Chilled meat has always been the main consumption form of fresh meat in developed countries because of its tenderness, juiciness, good flavor and tenderness during low-temperature ripening. With the continuous improvement of living standards in world countries, cold meat will be the inevitable trend of fresh meat consumption development in the future.

meat packaging solution
At present, the main packaging forms of fresh meat on the market are vacuum packaging, pallet oxygen permeable packaging and high oxygen modified atmosphere packaging. Pallet oxygen permeable packaging and hyperoxia modified atmosphere packaging are conducive to the formation of meat color, but these two kinds of packaging are prone to protein oxidation due to contact with oxygen. Protein oxidation is one of the hotspots in food research in recent years. Protein oxidation leads to changes in protein structure, thereby affecting the function of protein. It mainly includes changes in hydrophobicity, water holding capacity, solubility and hydrolysis characteristics of protein, which causes changes in meat flavor, color, elasticity, tenderness and gel quality. From the nutritional point of view, the digestibility and nutritional value of meat products will be affected by the decomposition or aggregation of oxidized proteins. Therefore, protein oxidation may be an important factor in regulating the quality of fresh meat during the transformation from muscle to fresh meat after slaughter.

The maturation of after-slaughter meat plays an important role in the formation of fresh meat quality, and the maturation process includes a series of complex physiological and biochemical changes. Among them, the degradation of myofibrillar protein and skeleton protein induced by calpain system is the key to the formation of fresh meat quality. Protein degradation can destroy the integrity of myofibril structure, change the original fixed structure of muscle cells, affect the conduction process of muscle cells, thus affecting the distribution of water content in muscle cells and tenderness of fresh meat [4]. However, the active sites of calpain include cysteine residues containing thiol groups. The ability of fresh meat to maintain its reductive state after slaughter is greatly reduced, resulting in a large number of reactive oxygen species gathering in the muscle, which can attack the active sites of calpain and make them inactive, thus affecting the degradation of key structural proteins [5]. Under different packaging methods, protein oxidation may regulate the biochemical and metabolic reactions in muscle, thus affecting the formation of meat quality. This paper reviews the common packaging methods of fresh meat, and elaborates the definition and mechanism of protein oxidation, revealing the mechanism of different packaging methods affecting the quality of fresh meat.

Fresh meat packaging

Food packaging technology is widely used in fresh meat preservation and preservation. These packaging technologies can inhibit the growth and reproduction of microorganisms, prevent secondary pollution, improve the tenderness, water holding capacity and color of fresh meat, inhibit protein oxidation and fat oxidation of fresh meat, reduce storage loss and drip loss, thereby improving market competitiveness. At present, the most common retail packaging methods for cold meat in domestic market are pallet oxygen permeable packaging, vacuum packaging, vacuum skin packaging and high oxygen modified atmosphere packaging.

1.1 Pallet Oxygen Permeation Packaging

Pallet oxygen permeable packaging is one of the most common packaging methods in meat market. Fresh meat is sliced after ripening. The following is packaged with a white polystyrene tray. The upper part is packaged with polyethylene film and attached to the tray through a packaging machine. The advantages of pallet oxygen permeable packaging are low cost, simple packaging, and the formation of consumer favorite bright red in a relatively short period of time. The disadvantage of pallet oxygen permeable packaging is that fresh meat has short shelf life and is prone to protein oxidation and fat oxidation. Fresh meat will fade gradually after 4-7 days.

1.2 Vacuum Packaging

Vacuum packaging is a common packaging method in the transportation of fresh meat. The fresh meat is packed in a polyethylene packaging bag, and the air in the packaging bag is taken out to isolate the fresh meat from the air, so as to keep the purple-red color of the fresh meat myoglobin for a long time. When fresh meat is removed from the packaging bag, the purple-red color of myoglobin immediately changes to the bright red color of myoglobin oxide. The advantages of vacuum packaging are that it can inhibit the growth and reproduction of aerobic microorganisms, control the oxidation of protein and fat, prolong the shelf life of fresh meat and maintain the stability of purple red meat. The disadvantage of vacuum packaging is that the product is easy to deform, juice is easy to exude, and the water holding capacity of fresh meat is reduced.

