## 3 Results and Discussion
### Effect of HPP on the Free Amino Acids Profile of Beef During Storage
Compared to days 1 and 7, the lysine content of the control meat increased, with the highest (\(p\leq 0.05\)) values on days 28 and 35 of storage, and then gradually decreased to the initial level (similar to that observed on day 1). In turn, the lysine content in HPP-treated meat, compared to day 1, increased significantly starting from day 14 of storage, reaching the highest values on day 35, but decreased in the following days. On day 56 of storage, its level was approximately 1.5-fold higher than on day 1. The decrease in the lysine content that occurred after day 35 could result from the decarboxylation reaction, which led to cadaverine production. As presented in our previous study [31], the concentration of this biogenic amine rapidly increased in the samples on day 35, in comparison to the values measured at the previous sampling points, and generally continued to increase until the end of the experiment (except 49 day for HPP300 meat, when a slight decrease to 106.73 mg/kg occurred, the value insignificantly lower by approximately 7 mg/kg compared to the content of cadaverine on 42 day for HPP300), causing a decrease in the lysine concentration. An increase in the lysine content during the 10-day beef storage period was found by [PERSON] et al. [42]. According to [PERSON] et al. [52], the decrease in the quantitative content or even loss of some amino acids can be explained by specific oxidation reactions, such as nitrosylation, chlorination, or the formation of thiol, bittyrosine, and oxo-histidine bonds. Lysine and threonine tend to form carbonyl compounds, while lysine and histidine react with some lipid peroxidation breakdown products, such as malondialdehydedehyde. The above-mentioned changes in the amino acid structure may then affect their susceptibility to the action of aminopeptides and therefore to an increased level of free amino acids observed after storage [4]. The histidine content in beef at the beginning of storage (day 1) ranged from 29.62 to 33.76 mg/kg, and on day 56, it ranged from 52.03 to 137.89 mg/kg (Table 2). An effect (\(p\leq 0.05\)) of HPP on the histidine content in beef was found (Table 2). Starting from day 14, the histidine content was lower (\(p\leq 0.05\)) in the HPP-treated meat than in the control meat. On days 49 and 56 of storage, the histidine content in the HPP500 beef was approximately 3-fold lower than that of the control meat (52.03 vs. 137.89 mg/kg on day 56). In contrast, [PERSON] et al. [4] reported that the meat treated with 500 MPa had a higher histidine content than unpressurized samples. The longer the beef storage time, the higher the histidine content. An increase (\(p\leq 0.05\)) was observed, compared to day 1, from day 7 of storage for the control and HPP300 meat, and from day 21 for the HPP400 and HPP500 meat. The dynamics of this increase was the highest for the control meat (more than 4-fold higher during 56 days of storage, up to 137.89 mg/kg) and decreased when higher pressures were applied (approximately 3-, 2-, and 1.5-fold higher for HPP300, HPP400, and HPP500 meat, up to 87.16, 65.97, and 52.03 mg/kg, respectively). The results obtained by [PERSON] et al. [44] and [PERSON] et al. [4] indicate that the histidine content increased during 21 and 14 days of beef storage, respectively. The tryptophan content, compared to the other free amino acids, was low and ranged from 10.90 to 12.64 mg/kg on day 1 of storage, while at the end storage, it ranged from 29.68 to 47.68 mg/kg (Table 2). Starting from day 7 of storage, the HPP500 meat had a lower (\(p\leq 0.05\)) tryptophan content than the control meat, and from day 21, it was also lower than the HPP300 meat. These differences were observed until the end of the storage time. The differences between the content of tryptophan in the control and HPP300 meat were insignificant for most of the analyzed storage times. On day 56 of storage, the HPP500 meat was characterized by more than a 1.5-fold lower content of tryptophan than the control meat (29.68 vs. 47.68 mg/kg). Also, [PERSON] et al.