Novel carcass suspension methods
The conventional way to suspend carcasses during chilling is by the hind legs using a hook passed behind the Achilles tendon. The weight of the carcass puts many muscles into tension so stretching them as they pass into rigor. This stretching may increase sarcomere lengths and produce more tender meat. While some muscles are in tension others are free to contract because of the antagonistic way in which many groups of muscles operate. If, instead of hanging the carcass from the Achilles tendon, it is hung from a hook placed into the obturator foramen, then the valuable m. longissimus dorsi,and the muscles on the outside of the hip, such as the m. semimembranosus and m.
semitendinosus, are stretched when they enter rigor. The obturator foramen is the hole in the skeleton of the pelvic girdle between the ilium, ischium and pubis bones. This is referred to by butchers as the aitch bone because of its shape. The process is called pelvic suspen- sion or hip free suspension. The stretching of the muscles results in more tender meat after cooking. It was originally described for beef carcasses in North America (Hostetler et al., 1970, 1975) and became known as the ‘Tenderstretch’ method. A disadvantage is that although some muscles become more tender, others toughen. However, these are usually either the less valuable ones, or muscles like the m. psoas, which is inherently very tender anyway so the slight toughening is of little importance. Another problem is that the carcasses are unconven- tional in shape after pelvic hanging and take up more space in the chillers. The different shape makes butchery more difficult and alters the appearance of the joints. It is possible to rehang small carcasses, such as those of pigs, from the Achilles tendon after rigor has developed while in pelvic suspension. This returns the carcasses to near-normal shape and gives the benefits of pelvic suspension without the disadvantage of altered carcass shape. It is, however, not a popular technique in practice because of the considerable extra labour required to rehang carcasses while in the chiller. Taylor (1996c) has given an interesting comparison of the benefits of pelvic suspension and electrical stimulation in pig carcasses.
main functions. It protects the meat from contamination and inhibits microbial growth, it reduces or eliminates evaporative weight loss and surface drying and it may enhance the appearance of the product.
Bacteria such as Pseudomonas can grow below 5°C under aerobic conditions. Including high concentrations of carbon dioxide in the pack restricts this growth and encourages the proliferation of lactic acid-producing bacteria, which are far less likely to cause spoilage.
This sort of packaging may therefore delay spoilage and prolong shelf life. Consumers like meat such as beef and lamb to appear bright red because they associate the colour with freshness. The red is produced by reaction of the muscle haem pigments with oxygen. Covering meat with plastic films of different gas permeabilities can affect this reaction and so alter the colour of the meat surface.
Reaction of the haem pigments with oxygen
To understand how this occurs it is necessary to discuss the way oxygen reacts with myoglobin and haemoglobin. Myoglobin (Mb), oxymyoglobin (MbO2) and metmyoglobin (met Mb), and their three haemoglobin equivalents, are the common forms of the pigments that occur in meat. The formation of oxymyoglobin from myoglobin involves the attachment of an oxygen molecule (O2) to the myoglobin molecule. Oxygen is referred to as a ligand. Various other molecules can act as ligands. Examples are carbon monoxide (CO), which reacts to form carbonyl compounds, and nitric acid (NO) which reacts to form the compounds associated with the characteristic pink colour of cured meat. The formation of metmyoglobin from myoglobin or oxymyoglobin does not involve reaction with a ligand. Instead, there is a change in the oxidation state of the iron atom at the centre of the haem molecule. In myoglobin and oxymyoglobin the iron is in the ferrous state (Fe2+) whereas this is oxidized to ferric iron (Fe3+) in metmyoglobin (Fig. 8.4).
The reduction of metmyoglobin only occurs to a limited extent, particularly in meat that has been aged for a long time post mortemand in which the reducing activity of the muscle enzymes is consequently low. The importance of these changes to the colour of meat is that the three compounds have different colours. Myoglobin is purple, oxymyo- globin is bright red and metmyoglobin is grey–brown. The colours of myoglobin and oxymyoglobin are analagous to those of the corres- ponding haemoglobin compounds that occur in deoxygenated venous blood (purple) and oxygenated arterial blood (bright red) in the living animal. To consumers the bright red of oxymyoglobin is desirable as the colour of fresh meat whereas the purple and particularly the grey–brown of the other two forms of the pigment are less desirable.