1.3 VSP Vacuum Skin Packaging

Vacuum skin packaging is a popular packaging method for high-grade meat and seafood in recent years. Vacuum skin packaging is to place fresh meat on the tray. High barrier packaging film is used to make the packaging film soft and close to the surface of fresh meat by heating and vacuuming. At the same time, the packaging film is tightly sealed with the bottom of the tray. The advantages of vacuum skin packaging are that it can effectively block oxygen, inhibit the growth and reproduction of aerobic microorganisms, prolong the shelf life of fresh meat; maximize water retention, prevent water loss; prevent protein oxidation and fat oxidation; improve the tenderness of fresh meat, make fresh meat tender and juicy; prevent wrinkles, eliminate the gap between packaging and fresh meat. In addition, the packaging product has compact volume, can effectively utilize space, convenient transportation and cost saving.

Definition and mechanism of protein oxidation

Protein oxidation is a by-product of reactive oxygen species (ROS) or oxidative stress, which acts on proteins and causes covalent modification of proteins, thus further altering the structure and function of proteins. Reactive oxygen species can react directly with proteins or with sugars and lipids first, and the oxidized products can modify proteins. In living animals, oxidative stress and antioxidant defense ability are in a dynamic equilibrium under normal conditions. However, during postmortem ripening, ROS accumulated continuously in vivo due to its ability to maintain its own antioxidant defense system, resulting in an increase in protein oxidation level. These ROS include hydroxyl radical (OH), superoxide anion radical (O 2-), alkyl peroxide radical (ROO -), singlet oxygen (1O 2), nitric oxide radical (NO -), hydrogen peroxide molecule (H 2) and hydroperoxide (ROOH), as well as active aldehydes and ketones, which are potential initiators of protein oxidation [17]. ROS is produced in vivo mainly through four ways: normal metabolism, catalysis of various endogenous enzymes, various exogenous substances and pro-oxidant proteins. The mechanism of protein oxidation is similar to that of fat oxidation. The oxidation process includes three stages: initiation, transfer and termination. ROS initiates protein dehydrogenation to produce protein free radicals (P -), which are further converted into peroxide free radicals (POO -) under aerobic conditions. Then one hydrogen atom is captured from another protein molecule to form alkyl peroxides, which are further reacted with peroxide free radicals (HO2 -) to form alkoxy radicals (PO -) and its hydroxyl derivatives (POH). Free radicals react with side chain amino acids or with residual side chains, peptide chain skeletons and some active functional groups of oxidized amino acids, resulting in intramolecular and intermolecular crosslinks of proteins, breakage of peptide chain skeletons or oxidative modification of amino acid side chains, eventually leading to loss of protein function and inhibition of enzyme activity. Among all side chain amino acids, cysteine and methionine are most easily oxidized under mild conditions, because they contain sulfur atoms which are the strongest nucleophilic reagents, and sulfur elements are easy to lose rich electrons leading to oxidation]. Tryptophan is also easily oxidized in the presence of metal ions, which is considered as a marker of early protein oxidation. Oxidation indirectly attacked the peptide chain skeleton of the side chain of proteins, resulting in the breakage of the skeleton or the change of the conformation of the secondary or tertiary structure of proteins. Cystine (disulfide bond) formed by oxidation of 2 cysteine and 2 tyrosine, and the aggregation or crosslinking of proteins between the side chains of 2 tyrosine and other amino acids caused by oxidation, thus changing the hydrolytic properties of some proteins. In addition, carbonyl compounds formed by oxidation react with lysine to form cross-linking or aggregation, which also changes the degradation function of protein [23].

Effect of Protein Oxidation on Fresh Meat Quality under Different Packaging Methods

3.1 Effect of Protein Oxidation on Meat Color of Fresh Meat under Different Packaging Methods

Color is one of the most important quality indicators to evaluate fresh meat. The quality of fresh meat color not only affects consumers’desire to buy, but also plays a very important role in regulating market prices. The color of fresh meat mainly depends on the proportion and content of deoxymyoglobin, oxymyoglobin and ferrimyoglobin in different chemical states. During the storage of fresh meat, the redox reaction of myoglobin is reversible. Both deoxymyoglobin and oxymyoglobin are easily oxidized and converted to feromyoglobin, which can be reduced to deoxymyoglobin under the action of reductase system. Therefore, the color change of fresh meat after slaughter depends on the auto-oxidation rate of deoxymyoglobin and the relative reduction rate of ferrimyoglobin. During postmortem ripening, the tray packaging system can cause myoglobin oxidation due to the high concentration of oxygen, thus affecting the color stability of fresh meat. The high oxygen modified atmosphere packaging also contains high concentration of oxygen, but the oxidized myoglobin of fresh meat forms oxymyoglobin on the surface of fresh meat, covering high iron myoglobin, thus prolonging the color of oxymyoglobin in a certain period of time, which is conducive to maintaining color stability.

