## 4 Discussion
[10] who showed that HPP at 450 or 600 MPa significantly increased hardness in rainbow trout in comparison to samples treated at 150 or 300 MPa. [PERSON] et al. [30] studied salmon, cod and mackerel for the sensory quality parameters including texture and colour, with an HPP treatment of 200 and 500 MPa for 2 min at 8-9 \({}^{\circ}\)C. The main findings were that hardness and lightness increased at 500 MPa and redness decreased. Same was true for CSRT-400 and CSRT-600 samples in our study. On the other hand, [PERSON] et al. [31] used a trained ten-person panel to evaluate mild smoked rainbow trout and fresh European catfish fillets treated with HPP at 200, 400 or 600 MPa for 1 or 5 min at room temperature and found only minor effects on the overall appearance, colour and texture of rainbow trout fillets. Instead, catfish tended to become paler and thus, a treatment with 600 MPa for 5 min for rainbow trout fillets was recommended. The challenge in comparing different studies investigating the effects of HPP on sensory properties of fish products is the variability in treatment parameters and fish raw materials. Interestingly, CSRT-200 was rated with the highest overall liking score among all CSRT samples in our study. The same sample was also rated as most juicy and freshest. Although overall liking was more strongly loaded together with appeal and pleasantness in the PCA, some positive correlation was also seen between overall liking and the PC with positive loadings of juiciness and freshness. Meanwhile, a small negative correlation was seen between overall liking and the PC with chewiness, hardness, dryness, and firmness. In the present study, WHC and SP were positively associated with intense red colour and negatively associated with light colour of CSRT samples. This was highlighted in samples treated at 600 MPa. Protein denaturation is related with colour changes of fish muscle, and it has been shown to contribute to fish surface lightness [32; 33]. In HPP-treated cod and salmon samples, HPP was found to reduce redness and to increase lightness, and especially denaturation of myosin was suggested to be a major factor [9]. Pressure of 300 MPa was suggested as a threshold for both protein denaturation and colour changes, as the treatment at 150 MPa did not cause significant changes in these variables in comparison to untreated control samples. In our analyses as well, samples treated with 0 or 200 MPa showed some positive correlation with intense red colour, WHC, and SP while being negatively associated with light colour. Against expectations, WHC did not correlate with juiciness, dryness, or moistness in our study. However, although WHC in HSRT samples did not vary at a statistically significant level, the trends in WHC of HSRT were in parallel with juiciness and moistness while showing an opposite trend with dryness. PCA on CSRT samples potentially indicated the drop in WHC when increasing the pressure from 0 to 200 MPa and the small recovery of WHC when increasing the pressure from 400 to 600 MPa. HPP may also cause shifts in fish muscle pH [34], and consequently, changes in taste and consumer liking [35]. In our study, the pH levels varied between 6.21 and6.27 in CSRT and between 6.36 and6.46 in HSRT, without any significant differences according to treatment pressure levels (Appendix B). This finding is in accordance with a previous study by [PERSON] and [PERSON] [36].