## 1 Introduction  
The global meat industry is based on red meat consumption, predominantly pork. According to OECD data, in highly developed countries, pork consumption in 2020-2023 was 22.8 kg/capita per year [1]. The production and distribution of pork meat is vitally important for the economy and agricultural sector of many countries, generating labour places and profits for producers and processors [2; 3]. Pork is a rich source of high-quality protein. It also delivers many nutrients, such as B group vitamins (i.e., thiamine, niacin, B\({}_{12}\) vitamin), iron, zinc, and selenium. Moreover, pork in used in many national and regional cuisines worldwide because of its taste and texture [4; 5]. Many researchers indicate that visual attributes of meat, including colour, are of key importance for the consumer's perception of the quality of meat and have a direct impact on the decision to purchase it, while sensory attributes (tenderness, juiciiness, and tastiness) determine the decision to purchase meat again [6; 7; 8]. The colour of raw meat and its changes occurring during storage are easily noticeable by consumers, which makes the colour a key indicator of the meat's quality and freshness [5; 9; 10]. Once the quality attributes of fresh meat do not meet consumers' expectations, the meat is withdrawn from retail, which leads to food waste through the loss of valuable and still safe sources of nutrients [5; 6; 7; 8]. Pork colour is predominantly influenced by muscle biology, metabolism, and myoglobin chemistry, while important roles are played by metabolic, contractile, and stress response pathways [10]. Meat quality is further affected by muscle fibre characteristics, notably fibre type and density. Specifically, meat lightness, a critical quality parameter, is associated with type I and IIB fibre proportion, which also inversely affects myoglobin content and perceived colour [11]. A significant role in shaping the colour of meat is played by myoglobin (Mb), which exists in three natural forms, depending on the degree of exposure to oxygen and the chemical state of iron. Under anaerobic conditions, myoglobin exists as deoxymyoglobin (DeoMb), giving the meat a characteristic purple-red colour. When meat is stored in the presence of oxygen, oxygen is bound to the myoglobin, forming oxymyoglobin (OxyMb), which makes the meat colour intense and bright red. During the storage process, myoglobin may be transformed into metmyoglobin (MetMb), which results in a change in the colour of the meat to an unattractive grey-brown or grey-beige. The proportion of different myoglobin forms determines the final meat colour, and it is a vital factor affecting the shelf life and aesthetics of meat products [3; 5; 12]. Modified atmosphere packaging (MAP) systems combined with a refrigeration process are used in the food industry to maintain the quality and extend the shelf life of meat and meat products. MAP uses combinations of various gasses, including oxygen, carbon dioxide, and nitrogen. The widespread use of MAP helps maintain product colour, especially through the use of high oxygen concentrations [13; 14]. Oxygen in MAP is used not only to improve the stability of the ideal red colour of meat but also to inhibit the growth of anaerobic microorganisms. Atmospheres containing the appropriate concentration of CO\({}_{2}\) effectively extend the shelf life of meat products by limiting the development of aerobic microorganisms. However, using high CO\({}_{2}\) concentrations (>50%) can cause problems, such as flavour degradation and packaging collapse. Therefore, nitrogen, which is an inert gas, is also used in MAP. It is used as a balancing gas, preventing excessive oxidation, deterioration of taste, and collapse of packaging [4; 13; 15; 16]. Packaging in a modified atmosphere with a high oxygen content (up to 80%) has become one of the most popular commercial methods of packaging chilled fresh meat to ensure a stable, attractive, \"bloomed\" colour of red meat.