## 3 Results and Discussion
### Effect of HPP and Storage Time on Lipid Oxidation of Beef  
As expected, lipid oxidation processes in beef were enhanced by the HPP treatment, which was reflected in higher TBARS index values in HPP-treated beef compared to the control beef (Figure 2). On day 1 of storage, the HPP-treated meat had higher TBARS index values than the control meat (0.5 vs. 0.3 mg MAD/kg product). However, the observed differences were not significant. From day 14 to day 56, higher (\(p\leq 0.05\)) TBARS index values were observed for HPP400 and HPP500 meat than for the control meat. On the 56 th day of storage, TBARS index values of HPP500 meat were more than three times higher than in the control group (1.9 vs. 0.6 mg MAD/kg of product, respectively). Our studies confirmed that the exposure of meat to HPP can intensify lipid oxidation, especially in the pressure range from 300 to 500 MPa, which is within the range quoted in the literature [1; 22]. In the study by [PERSON] et al. [1], HPP treatment at higher pressures resulted in high TBARS values in some beef samples. In contrast, [PERSON] et al. [32] reported that the TBARS value of beef muscle (stored for 7 days at 4 \({}^{\circ}\)C and treated at 20 \({}^{\circ}\)C) was the highest at 400 MPa (6.71 mg MDA/kg), and then decreased at 600 MPa (5.25 mg MDA/kg) and 800 MPa (4.08 mg MDA/kg). Figure 2: Changes in TBARS index values of beef during 56 days of storage (average \(\pm\) standard deviation; \(n=3\)). The symbols identifying the treatments are as follows: control—not treated by HPP; HPP300, HPP400, or HPP500—HPP-treated at 300, 400, or 500 MPa. Different uppercase letters (A–C) within the same column indicate that average values are significantly different (\(p\leq 0.05\))—differences between treatments for a particular day of storage. Different lowercase letters (a–e) within the same row indicate that average values are significantly different (\(p\leq 0.05\))—differences between days of storage for a particular treatment. With the extension of storage time, a significant (\(p\leq 0.05\)) increase in the TBARS index values of skin-packed beef was observed (Figure 2). In the case of the control and HPP300 meat, the increase was relatively slight, in the range of 0.2-0.3 mg MAD/kg of product (to the level of 0.5-0.8 mg/kg product on days 49-56). For beef treated by 400 and 500 MPa, the TBARS index values increased from 0.5 (day 1) to 1.2 and 1.9 mg MAD/kg product (day 56), respectively (Figure 2). Compared to day 1, an increase (\(p\leq 0.05\)) in the content of fat oxidation products was observed on days 21-56 of storage; compared to day 7, a similar increase was observed on days 35-56; and compared to day 14, a similar increase was observed on days 56 for the HPP400 meat. Compared to day 1, the application of 400 MPa pressure resulted in more than 2-fold higher TBARS index values on the last day of storage. The rate of changes in the TBARS index values was the highest for the HPP500 meat, especially at the end of storage. An increase (\(p\leq 0.05\)) in the value of this index was observed on days 21-56 of storage compared to its level on days 1 and 7. From day 28 of storage, another significant (\(p\leq 0.05\)) increase in the value of this index was observed compared to its level from days 1-14. The highest TBARS values for the HPP500 meat were found on day 56 of storage and were almost four times higher than the results obtained on day 1 (1.9 vs. 0.5 mg MDA/kg of product, respectively). [PERSON] et al. [33] also observed a significant increase in the oxidation of HPP-treated beef by approximately 2- and 3-fold on the 5 th and 10 th day of storage, respectively, compared to day 0. A significant increase in TBARS index values was observed on 10 th day for the non-pressurized samples, while for the HPP-treated samples, a significant increase in TBARS index values was observed after 5 days of storage. The effect of pressure was particularly noticeable at 450 and 600 MPa. In the study by [PERSON] et al.