies even after 56 days of storage; however, for the HPP500 meat on day 56, the TBARS index was 1.9 mg MDA/kg of product (Figure 2). Among the hypotheses regarding the prooxidant effect of high pressure on meat lipids, [PERSON] et al. [14] mentioned the results of studies that indicated that HPP can modify the physical state of lipids by accelerating their crystallization, and thus facilitate oxygen penetration. Moreover, the authors refer to the potential relationships between the oxidation of proteins and lipids in meat exposed to high pressure. The HPP changes the conformation of heme proteins (myoglobin and hemoglobin) and then causes greater exposure of unsaturated fatty acids to contact with heme groups, acting as a catalyst for lipid oxidation. Effect of HPP and Storage Time on the L\({}^{*}\), \(a^{*}\), and \(b^{*}\) Color Parameters of Beef  
The HPP treatment significantly (\(p\leq 0.05\)) influenced the color parameter values of L* (lightness) of beef, and these changes were observed on each of the analyzed days of storage (Table 1). The control meat had lower (\(p\leq 0.05\)) values of this color parameter compared to HPP-treated beef. Moreover, with increasing pressure, a greater effect on the lightness of the beef color was observed, as the highest values of the color parameter L* were found for HPP500 meat. A significant (\(p\leq 0.05\)) effect of storage time on the L* color parameter values was observed only for the control meat. Compared to days 1-14, a significant increase in L* color parameter values was observed on days 21-56 of storage. On the other hand, the lightness of the HPP-treated meat has changed slightly (46.6-52.4 for HPP300, 51.0-56.0 for HPP400, and 53.8-56.4 for HPP500 meat, respectively) throughout the storage time (Table 1). The lightening of the color of red meat treated with HPP has been confirmed in numerous studies [5; 9; 10; 21]. According to [PERSON] et al. [36], the significantly lighter color of meat after applying a pressure of \(>\)200 MPa compared to unpressurized meat was caused by the denaturation and coagulation of muscle proteins. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{**Color Parameter**} & \multicolumn{1}{c}{**Treatment**} & \multicolumn{1}{c}{**Storage Time (Day)**} & \multicolumn{1}{c}{**Storage Time (Day)**} & \multicolumn{1}{c}{**} & \multicolumn{1}{cThere was no significant (\(p>0.05\)) effect of the applied pressure on the color parameter values of a* (redness) on the 1 st and 7 th day of storage of the analyzed skin-packed beef. However, from the 14 th day of storage, a significant (\(p\leq 0.05\)) influence of the pressure at the level of 500 MPa on the values of this color parameter was observed. From this day of storage to day 56, the HPP500 meat was characterized by the significantly lowest values of the a* color parameter. The storage time did not differentiate a* color parameter values, regardless of the treatment (Table 1). [PERSON] and [PERSON] [21] found that processing beef with HPP at a pressure of not less than 400 MPa significantly reduced a* (redness) values. In the studies of [PERSON] and [PERSON] [9], the redness of ground beef decreased after HPP. In the study by [PERSON] et al. [5] for DFD meat (dark, firm, dry), the redness (a*) value showed an increasing tendency with increasing storage time, which was not observed in our studies. An effect (\(p\leq 0.05\)) of HPP treatment on the b* (yellowness) color parameter of beef was observed (Table 1). On days 1-14, control beef samples had lower (\(p\leq 0.05\)) values of these color parameters than HPP-treated beef. From day 21 to the end of storage time, there were no differences in b* color parameter values between the control and HPP-treated beef.