## 4 Discussion
The results could also indirectly indicate that that degree of muscle fibre shrinkage in M samples was lower than in SV. In the present study, significant differences in the colour between raw and cooked fillets were noted. The increase in lightness as a result of muscle tissue cooking results from protein precipitation and solubility reduction [18]. The colour of raw and cooked fish muscle tissue results from myoglobin content and its forms [19] as well. Thus, colour changes during cooking are also caused by myoglobin denaturation and oxidation [20; 21], which occurs more easily in samples cooked under atmospheric conditions [22]. In this study, a reduction in redness was noted in both M and SV treatments as compared with raw fillets, and there was a trend towards lower a* values in M samples (\(p>0.05\)) cooked under atmospheric conditions in a microwave oven. Changes in the redness between raw and cooked fillets as a result of cooking may also be partially explained by cooking loss (leakage), which carried away some of the myoglobin and haemoglobin from the muscle tissue [21]. The cooking method did not affect sensory quality of the wild pikeperch fillets, which might be explained by a relatively high temperature used in both treatments, which caused changes in muscle tissue proteins, including a shrinkage of protein structure. At temperatures over 60 \({}^{\circ}\)C, which were used in this study in both M and SV treatments, shrinkage occurs both in diameter and in the longitudinal axis of sarcomeres [17]. Although significant differences in cooking loss were noted, suggesting a higher shrinkage of muscle fibres in SV samples, no negative effect on the texture was noted and the M and SV samples were similarly scored. As in the present study, a good sensory quality of SV-cooked (65 \({}^{\circ}\)C for 46 min.) seabream was reported by [PERSON] et al. [23]. Results obtained thus suggest that both thermal treatments with the parameters proposed in this study could be used for preparing pikeperch without any adverse effect on the sensory quality. It was proved that microwave cooking was more effective than sous-vide cooking in reducing microbial load and decreased the TVC by 5 to 6 folds, while in the case of sous-vide, it was a 4-fold reduction. This resulted from the different temperature obtained in the centre of the products, which was much higher in M samples (83 \({}^{\circ}\)C vs. 65 \({}^{\circ}\)C in M and SV samples, respectively), although the time in which the process was conducted was much shorter (3 min. vs. 40 min., respectively). The increase in microbial reduction as a result of increased temperature and time of heat treatment was also noted by [PERSON] et al. [24] in sous-vide rainbow trout. Microwave cooking and sous-vide were effective in reducing _Staphylococcus_ spp. and total _Enterobacteriaceae_ below the detection level (100 CFU/g). The number of _Enterococcus_ spp. and _Listeria monocytogenes_ in the raw pikeperch muscle tissue was below the detection level, and the bacteria were detected in neither M nor SV samples. A similar result in terms of TVC reduction to that obtained in the present study for SV samples was reported by [PERSON] et al. [25] in Russian sturgeon sous-vide cooked at 60 \({}^{\circ}\)C for 10 min. In their study, as a result of heating, the number of bacteria decreased from 4 log CFU/g to 2 log CFU/g, and this treatment turned out to be the most effective compared to those carried out at lower temperatures (40 \({}^{\circ}\)C and 50 \({}^{\circ}\)C). The use of sous-vide at 60 \({}^{\circ}\)C for 10 min. enabled to preserve fish products--after 9 days of storage under refrigerated conditions (4 \({}^{\circ}\)C), the TVC were below a spoilage limit of 7 log CFU/g [25; 26]. [PERSON] et al. [27] used a higher temperature and a longer time (80 \({}^{\circ}\)C for 45 min) than was used in the present study and by [PERSON] et al.