## 3 Results and Discussion
### Effect of HPP on the Free Amino Acids Profile of Beef During Storage
The dynamics of this increase was the highest in the control and HPP300 meat (approximately 9-fold increase in content during 56 days of storage). The higher the pressure, the lower the dynamics of growth (for HPP400 meat, about a 6-fold and for HPP500 meat, about a 4-fold increase). An approximately 3-fold increase in the leucine content was observed during 10 days of beef storage by [PERSON] et al. [42]. The phenylalanine content on day 1 was 37.30-50.05 mg/kg, and on day 56 of storage it ranged from 168.09 to 312.76 mg/kg. An effect (\(p\leq 0.05\)) of HPP on the phenylalanine content in beef was observed. From day 1 to 14 of storage, the meat treated with HPP was characterized by a higher or significantly higher content of this amino acid compared to the control meat. This difference was significant from day 35 to the end of the experiment. Starting from day 42, the meat treated with 400 and 500 MPa was characterized by a significantly lower content of this amino acid than the control and HPP300 meat, and on day 56, it was approximately 2-fold lower for HPP500 meat (168.09 vs. 284.21 mg/kgor 312.76, respectively). [PERSON] et al. [32] found that an increase in the phenylalanine and threonine content in lamb meat after the HPP treatment could be explained by the high overall autolytic activity of the meat that occurs after the HPP treatment, especially at pressures from 100 to 300 MPa. The phenylalanine content in the HPP300 samples starting from day 42 was the highest among treatments, which was similar to the trend observed for leucine. Similarly, the reduced phenylalanine concentration in the control during storage, compared to HPP300 samples, could result from the microbial decomposition of phenylalanine [49]. Similarly to the previously analyzed amino acids, in the case of phenylalanine, an increase (\(p\leq 0.05\)) was also observed in its content during meat storage (Table 2). The dynamics of this increase was higher for the control and HPP300 meat (approximately 8- and 6-fold increase compared to day 1, respectively, up to 284.21 and 312.76 mg/kg on day 56), and lower for the HPP400 and HPP500 meat (approximately a 4-fold increase compared to day 1, up to 217.75 and 168.09 mg/kg on day 56, respectively). [PERSON] et al. [42] found an almost 8-fold increase in the phenylalanine content during a 10-day beef storage period. [PERSON] et al. [44] showed that contents of leucine, isoleucine, serine, threonine, valine, and phenylalanine increased 3-fold in beef stored for 3 weeks. On day 1 of storage, beef had a lysine content in the range of 70.58-80.75 mg/kg, and on day 56, it was in the range of 69.91-140.55 mg/kg (Table 2). The use of HPP caused an increase (\(p\leq 0.05\)) in the lysine content of beef compared to the control meat (except day 1). The significantly lower lysine concentrations found in control samples could follow the microbial production of cadaverine, which is formed through the decarboxylation of lysine. As shown in our previous studies [31], the cadaverine content was significantly higher in the control meat than in HPP samples throughout the experiment (except day 35). The application of HPP caused a reduction in the number of bacteria and slowed down their growth in comparison to the control meat, in which the activity of microorganisms able to decarboxylate lysine putatively occurred. Data on the inhibitory effect of HPP on the growth of microflora in food, as well as the dependence of this effect on the level of applied pressure, are commonly available in the literature [50; 51]. [PERSON] et al. [4] showed that the amino acid content in meat increased under the influence of moderate pressure (200 and 300 MPa), whereas it decreased under the influence of higher pressure (500 MPa) in comparison to control samples; for moderate pressures, most of the analyzed free amino acids had an opposite trend to that observed in our studies. It was found that the storage time had an effect (\(p\leq 0.05\)) on the lysine content in beef.