When purchasing fresh meat, consumers judge the acceptability of the product largely on the appearance, mostly color, of the exposed muscle tissue. Dull and/or discolored fat and darkened cut bone surfaces can detract from meat cut appearance when the muscle tissue appears attractive, but good appearances of fat and bone cannot compensate for a degraded appearance of the muscle tissue (22).
Muscle tissue appearance is determined by the chemical state of the muscle pigment, myoglobin, as shown in Figure 1. In the absence of oxygen, the pigment is in the deoxymyoglobin state, which has a dark, purplish-red color (23). On exposure to air, the pigment is rapidly oxygenated to form oxymyoglobin, the bright red that consumers have been taught to expect and find attractive. Deoxymyoglobin and oxymyoglobin, which are both in the reduced state, can oxidize to metmyoglobin, which has a dull brown color associated with deterioration of quality. Oxidation of deoxymyoglobin can be more rapid than oxidation of oxymyoglobin. That results in the faster oxidation of myoglobin at low (highest at 4 mm partial oxygen pressure) than at higher concentrations of oxygen (24).
Myoglobin oxygenation is rapid, and the fraction of the pigment in the oxygenated form increases with increasing oxygen concentration. Metmyoglobin is more stable and is
Reduced Fe Oxidized Fe desirable undesirable
Figure 1. Fresh meat color triangle.
Reduced Fe Oxidized Fe desirable undesirable
slowly converted to deoxymyoglobin by enzyme-mediated reactions termed metmyoglobin-reduction activity (25). Muscle tissue that is deficient in the enzymes that mediate metmyoglobin reduction or in the reduced cofactors necessary for the reduction reaction will be unable to reconvert metmyoglobin, which will persist once it is formed. Muscles vary widely in metmyoglobin-reduction activity. Those that tend to have a high activity, such as the longissimus dorsi, are more color-stable in air, their red color persisting for 3 or 4 times as long as color of unstable muscles of low metmyoglobin-reduction activity, such as the psoas. Metmyoglobin-reduction activity dissipates during storage of muscle. After lengthy storage, the color stability of initially color-stable muscles is similar to that of those muscles that were initially of relatively poor color stability (26).
Maintaining meat at as low a temperature as possible without freezing will slow pigment oxidation and increase the depth of the oxygenated surface layer. Two obvious means of preserving muscle color are to increase the proportion of oxidation-resistant oxymyoglobin by high-oxygen MAP or to largely exclude oxygen using ultra-low-oxygen MAP.
High-oxygen MAP is used mainly for retail product. If used for poultry, high-oxygen atmosphere may be inappropriate because of the meats' limited myoglobin level. The product may be packaged in gas barrier trays that contain the preservative atmosphere and are sealed with a lidding film of low gas permeability. Alternatively, several trays of product overwrapped with a film of high gas permeability may be master packaged in a bag of low gas permeability. The gas with which packs are filled may have a composition of about 65% 02, 25% C02, and 10% N2. The C02 is required to retard the growth of aerobic spoilage bacteria. Nitrogen has no preservative function for the meat and may be omitted, but it is often included to guard against the collapse of sealed trays that may result from the dissolution of C02 into the muscle.
The deoxymyoglobin chemical state predominates in normal muscle before cutting. Upon exposure to oxygen, usually in air, the oxygen diffuses further into the muscle with time, creating an oxymyoglobin layer on the meat surface. Diffusion into the muscle is more rapid in lower-pH muscle due to a more open muscle structure (27) and less scavenging of oxygen by muscle enzyme systems. Higher-oxygen tension at the meat surface, such as in high-oxygen MAP or by hyperbaric oxygen pressures, will drive the oxymyoglobin layer more deeply into the muscle, with the remaining muscle in the deoxymyoglobin state because of very low partial oxygen pressures in the meat-cut interior. At some variable time in display, metmyoglobin-reduction activity at the interface of the oxydeoxymyoglobin interface will be exhausted, so a layer of brown metmyoglobin begins to form, then becomes broader and begins to move toward the meat surface, diminishing the depth of the oxymyoglobin layer. At some point the brown metmyoglobin is close enough to the meat surface at some locations to contribute brown to the visual color as viewed on the meat surface, or as detected by a reflectance spectrophotometer. Because both the oxidative-reductive state and partial oxygen pressure may vary at different meat locations, the brownness is frequently spotty and not uniform, especially in ground meat where variable amounts of oxygen have been incorporated into the meat.
