relatively weak contraction, if at all, associated with normal rigor. The contraction is probably stimulated by the rapid and massive release of calcium ions from the sarcoplasmic reticulum on thawing. The effect is greater with muscles that have been frozen very quickly (slow freezing may of course produce cold shortening before freezing). The breakdown of ATP is probably through the activation, by freezing and thawing, of the contractile (actomyosin) ATP-ase, rather than the non- contractile ATP-ase that causes ATP breakdown in normal rigor development. This leads to the strong contraction seen in thaw rigor and accounts for the very high ATP-ase activities ("10) seen.
As in cold shortening, physical prevention of contraction and shortening of muscles can reduce the toughening effects. Physical prevention may be through restraint on the carcass or by thawing being so slow that ice prevents contraction. At temperatures below freezing the activity of actomyosin ATP-ase and the levels of ATP are gradually reduced in storage so that on thawing the effects seen in ‘normal’ thaw rigor may also be reduced.
Heat shortening
If muscles are stimulated, and allowed to contract and shorten, at high temperatures, without subsequent relaxation, then they may subse- quently become tough if they enter rigor in this state (Locker and Daines, 1975). This so-called heat shortening may be a problem if hot processing is practised.
normal conditions. Electrical stimulation may reduce this time to only 1–2 h. The effectiveness of electrical stimulation for lamb carcasses was demonstrated by Chrystall and Hagyard (1976), and for beef carcasses by Daveyet al.(1976). Table 8.1 shows the improved tender- ness of lamb muscles caused by stimulation.
As well as preventing cold shortening, electrical stimulation also appears to tenderize the meat per se and improves appearance and possibly flavour. This is even true in pork, which is rarely cold shortened by normal chilling rates (Tayloret al., 1995). The reasons for these effects are not completely understood. It has been suggested that tenderization could be caused by several mechanisms. The intense contractions might physically disrupt and so weaken the muscle structure (Savell et al., 1978). Calcium released during contraction might stimulate the calpains, at a time when muscle temperature and pH were high, and cause greater proteolytic breakdown. Rather less likely, the lysosomes might be disrupted, allowing the release of their enzymes, which could also cause proteolytic breakdown. The colour of beef from electrically stimulated carcasses is brighter, perhaps because the rapid muscle acidification leads to some protein denaturation, causing greater reflectance of light from the meat surface. The occur- rence of ‘heat ring’ is also reduced. These effects improve beef carcass grades in North American systems (Smith, 1985).
The influence of electrical stimulation on potentially dark cutting meat is a little unclear. Inadequate muscle glycogen concentrations at slaughter should reduce or remove any benefits of faster glycolysis and consequent pH fall. However, there is evidence that electrical stimula- tion does influence the biochemistry and ultrastructure of dark cutting beef (Fabiansson et al., 1985) and could potentially have beneficial effects on colour, although work by Dutsonet al.(1982) indicated none in practice.
Electrical stimulation may improve flavour by affecting the concentrations of flavour precursors and enhancers, such as nucleotides, in the muscles. Mikami et al. (1993) have shown that Table 8.1. The effect of electrical stimulation on the texture of lamb muscle (from Chrystall and Hagyard, 1976).
Muscle Unstimulated carcasses Stimulated carcasses
m. longissimus dorsi 77 37
m. biceps femoris 49 21
m. semimembranosus 76 36
m. gluteus medius 57 22
Texture was assessed by shear force measurements made with a tendermeter. Shear force values of 30–40 were considered marginally tender with 40 the maximum acceptable level for palatability. Values above 50 were very tough.
electrical stimulation of beef leads to the accumulation of peptides and glutamic acid which are thought to contribute to better flavour. The flavour enhancement has been seen in beef (Smith, 1985) and pork (Warrisset al., 1995a).
Numerous different forms of electrical stimulation have been developed. The voltages used normally range from 20 to 1000 V.
However, low- and high-voltage stimulation systems are generally differentiated (Table 8.2). Low-voltage stimulation is usually considered to be up to 100 V, high-voltage greater than about 500 V. The time of application can be almost immediately after death to up to 60 min post mortem. The current can be applied through various combinations of electrodes but usually flows between the muzzle or nose of the animal, or the chest, and the hind legs. The electrodes are often clips or rubbing bars. Low voltage systems are usually applied immediately after exsanguination and work by stimulating the musculature via the still- living nervous system. High-voltage systems can be applied much later and do not rely on the animal’s nervous system but stimulate the muscles directly. Usually the current is applied as a series of pulses of 1–2 s duration and for a period of up to 90 s.
The advantage of high-voltage stimulation is that it is less reliant on accurate timing of the application. However, stringent safety precautions are required and the investment to fulfil these may be too expensive to justify in small, low-throughput plants. Low-voltage systems obviously also require attention to safety but the necessary compliance costs are generally lower. However, good electrode contact is more critical than in high-voltage stimulation and the final results may not be as consistent.
