## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c} \hline
**Treatment/Storage Time** & **1 Day** & **14 Days** & **28 Days** \\ \hline
**Nontreated** & 4.1 \(\pm\) 0.1 & 5.3 \(\pm\) 1.2 & 6.5 \(\pm\) 1.1 a  \\
**HPP treated at 200 MPa** & 3.9 \(\pm\) 0.2 & 5.0 \(\pm\) 0.8 & 5.3 \(\pm\) 1.5 \\
**HPP treated at 400 MPa** & 2.9 \(\pm\) 0.4 & 3.5 \(\pm\) 0.9 & 3.9 \(\pm\) 1.7 \\
**HPP treated at 600 MPa** & 1 & 1 & 2.0 \(\pm\) 0.5 a  \\ \hline \end{tabular} Values are the mean \(\pm\) standard deviations of three replicates. Values followed by the same letter are significantly different (\(p<0.05\)). 1 Below the detection limit (100 cfu/g).  
\end{table} 
Table 2: Numbers (log\({}_{10}\) cfu/g) of _Listeria monocytogenes_ colonies in inoculated warm-smoked rainbow trout samples at three (1, 14, and 28 days) different storage timepoints. vacuum-packed cold-smoked rainbow trout [21]. Smoking decreases the pH of the fish [18], and low pH (4.5) was observed to enhance the reduction of _L. monocytogenes_ at the 200 MPa HPP treatment compared to higher pH [22]. Overall, a good microbiological quality of fresh rainbow trout fillets was reached by HPP treatment at 450 MPa and 600 MPa for 15 min as the microbial growth of total aerobic bacteria was not observed after 6 days of storage at 4 \({}^{\circ}\)C [23]. In the present study, an acceptable level of _L. monocytogenes_ was reached after the treatment at 600 MPa for 3 min for smoked RTE fish products. However, [PERSON] et al. [14], indicated in their study that high pressure combined with short holding time is the most effective way to inactivate _L. innocua_ in cold-smoked salmon, and the microbial safety of a fish product can be ensured using the treatment at 700-900 MPa for 10 s.  
The inactivation of _L. monocytogenes_ was observed to increase with higher pressure and longer holding time [13]. In our study, we did not test the influence of different holding times, but higher-pressure treatment increased inactivation of _L. monocytogenes_ in both cold-smoked and warm-smoked rainbow trout samples. Based on the results by [PERSON] et al. [17], HHP treatment at the low-pressure level, together with a low amount of liquid smoke, could be a promising method for decreasing _Listeria_. For example, the amount of nonresistant _Listeria_ strains can be reduced more than four logarithmic times of cfu/mL with the combined effect of the HHP treatment and liquid smoke [17]. Salmon fish species have good nutritional fat compositions, including a high amount of unsaturated fatty acids and especially n-3 fatty acids. [PERSON] et al. [24] observed that HPP is a mild treatment for fatty acids in salmon samples, as it does not change the n-3 and n-6 polyunsaturated fatty acid profile. Furthermore, the treatment at 300 MPa for 15 min decreased susceptibility to oxidation of the samples compared to control samples, thus improving quality and extending shelf-life during the storage time [24]. However, the structure of proteins can change during the pressure treatment, for example causing denaturation, aggregation, or gelation of proteins [25]. Thus, HPP treatment may also lead to undesirable changes in protein structure while changing the sensorial and texture properties of the product. [PERSON] et al. [26] observed that significant protein denaturation in rainbow trout samples occurred at pressures of 400-600 MPa at holding times of 3 or 6 min, negatively affecting protein structure. However, appropriate specific parameters (pressure, time, temperature) of the treatment could extend the shelf life of the product without changing the sensorial properties, but the parameters should be optimised on a case-by-case basis.