## 4 Discussion  
For HSRT samples, LP was found to be highest in HSRT-200, showing LP of 75%, which was significantly higher than LP of 68% in HSRT-600. Previous studies have reported similar levels of in vitro proteolysis in fish muscle: depending on the pre-treatment and/or cooking methods, pancreatic digestibility of fish proteins have been reported to vary between ca. 56% and 80% [8,24]. HPP treatment of 200 MPa at 25 \({}^{\circ}\)C for 20 min has been found to increase pancreatic digestibility of cod proteins in comparison to fresh samples [8]. Although there are considerable differences in the study designs and raw materials, it could be speculated that 200 MPa might be a safe and even beneficial pressure level regarding fish protein digestibility. Based on the present results, it can also be suggested that physical protein-related variables did not affect LP. While CSRT samples showed significant differences in SP and WHC, but no significant differences in LP, HSRT samples did not vary in SP or WHC, but demonstrated small, yet significant changes in LP according to applied HPP treatment. Indeed, WHC in CSRT samples was significantly reduced when 400 and 600 MPa pressure levels were used. [PERSON] et al. [17] reported reduced WHC of sea bass fillets when the samples were treated at 250 or 400 MPa for 0-30 min instead of 100 MPa (6 \({}^{\circ}\)C). In comparison to control samples, 100 MPa did not have a significant effect on WHC. Reduced WHC in HPP-treated muscle samples was probably due to protein denaturation and degradation [25]. This is supported by the fact that protein solubility was reduced in CSRT samples, but not in HSRT samples, suggesting protein denaturation in CSRT as a result of HPP. Pressures higher than 200 MPa have been observed to reduce solubility of sarcoplasmic proteins in beef longissimus muscle [26]. Furthermore, actin, a myofibrillar protein, has been suggested to be susceptible to denaturation by HPP in high-fat fish [9], such as rainbow trout. Protein bands in SDS-PAGE indicate that sarcoplasmic proteins [27] in HSRT were already immobilized at the smoking phase. Instead, in CSRT-0, many sarcoplasmic and myofibrillar proteins [28] are still soluble, while especially sarcoplasmic proteins, but also some myofibrillar proteins are insolubilized by HPP at 400 and 600 MPa. Furthermore, high pressures may potentially break down muscle proteins [26]. For CSRT, this could explain the increase in band intensity in SDS-PAGE at the lower end of the molecular weight spectrum with higher HPP treatment intensity. [PERSON] et al. [29] studied rainbow trout samples with variable pressures (100, 200, 400 or 600 MPa) either for 3 or 6 min and found that the use of lower pressure levels 100 or 200 MPa did not have a major influence on myosin or actin, but instead, higher pressures 400 or 600 MPa did. These results are well in line with ours, although in their study, the ambient temperature during HPP treatment was approximately 35-47 \({}^{\circ}\)C, which remarkably differs from our study, where the processing temperature was 4 \({}^{\circ}\)C.  
CSRT underwent more significant changes in texture and appearance according to HPP treatments in comparison to HSRT. For both samples, juiciness, dryness, and moistness were significantly affected by HPP. Appearance was significantly altered only in CSRT. Hardness was significantly increased in CSRT-600 in comparison to CSRT-0 and CSRT-200 \begin{table}
\begin{tabular}{c c c c c c c} \hline  & \multicolumn{6}{c}{**Component**} \\  & **1** & **2** & **3** & **4** & **5** & **6** & **7** \\ \hline Appeal & & & & 0.829 & & & \\ Overall liking & & 0.356 & \(-\)0.306 & 0.727 & & & \\ Pleasant & & 0.529 & & 0.705 & & & \\ _400 MPa_ & & & & & \(-\)0.947 & & \\ Matt surface & & & & & & \(-\)0.849 & \\ Shiny surface & & 0.439 & & & & 0.757 & \\ _0 MPa_ & 0.497 & & & & & 0.808 \\ _200 MPa_ & 0.459 & & & & & \(-\)0.789 \\ \hline \end{tabular}
\end{table} 
Table 4: _Cont._samples, which is in line with the study of [PERSON] et al.