## 3 Results and Discussion
### Effect of HP on the Physicochemical Characteristics of Sea Bass Fillets  
The pH value of untreated and HP-treated sea bass fillet did not significantly differ among aerobic and anaerobic storage conditions or various processing temperatures (Table 3). However, it did exhibit significant differences (\(p<0.05\)) with respect to different applied pressures (statistical mean pH value: 6.58 and 6.73 for HP-treated samples at 100-300 and 400 MPa, respectively) and processing times (statistical mean value: 6.65 and 6.59 for HP-treated samples for 3 and 10 min, respectively). However, during storage, none of the untreated or HP-treated samples showed significant changes in pH value [40,51] as this parameter is directly dependent to the type of demonstrating dominant growth at each sampling interval, leading to either a slight increase when _Pseudomonas_ spp. are the dominant microorganisms or a slight decrease when species such as _Enterobacteriaceae_ spp., LAB, or _Shewanella_ spp. dominate [52]. Among the calculated textural attributes of sea bass fillets, those of hardness and adhesiveness showed the highest variations between tested samples (Table 3). Cohesiveness and the index of elasticity (springiness) showed no differences between samples. Regarding the parameters of gumminess and chewiness, as their values are numerical products of the hardness, they followed the same trend as the hardness. Thus, the textural parameters of both hardness and adhesiveness, which are also strongly related to the sensorial acceptance of the fillets, discussed later, were found to be strongly affected by all the designing parameters, i.e., pressure, temperature, and time. In general, untreated fillets and those treated at low pressures (100 MPa) exhibited the lowest hardness and highest adhesiveness in comparison to all other fillets, indicating that HP processing fish fillets at pressures below 200 MPa results in fish fillets with very similar characteristics to raw ones (similar results have been obtained for HP processing at 150-180 MPa within the context of this study), which is in accordance with previous studies [12]. The HP treatment of fish fillets at pressures above 200 MPa showed increasing values of hardness as the applied pressure increased up to 400 MPa, which almost doubled in the case of the fillets pressurized at 400 MPa, when compared to untreated ones or those treated at 100 MPa (statistical mean values of hardness: 182, 231, 272, and 317 g for sea bass fillets treated at 100, 200, 300, and 400 MPa, respectively). It has been reported that the increase in hardness is related to the inactivation of the proteolytic enzymes in HP-treated fish samples, inducing denser structures, especially when pressures above 200 MPa are applied [43]. Regarding the effect of the processing temperature, it was observed that the fillets which were HP treated in combination with low temperatures (3 or 7 \({}^{\circ}\)C) showed a significantly (\(p<0.001\)) higher hardness than those treated in an ambient temperature (statistical mean value of hardness: 278 and 240 g for sea bass fillets treated at 3 and 25 \({}^{\circ}\)C, respectively). Additionally, it was also observed that the increase in processing time led to higher values of hardness (statistical mean value of hardness: 198 and 278 g for sea bass fillets treated for 5 and 10 min, respectively). The effect of HP treatment on the increase in hardness values of the fish were also reported for a variety of fish products, e.g., salmon, cod, hilsa, mackerel, sea bream, rainbow trout, and maih maih [12; 27; 52; 53; 54; 55; 56], particularly at pressures above 200 MPa. At these conditions, the denaturation of fish proteins is more likely to occur and is in direct relation to the food matrix since it has been reported that, e.g., the hardness of salmon is less susceptible to pressure changes compared to cod, especially when low pressures are applied [12].