## 4 Discussion
### pH and Water-Holding Capacity  
The pH values recorded at the beginning of the experiment (day 0) were typical for meat of normal quality, without PSE (pale, soft, exudative) and DFD (dark, firm, dry) defects [10]. The storage time, but not the gas concentration in the packaging, had a significant impact on the pH increase. During the first stage of cold storage, i.e., 0-5 days, pH values decreased as a result of glycolysis processes still taking place in the muscles and the accumulation of lactic acid and ATP decomposition products (mainly inorganic phosphate). With the extension of the refrigeration time, from the 5 th to the 15 th day, there was an increase in the pH value, which is related to the autolysis of proteins related to the action of microorganisms and the formation of trimethylamine and ammonia. This is a typical process for the pH during meat storage, which was reported in many studies [47; 48; 49; 50]. The present study showed that different oxygen concentrations do not have a significant effect on the pH of pork muscles during storage, which stays in agreement with the results of [PERSON] et al. [23] However, modified atmosphere composition might affect meat pH, as shown by [PERSON] et al. [51], who reported that the meat stored under a high-oxygen MAP (80% O\({}_{2}\)/20% CO\({}_{2}\)) was less acidified than that stored with the same CO\({}_{2}\) concentration but without oxygen. It was associated with the growth of _Leuconostoc_ sp., which produces acetoin and diacetyl as the end products and does not acidify the meat as much as, for example, homofermentative lactic acid bacteria (i.e., _Lactobacillus_ sp. ), which grow more intensively when the CO\({}_{2}\) concentration is over 80%. Therefore, the lack of differences between MAP used in the present study, containing 75% and 55% O\({}_{2}\), might be explained by a similar microbiota composition in HO and LO samples. The water-holding capacity of meat (WHC) is another key aspect affecting its quality. WHC is defined as the ability of meat and meat products to bind water [52] during cutting, grinding, and pressing, as well as during transport, storage, processing, and cooking [36] and it is directly related to the pH value of fresh meat [53]. In the present study, the WHC changed during storage, which may result from processes related to changes in the structure of proteins degraded during meat storage [54]. However, there was no effect of increasing the oxygen concentration by 20% in packaging on free water, purge loss, and cooking loss. In contrast, other reports showed the impact of increasing the oxygen content on the WHC of meat samples during storage [25; 46; 55; 56] as a result of increased dynamics of lipid and protein oxidation. However, the increase was noted when the differences in the oxygen concentrations were greater than those used in the present study, e.g., 20% vs. 50% vs. 80% O\({}_{2}\)[25] and 46% vs. 70% O\({}_{2}\)[55]. It might be assumed that the difference in the oxygen concentration used in the present study was too small to produce changes in WHC and pH caused by oxidation. The oxygen concentration reduction from 75% to 55% in MAP investigated in the present study failed to fight against the colour deterioration of pork loin. Therefore, to obtain successful results, this intervention should be combined with some other treatment, such as active packaging or natural colourants with antioxidant potential. [PERSON] et al. [57] developed PVA-active films containing NaNO\({}_{2}\) and used it for pork neck packaging, whereas [PERSON] and [PERSON] [58] extended the colour stability of fresh beef by using FreshCase(r) (pouches made of nitrite-embedded film, NEF, Curwood Inc., Division of Bemis Company Inc., Neenah, WI, USA). Therefore, active packaging might effectively improve the colour stability of meat stored under MAP. Another possibility might be using betanin as a potential natural pigment and antioxidant [59].