## 3 Results and Discussion  
### pH  
Post mortem pH is usually characterized by two measured values: 45 min as the initial and 24 h after slaughter as the ultimate pH [28]. Figure 1 shows the change in pH post mortem. The CO2 stunning resulted in a slightly lower initial pH compared to electrical stunning (\(p\) > 0.05). This might be caused by inhaled CO2 during stunning which led to slight acidification in the blood and muscle. Studies found that inhalation of 80% or 90% of CO2 for 15, 45, or 58 is increases the pressure of CO2 (mmHg) in the blood which results in pH decreasing [5; 6; 29]; furthermore, CO2 gas stunning accelerated early post mortem pH decline compared to electrical stunning [30]. The CO2 stunning resulted in muscle with significantly (\(p\) < 0.01) lower pH compared to electrical stunning post mortem. The pH decreased to pHCO2 5.46 \(\pm\) 0.07 and pHelectric 5.75 \(\pm\) 0.20 in the case of electrical stunning as the ultimate pH. These results are consistent with earlier data from the literature which found that CO2-stunned m. quadriceps femoris showed slightly lower ultimate pH than electrical stunning [31; 32]. The more rapid pH decrease in the case of CO2 stunning is attributed to increased anoxic convulsion which leads to an increased ATP depletion and more rapid lactic acid production [33]. Furthermore, inhaled CO2 causes acidification of the blood and reduces the muscle cells' capacity to maintain intracellular pH [13]. The suggestion in [34] of a quick or moderate pH decrease was not observed for PSE or DFD; the GM pH decreased normally over time.  
The pH 6.0 and temperature (TPH6.0) combination is important for the meat quality. Furthermore, certain regions in the pH/T combinations have to be avoided, which are called pH/T windows. One of them is TPH6.0 \(<\) 12 \({}^{\circ}\)C and the other one is when TPH6.0 \(>\) 35 \({}^{\circ}\)C. The first one leads to cold shortening and the latter to heat shortening of meat [35]. The post mortem pH values were plotted in a pH/Temperature diagram with non-linear estimation of pH as shown in Figure 2. The 'ideal pH/Temperature window' and 'cold shortening window' based on [35] are also shown as solid lines. The CO2 prediction model showed R2 = 0.7584-0.9547 fitting and the electrical R2 = 0.5221-0.9407. The pH decreasing indicates the difference between the CO2 and electrical stunning methods. The predicted pH data show the CO2-stunned meats reached the 'ideal window' at 35 \({}^{\circ}\)C \(<\) pH6.0 \(<\) 12 \({}^{\circ}\)C suggested by [36]. However, in eight cases of electrical stunning, the predicted pH crossed the boundary between 'ideal' and 'cold shortening' and three of them showed pH\({}_{\text{u}}\) > 6.0. In the latter cases (pH\({}_{6,0}\) < 12 \({}^{\circ}\)C), cold shortening may have occurred. Besides these, the predicted pH\({}_{\text{u}}\) values were in the range of 5.4-5.8 which is midway between the onset of rigor pH 6.0 and the isoelectric point (pl) which offers a good compromise between the water holding capacity and toughness of the meat. When pH reaches the pI of main proteins (e.g., myosin, pI = 5.4), the negative and positive charges are equal to each other and the protein has a minimal net charge. As a result, the protein's negative and positive groups attract one another which results in minimal swelling of myofibers and results in a poor water holding capacity of meat [37].  
Figure 1: Post mortem pH changes in CO\({}_{2}\)- and electrically stunned pork m. gluteus medius. Data points present the averages and error bars show the standard errors.  
Figure 2: The change in pH as a function of temperature and the spline model predictions for the pH of CO\({}_{2}\)- and electrically stunned pork m. gluteus medius. The solid lines show the ‘ideal’ and ‘cold shortening’ windows.