A freshly cut surface of meat is purple because the pigment is in the deoxygenated form. On exposure to air, the myoglobin in the surface to a depth of 2–6 mm (or more) reacts with oxygen to form the desirable,
bright red oxymyoglobin, the process taking between about 15 min to 1 h. This is known as ‘blooming’. The depth of the oxymyoglobin layer depends on the extent of penetration of oxygen from the atmosphere.
The oxygen oxidizes various reduced substances, particularly coenzymes, present in the muscle. Below the layer of oxymyoglobin a very thin layer of oxidized metmyoglobin forms (Fig. 8.5). This is because oxidation is favoured over oxygenationat low partial pressures of oxygen. Metmyoglobin formation is maximal at partial pressures of oxygen between 6 and 7 mm in beef semitendinosus muscle stored between 0° and 7°C (Ledward, 1970). Oxymyoglobin is more resistant to oxidation than is deoxygenated myoglobin.
The depth of the oxymyoglobin layer varies slightly between muscles because of their different metabolic characteristics, particularly the activity of the various enzyme systems, which continue to be active for a time after death of the animal. The layer is thinner in muscles with high activities of reducing system enzymes, particularly the cytochromes. The reducing activity is high in fresh meat and low in aged meat. It also decreases with temperature more than does the ease with which oxygen diffuses through the tissue. The thickness of the oxymyoglobin layer is therefore greater, and meat colour is brighter, at lower temperatures of storage.
After about 2 or 3 days in air the oxymyoglobin at the meat surface gradually starts to oxidize to brown metmyoglobin. When around 20%
of the surface pigment has oxidized, the change in colour of the meat can be enough to cause consumer discrimination. Colour stability is very sensitive to temperature. The difference between storage of meat at 0°C compared with 5°C is significant, browning of the surface being post- poned from perhaps 48 h to 1 week. The factors that affect discoloura- tion of meat have been reviewed by Renerre (1990) and Kropf (1993).
Controlling gas atmospheres
The major role of plastic films in influencing the appearance of meat is to control the gas atmospheres in contact with its surface and so
Oxymyoglobin (Fe2+)
oxidation reduction Myoglobin (Fe 2+)
oxygenation deoxygenation
Metmyoglobin (Fe3+)
Fig. 8.4. The relationships between myoglobin, oxymyoglobin and metmyoglobin.
influence the reaction of the haem pigments. The gases of importance are oxygen, carbon dioxide and nitrogen. Oxygen oxygenates or oxidizes the pigments and influences the differential growth of aerobic and anaerobic bacteria. Carbon dioxide inhibits undesirable bacterial growth (Gill and Penney, 1988) mainly by extending the lag phase (see Chapter 9). Nitrogen is inert and is sometimes used as a ‘ballast’ gas to overcome the problem of the high solubility of carbon dioxide in water, and therefore in meat. If large amounts of carbon dioxide dissolve in the meat, the pack may collapse. Carbon dioxide and oxygen are often used for fresh red meats. For cooked, cured and processed meats, the gases are carbon dioxide and nitrogen, oxygen being excluded to prevent the development of rancidity and colour fading. Different types of plastic film have different properties of gas, and, to a lesser degree, water permeability. Taylor (1996a) lists the oxygen and water transmission properties of a variety of plastics used in meat packaging.
Different plastics can be laminated together to benefit from their combined properties – polyethylene for its sealing ability, ethyl vinyl alcohol or polyvinylidene chloride for gas impermeability, and nylon for strength.