Electrical stimulation is generally only applied to carcasses from sheep and cattle. Cold shortening is less of a potential problem with pork carcasses because of the much more rapid rigor development.
However, with very fast chilling procedures, stimulation may be beneficial. It appears that timing of the stimulation is important (Taylor, 1996c). Stimulating pigs within 5 min of slaughter results in very rapid pH fall in the musculature leading to loss of large amounts
Table 8.2. Low- and high-voltage electrical stimulation.
Low voltage High voltage
20–100 V 500–1000 V
#1 amp > 5 amps
For up to 20 s For up to 90 s
Applied immediately after Applied up to 60 min post mortem exsanguination (within 5 min (but more usually earlier than this) of stunning)
Stimulates muscles via nervous system Stimulates muscles directly
of drip from the meat, reminiscent of the PSE condition. By contrast, stimulating at 20 min after slaughter seems to prevent the excessive exudation. Stimulation even at 20 min post mortemwithout very rapid chilling results in paler, more watery meat from pigs, as might be expected in view of their predisposition to PSE (Table 8.3).
Hot processing
Conventional systems chill the whole carcass to a temperature of 7°C or less before subsequent cutting into smaller parts and further processing. This can be wasteful of chiller capacity, and energy consumption, if, on butchery, much bone and fat is removed and discarded since it will have been cooled unnecessarily. Additionally, because of their size and irregular shape, whole carcasses cool unevenly.
A solution is to process the carcass when it is still hot, before chilling.
Because the processing largely involves removal of muscles from the skeleton it is often referred to as hot boning, hot deboning or hot cutting. As well as saving refrigeration space and energy, the system may require less labour and reduce the time needed to produce
Table 8.3. Effect of electrical stimulation on conventionally chilled pig carcasses (from Warriss et al.,1995a).
Unstimulated Stimulateda Temperature in the m. longissimus dorsi at
45 min post mortem 35.5 36.0
pH at 45 min post mortem 6.31 5.92
Ultimate pH 5.55 5.51
Lightness (L*) 53.3 55.2
Percentage drip loss 7.1 9.3
a Carcasses were stimulated 20 min after exsanguination using 700 V applied for 2 min at 12.5 cycles s!1. After slaughter the carcasses were held at ambient temperature for 2 h before chilling at 2°C. All differences between stimulated and unstimulated carcasses were statistically significant.
Table 8.4. Advantages and disadvantages of hot processing.
Advantages Disadvantages
Reduced refrigeration costs Abnormal shape of joints
Energy saving Difficulty of handling and butchering pre-rigor carcasses
Increased meat yield Decreased tenderness
More uniform colour Carcasses cannot be graded using Better water-holding capacity, less drip normal post-chill systems
marketable meat. In regard to yield and meat quality there are advan- tages and disadvantages (Table 8.4).
Meat yield can be improved by up to about 2% (Tayloret al., 1981) because of the reduction in evaporative and drying losses. The meat also tends to have better WHC, so loses less drip during storage and colour is more uniform. Both these effects can be attributed to faster, more uniform cooling of the smaller meat pieces, compared with whole carcasses. Because the muscles are removed before they have gone into rigor, their shape is not fixed by the skeleton and the presence of other muscles of the carcass. The corresponding joints of meat do not therefore have the characteristic shapes associated with normal post-rigor butchery. This can be a disadvantage as the appearance is novel to the consumer but, on the other hand, allows the processor to control the shape of the final joint by appropriate packag- ing. Handling soft pre-rigor carcasses and meat is more difficult for butchers used to traditional techniques developed for dealing with firm bodies of meat.
A more serious disadvantage of hot processing is that the meat may be slightly tougher. This is true for both beef (Babiker and Lawrie, 1983) and pork (Van Laack and Smulders, 1989). There are several likely reasons for this. The meat may heat shorten because of the stimulation associated with boning and manipulation pre-rigor. It may cold shorten because of the speed with which it can be cooled when compared with whole carcasses. Lastly, this relatively rapid cooling may reduce the tenderizing effects of the proteolytic enzymes involved in the normal conditioning processes. This is because they are working at a high temperature for only a short time.
A compromise solution to enable the processor to take advantage of hot processing while minimizing the disadvantages may be to hot- process some parts of the carcass and to retain normal processing for others. Hot processing is especially suitable for beef forequarter meat, which has inherently lower value. In contrast the high-value hind quarter cuts may benefit from conventional processing where the traditional appearance of joints is more important and palatability is at a premium. Hot processing could influence the microbiological charac- teristics of the meat. Exposure of a large surface area allows the poten- tial for the contamination by bacteria from the operative’s hands and tools, and contamination at a time when the meat temperature is high and the meat surface is wet or sticky. This might predispose hot- processed meat to faster spoilage or the growth of pathogenic bacteria associated with food poisoning. However, provided that the hot processing is carried out with due regard for hygiene, and that chilling is rapid, these concerns may be unfounded (Gilbertet al., 1977; Taylor et al., 1981).
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.