3.2 Effect of Protein Oxidation on Water Holding Capacity of Fresh Meat under Different Packaging Methods

Water holding capacity is the ability of fresh meat to maintain its original moisture and add moisture when it is subjected to external forces. It not only affects the tenderness and color of fresh meat, but also affects the juiciness and flavor of fresh meat, and directly affects the economic benefits of meat processing enterprises. Water holding capacity is usually evaluated by drip loss, storage loss, centrifugal loss and cooking loss. It is generally believed that myofibrils mainly endow the muscle with water holding capacity and hydration capacity. Water in muscle fibers mainly exists between myofibrils and myofibrils and is maintained by capillary force. Protein oxidation can inhibit the activity of calpain, which leads to the decrease of calpain activity and autolysis rate, which directly affects the degradation of intermuscular linear protein and the decrease of water holding capacity of fresh meat.

Traore et al. found that there was a significant correlation between protein oxidation and drip loss in fresh pork, which indicated that the cross-linking of protein oxidation could cause the decline of water holding capacity of fresh pork. Huff-Lonergan et al. [4] found that protein oxidation affected the degradation of intermuscular linear protein in fresh meat, which increased the storage loss and drip loss of fresh meat, resulting in the decline of water holding capacity. Wang Juan et al. confirmed that lower calcium activator activity could reduce its own degradation rate, leading to the degradation of intermuscular linear proteins and water holding capacity. Lindahl et al.  showed that the oxidation of beef protein inhibited protein degradation, but also destroyed the orderliness and structural integrity of skeletal muscle cells, weakened the binding ability of myofibrils to water, resulting in a decline in water holding capacity. Lund et al. studied the changes of water holding capacity of pork after 14 days of refrigeration at 4 C using high oxygen modified atmosphere packaging and vacuum packaging. The results showed that high oxygen modified atmosphere packaging intensified protein oxidation, led to myosin cross-linking and aggregation, and ultimately led to the decline of water holding capacity of pork. Lagerstedt et al. studied the changes of water holding capacity of beef after 15 days’refrigeration at 4 C using high oxygen modified atmosphere packaging and vacuum packaging. The results showed that compared with vacuum packaging, the water holding capacity of beef packaged in high oxygen modified atmosphere decreased significantly due to the high degree of protein oxidation during refrigeration. Thus, the effect of protein oxidation on the water holding capacity of fresh meat under different packaging methods also showed significant differences.

3.3 Effect of Protein Oxidation on Fresh Meat Tenderness under Different Packaging Methods

Tenderness is the most important quality index to evaluate the quality of fresh meat. It directly affects consumers’satisfaction with fresh meat and purchase behavior. In recent years, different packaging methods have been used to treat fresh meat, and the mechanism of protein oxidation on the quality of fresh meat has become a research hotspot. Protein oxidation inhibits the activity of calcium-activated enzymes during postmortem beef cold storage and maturation, which is not conducive to the degradation of cytoskeletal proteins such as myofibrillar protein and myonectin, intermyosin and troponin-T, and plays an important role in regulating beef tenderness. Myosin is the most abundant protein in myofibril and plays an important role in the structure of meat. Myosin is highly sensitive to metal-catalyzed oxidation, especially the light enzymatic myosin in myosin is most sensitive to oxidation, which easily forms disulfide bond crosslinking. Myonectin is the largest skeleton protein found in mammalian tissues, which can maintain the integrity of myofibril structure. During the ripening process of fresh meat, the degradation of myofibrillar proteins weakens the integrity of the longitudinal structure and muscle structure of myofibrillar proteins, thus promoting the increase of the small fragmentation index of myofibrillar fibers and improving the tenderness of fresh meat. Intermyosin is a very important cytoskeleton protein, which plays an important role in improving tenderness of fresh meat. Intermuscular linear proteins are easily degraded during postmortem ripening. In beef, the main degradation product is about 38 kDa polypeptide, which weakens the structure of muscle fibers and is conducive to the improvement of tenderness of fresh meat . Troponin is a globular protein. The degradation of troponin-T after slaughter plays a very important role in improving the tenderness of fresh meat. The 28-32 kDa polypeptide products produced during the ripening process after slaughter have a significant correlation with beef tenderness.