Using very gas-permeable plastic film to overwrap meat in trays allows oxygen from the air to penetrate the pack easily and react with myoglobin to give bright red oxymyoglobin. The colour is, however, stable for only a short time – often only 1 or 2 days – before oxidation to metmyoglobin becomes a problem. To extend shelf life, modified atmosphere packaging (MAP) is used. In MAP the atmosphere over the meat is modified by inclusion of more oxygen, or more carbon dioxide, than in air. Although the gas atmosphere is fixed when introduced into the pack it may change over the life of the product because of interaction Fig. 8.5. Schematic cross section of muscle showing layers of myoglobin, oxymyo- globin and metmyoglobin in relation to the surface in contact with atmosphere.
with the meat. A relatively impermeable plastic film is employed and the pack is filled with a gas mixture containing, for example, 60–80%
oxygen in carbon dioxide. The enhanced oxygen concentration (compared with the 20% in normal air) encourages penetration of oxygen into the meat for a much greater distance from the surface. The depth of oxygen penetration is proportional to the square root of its concentration. A fourfold increase in the oxygen level from 20 to 80%
therefore doubles the depth of the oxymyoglobin layer. This thicker layer of bright red oxymyoglobin might reach 10 mm after a day’s storage and delays the progress of metmyoglobin formation so that shelf lives of a week at 1°C are possible. The high carbon dioxide concentrations in the pack inhibit undesirable microbial growth and so also prolong microbiological shelf life.
A potential concern is that the increased shelf life could enable the growth of pathogenic bacteria to levels that would make the meat unsafe. This is especially true for cooked, ready-to-eat products, and for bacteria that can grow at refrigerated temperatures such as Listeria monocytogenes, or those that grow at low oxygen concentrations, or anaerobically such as Clostridium botulinum and Clostridium perfringens. Inhibition of non-pathogens can also encourage the proliferation of pathogens.
To prevent the unsightly accumulation of exudate from the muscle when displayed for the longer times possible with modified atmosphere packaging, the trays in which the meat sits are often furnished with an absorbent pad which soaks up any drip that is formed. In these packs it is important that there is a space between the top meat surface and the overwrapped plastic film to allow the gas mixture access to the meat. These more stringent packaging requirements can make modified atmosphere systems expensive compared with simple overwrapping.
In vacuum packing, joints are sealed in bags in which the plastic is effectively impermeable to gases. Before sealing, residual air is removed from between the meat and the plastic by carrying out the process under a partial vacuum. The plastic pack or pouch is therefore very closely applied to the meat surface. Enzyme systems in the muscle continue to use any available oxygen. Oxygen penetrating the surface is consumed and carbon dioxide is given off. This process
‘mops up’ or scavenges any residual oxygen in the packs, which quickly become in effect anaerobic. The layer of oxymyoglobin at the surface of the meat is therefore non-existent or extremely thin. Instead, the normal oxidized layer of metmyoglobin layer increases in thickness and, together with the underlying deoxygenated myoglobin, produces a dark purple appearance. This makes the method less suitable for retail display but ideal for storing and transporting boneless primal joints of meat where the appearance is unimportant. As oxygen is consumed so carbon dioxide is generated. This inhibits spoilage by
undesirable Pseudomonad growth and encourages lactic acid-forming bacteria. With good hygiene, vacuum-packed meat can be stable for 5–6 weeks, or even more, when kept at 1°C. On removal from the packs and cutting into retail joints or steaks the meat surface ‘blooms’
normally.
The size of the meat cut or joint can influence the effectiveness of vacuum packing. In retail-sized packs the ratio of meat volume to residual air volume may be undesirably small. Vacuum packing is therefore of most use for larger joints at the wholesale level. The same is true of controlled atmosphere packaging (CAP). This is like MAP but the atmosphere is periodically monitored and controlled at its initial specified level, if necessary by the addition of fresh gas. It is therefore suitable for bulk storage purposes.
A key advantage of MAP is in allowing the development of centralized cutting operations, removing the need for butchery facilities at the retail level. It also allows ‘portion control’ and the pack can be made attractive to the consumer to aid marketing. An obvious requirement of all packaging systems is that the packs are sealed.
Stringent quality control systems must therefore be in place to identify leaking packs. Excellent reviews of both modified atmosphere and vacuum packaging are given by Taylor (1996a, b).