## 3 Results and Discussion  
### Gas Changes  
Gas (CO\({}_{2}\), O\({}_{2}\)) concentration (%) in MAP and ACT-MAP gutted sea bass package headspace during the isothermal storage (0, 5, 10 \({}^{\circ}\)C) are presented in Figures 1 and 2, respectively. The CO\({}_{2}\) concentration in the MAP gutted sea bass package headspace decreased (from the initial value of 50% to the value of 40% approximately) up to 15, 11, and 4 days of storage at temperatures of 0, 5, and 10 \({}^{\circ}\)C, respectively, and then increased (to the initial value of 50% approximately) due to the metabolic activity of the spoilage bacteria. The CO\({}_{2}\) concentration in the ACT-MAP gutted sea bass package headspace initially decreased (e.g., from the value of 50% to the value of 46% at a storage time of 8 days and storage temperature of 0 \({}^{\circ}\)C; Figure 1), which was attributed to the CO\({}_{2}\) dissolution in the fish flesh. Afterwards, the CO\({}_{2}\) concentration increased, reaching the maximum values, indicating that the CO\({}_{2}\) emitter started to produce CO\({}_{2}\) due to the contact of the emitter ingredients in the liquid absorber pad and the liquid loss. The highest CO\({}_{2}\) concentration values (60.5%-63.5%) were observed at the end of the storage period at all the tested temperatures. [PERSON] et al. (1995) observed that the CO\({}_{2}\) concentration in the MAP + CO\({}_{2}\) emitter packaged cod fillets decreased to 40% approximately within a few days, and then increased to a maximum stable value of 50%. [PERSON] et al. [20] reported similar CO\({}_{2}\) gas behavior for an MA + CO\({}_{2}\) emitter (60% CO\({}_{2}\)-40% N\({}_{2}\)) packaged salmon fillets. For the MAP (g/p: 3/1) samples, the CO\({}_{2}\) concentration decreased to 40% (Day 4) and then increased. For the MAP + CO\({}_{2}\) emitter (g/p: 1/1), the CO\({}_{2}\) concentration initially decreased to 45% (Day 1) and then increased and reached 65-70% during storage. They also observed that CO\({}_{2}\) concentrations above 60% in the package headspace caused inflation or bulging [19; 20; 21]. [PERSON] et al. [22] demonstrated that the MAP + CO\({}_{2}\) emitter packaged cod loins presented 35-37% CO\({}_{2}\) concentration in the package headspace (g/p: 1.6/1.0) when the absorption of CO\({}_{2}\) was stabilized (Day 1). For the MAP cod samples (g/p: 1.6/1.0), the CO\({}_{2}\) concentration decreased to the stable value of 26-27%.  
Figure 1: _Cont._  
Figure 1: CO\({}_{2}\) concentration in the sea bass package headspace during the isothermal storage at (**a**) 0 \({}^{\circ}\)C, (**b**) 5 \({}^{\circ}\)C, and (**c**) 10 \({}^{\circ}\)C (grey bars ACT-MAP, back bars MAP). MAP: modified atmosphere packaging, ACT-MAP: active modified atmosphere packaging with CO\({}_{2}\) emitters.  
Figure 2: _Cont._  
With regard to the O\({}_{2}\) concentration, it showed a descending trend (Figure 2), while at the end of the storage period, the O\({}_{2}\) concentration exhibited zero levels, which was related to the increased microbial population of the total viable counts, for both MAP and ACT-MAP gutted sea bass samples (\(p>0.05\); no statistically significant differences between ACT-MAP and MAP samples for most cases).