Scientists used to store the longissimus dorsi muscle in high oxygen packaging and vacuum packaging, and stored at 14 d at 4 C for studying the effect of protein on oxidation and meat texture. The results showed that when compared with vacuum packaging, the protein oxidation degree increased significantly, the myosin heavy chain formed cross-linked and the free thiol content decreased significantly, resulting in the tenderness of meat. The juiciness was significantly reduced. Scientists studied the changes of shear stress of beef after 15 days’refrigeration at 4 C by means of high oxygen modified atmosphere packaging and vacuum packaging. The results showed that compared with vacuum packaging, the tenderness and juiciness of beef steak packed with high oxygen modified atmosphere decreased significantly due to the high degree of protein oxidation during refrigeration. Scientists studied the tenderness change of beef after 2 days storage at 4 C using high oxygen modified atmosphere packaging and vacuum packaging. The results showed that compared with vacuum packaging, protein oxidation resulted in intramolecular cross-linking in the tail region of myosin heavy chain and cross-linking between myosin heavy chain and myosin protein, which resulted in a significant decrease in beef tenderness. Scientists studied the tenderness of steak after high oxygen modified atmosphere packaging and vacuum packaging for 4, 8 and 14 days. The results showed that compared with vacuum packaging, the carbonyl content of protein increased significantly, the free thiol content decreased significantly, and the oxidation of protein eventually led to the tenderness reduction of steak. Scientists Studied the effects of hyperoxia modified atmosphere packaging, tray packaging and vacuum packaging on beef tenderness during refrigeration. Compared with vacuum packaging, protein oxidation under hyperoxia modified atmosphere packaging and tray oxygen permeable packaging severely inhibited the activity of u-calpain, thus delaying the degradation of skeletal proteins such as troponin-T and intermuscular line protein, which eventually led to hyperoxia modified packaging. The tenderness of beef declined. Chen Lin and other stored the longissimus dorsi muscle in vacuum packaging and high oxygen modified packaging and stored at 6 d at 4 degree temperature. The results showed that protein oxidation inhibited the activity of pork calcium and activated calcines in pork after high oxygen packaging, thus hindering the degradation of pork intermuscular protein, muscle calcium and white protein and the degradation of annexin. Scientists showed that after 9 days of refrigeration at 1 C, the protein oxidation of modified atmosphere packaged steak was aggravated, myosin and myonectin formed cross-linking, which directly affected the activity of calpain and the degradation of myonectin, resulting in a significant decrease in tenderness. Scientists studied the tenderness changes of beef after 9 days of cold storage at 1 C using high oxygen modified atmosphere packaging and vacuum packaging. It was found that protein carbonyl content caused by protein oxidation of high oxygen modified atmosphere packaging beef increased significantly compared with vacuum packaging, which resulted in the decrease of tenderness and juiciness of beef. This indicated that there was also a certain relationship between tenderness, juiciness, protein oxidation and degradation of beef. Relevance. Therefore, compared with vacuum packaging, fresh meat packaged in tray oxygen permeable packaging and hyperoxia modified atmosphere packaging has a high degree of protein oxidation during cold storage and maturation, which inhibits the activity of calcium activator, resulting in a decrease in the degradation of structural proteins, and ultimately leads to a decrease in tenderness.

Conclusion

In the process of fresh meat refrigeration storage, high oxygen modified atmosphere packaging is beneficial to improve the color stability of fresh meat. However, due to the high degree of protein oxidation, it can inhibit the activity of calcium activator and further inhibit the degradation of skeleton protein, which is not conducive to centrifugal loss and tenderness improvement. Vacuum packaging is conducive to the improvement of tenderness of fresh meat due to the low degree of protein oxidation, but its water holding capacity. The tray oxygen permeable packaging can improve the color stability of fresh meat in a short time, but because of the high degree of protein oxidation, it also inhibits the activity of calcium activator, thus further inhibiting the degradation of skeleton protein, which is not conducive to the improvement of water holding capacity and tenderness. Vacuum skin packaging can not only improve the tenderness of fresh meat, but also effectively prolong the shelf life and tenderness of fresh meat. Improve color and inhibit protein oxidation and fat oxidation. Protein oxidation may be an important factor in regulating the quality of fresh meat during the transformation from muscle to meat after slaughter. However, there are few studies on the identification of protein oxidation sites under different packaging conditions and the nutritional value of protein oxidation to fresh meat protein. Further studies are needed to enrich the theory of quality formation of fresh meat during cold storage and ripening, and provide an important theoretical basis for the production of high-quality fresh meat.