High-oxygen MAP systems often have atmospheres of 20% carbon dioxide and up to 80% oxygen and can produce and maintain a desirable red color in beef for up to 9 days and depress the formation of metmyoglobin by driving oxygen deep within the surface of the meat. Beef loin eye steaks in high-oxygen (70%) packages with 30% carbon dioxide had brighter, more desirable visual color scores and higher CIE a* (redness) values than vacuum controls (28) but with undesirable color beginning at 10 days of storage. Another study found that beef loin eye steaks were stable in high oxygen (70%) up to 14 days (29). Ground beef in a similar treatment was stable for only 6 days (30). Color life varies between species as well as muscle type. Pork loin chops in high oxygen had acceptable saturation indices and display color for 8 to 12 days with off-odor, not color, in beef steaks and pork the limiting factor to shelf life (31). High-oxygen-packaged beef loin eye steaks from loins in vacuum less than 7 days postmortem had much better display color stability than steaks packaged from loins vacuum aged for 14 or 28 days.
Carbon monoxide has been used in beef MAP systems to maintain a bright cherry-red color. Carbon monoxide binds strongly to myoglobin and hemoglobin, forming car-boxymyoglobin and carboxyhemoglobin, respectively, pigments that are stable bright-red compounds (1). Round steak (pieces) stored at 0, 5, and 10°C in carbon monoxide from 0.5 to 1.0%, with the balance as nitrogen, had increased color and odor shelf life up to 20 days over that stored in air. Beef loin eye steaks at 4°C in a 0.4% carbon monoxide, 60% carbon dioxide, and 40% nitrogen system, compared with a high-oxygen (80%) system or vacuum packaging, were brighter red than the other two treatments and had better display color stability (28). Carbon monoxide at 0.4 to 0.5% is currently utilized safely in some domestic European fresh-meat markets. Low-oxygen packaging systems lack the benefit of high oxygen to cause oxymyoglobin to be formed more deeply into the meat. Low-oxygen systems often have too much oxygen for the meat to naturally scavenge and therefore may not discourage the formation of metmyoglobin.
In MAP systems with no oxygen in the gas mixture (ultra-low oxygen), a slight amount of residual oxygen frequently occurs due to small pockets of air not removed initially by the evacuation or flushing process. These small concentrations of oxygen frequently cause major discoloration, reduced color stability, or blooming ability problems. Initial oxygen concentrations >0.15% (1,500 ppm) seriously compromised the color stability of beef and lamb. Pork was affected by residual oxygen concentrations >1.0%. At low oxygen partial pressure, oxidation of oxymyoglobin to metmyoglobin is highly favored. Oxygen concentrations between 0.5% and 2% most rapidly revert myoglobin to metmyoglobin (24). Beef or lamb stored in such atmospheres may be initially discolored by the residual oxygen, but fresh red meat retains a limited reducing capacity by which it can recover from this initial discoloration. Compared to pork, beef and lamb were more sensitive to residual oxygen. Provided the amount of oxygen did not exceed 0.5%, fresh meat will naturally scavenge most residual oxygen, but more muscle scavenging will reduce later ability to bloom (oxygenate). Constant exposure to low concentrations of oxygen will have detrimental effects on reblooming ability and color stability. Therefore, if the residual oxygen could be removed earlier, the meat would have less discoloration, better rebloom, and improved color stability.
Rapid scavenging of oxygen in an ultra-low-oxygen package is important if the meat is expected to bloom with acceptable speed and completeness upon later exposure to oxygen. Muscle oxygen scavenging may not be fast enough, and added chemical scavengers may be necessary.
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