In conventional chilling systems the carcasses are placed in chill rooms held at about 1°C as soon as possible after they have been dressed, washed and inspected. Because of the large amount of hot carcass meat, and therefore the large amount of heat that needs to be dissipated, the air temperature in the chiller may rise above 1°C for periods while hot carcasses are being loaded. Modern refrigeration systems can, however, allow much lower chiller temperatures to be achieved – well below freezing point – so that much faster chilling is possible. ‘Blast chilling’ for 1 h using air at !25°C can reduce pork carcass temperatures to 24°C at 2 h post mortemcompared with 32°C in carcasses subjected to a standard 1°C chill (McFarlane and Unruh, 1996). In some systems the surface of the carcass is frozen by very cold air when combined with high air speeds. After such extreme blast chilling the carcasses may be tempered at higher temperature (1°C).
This allows equilibration to a higher uniform meat temperature equivalent to that achieved over a longer time in normal chilling.
Normal practice is to achieve a meat temperature of 7°C or below before further handling such as butchery, or transport from the plant.
Rarely, carcasses are not chilled immediately after washing and inspec- tion but are held at relatively high temperatures while they are hot processed (see p. 166).
The influence of temperature on muscle metabolism
The rate of cooling of meat has other implications besides its effects on microbiology, weight loss and WHC. Because the activity of enzymes is temperature dependent, different cooling rates can affect the rates of pH fall through lactic acid production, the disappearance of creatine phosphate and adenosine triphosphate (ATP), and the speed of onset of rigor mortis.
Between the in vivopH of about 7 and a value around 6.1, the rate of pH fall in beef muscles is approximately linear with time. The rate of fall depends on the muscle temperature. The minimum rate occurs at about 10°C. As the temperature gets closer to 0°C the rate increases.
Also, as the temperature increases to 37°C the rate increases, produc- ing overall a curvilinear relationship between rate of pH fall and temperature (Fig. 8.1). There is evidence that the increased rates above and below about 10°C are both caused by activation of the actomyosin ATP-ase that results in muscle contraction. However, the increase above 10°C is caused by increasing activation of calcium-independent ATP-ase but the increase below 10°C is caused by a calcium dependent ATP-ase. The calcium ions that stimulate this come from the sarco- plasmic reticulum, which loses its ability to sequester calcium at low temperatures.
Temperature also influences the rate of ATP depletion post mortem and therefore the time of onset of rigor mortis. Characteristically, ATP depletion follows a biphasic pattern (Fig. 8.2). There is an initial
‘delay’ phase where ATP that is used up by the normal energy- consuming processes in muscle is wholly or partly replenished by resynthesis from creatine phosphate and through glycolysis. This is followed by a phase in which the processes of resynthesis cannot maintain the ATP concentration, which consequently falls in a more or less linear fashion. The length of the delay phase is temperature dependent with a maximum at about 10–15°C. At lower and higher temperatures the delay phase is shorter. The rate of the second phase, where ATP levels fall significantly, is apparently relatively independent of temperature.
A shortening of the delay phase reduces the time to the onset of rigor.
If the muscle is maintained at temperatures around 0° or 30°C there is effectively no delay phase and rigor occurs relatively rapidly. In practice, interpretation or prediction of the course of events is complicated by other factors. Normally, the temperature of muscles is falling progres- sively with time after death and the fall will not be exactly the same throughout either the muscle, or the carcass, since temperature gradients will be set up during chilling. Muscles differ in their fibre composition and tendency towards oxidative or glycolytic metabolism. This can affect the rate of acidification and rigor development. Events immediately Fig. 8.1. Relation of rate of pH fall to muscle temperature in beef muscle (based on Jeacocke, 1977a).
pre-slaughter, such as prolonged fasting, or struggling at slaughter, can deplete glycogen reserves and reduce the length of the delay-phase of ATP depletion.
Three practical implications of different rates of carcass cooling, and therefore muscle temperatures post mortem, are cold shortening, thaw rigor and, to a lesser degree, heat shortening. Cold shortening and thaw rigor are consequences of wanting to cool muscle too quickly.
Cold shortening
If muscle is cooled below about 10°C before the onset of rigor the subsequent meat is tough after cooking. The phenomenon is referred to as cold shortening (Locker and Hagyard, 1963). Above freezing, the effect is greater the lower is the temperature. The problem is therefore likely to be most acute in situations where rigor development is delayed and where small volumes of tissue are being chilled. Species such as chickens and pigs, because they have relatively rapid rates of rigor onset, are less prone to cold shortening even though their carcasses are small (Møller and Jensen, 1993). Beef carcasses tend to cool slowly because of their bulk and are again less prone. Lamb carcasses are small enough to cool rapidly and generally do not enter rigor quickly. They are therefore most prone to shortening. This is an oversimplification since modern, very fast chilling regimes can cool some muscles, particularly those near the surface of the carcass, rapidly enough to shorten in all species. Different muscles in the carcass will Fig. 8.2. The biphasic pattern of ATP depletion post mortem.
also be differently predisposed to cold shortening because of inherent metabolic differences. So, while the m. longissimus dorsi is very prone to cold shortening, the m. psoas is hardly affected. Variation in ante- mortem handling may also deplete glycogen stores or speed up post- mortem glycolysis.
The mechanism of cold shortening is thought to be stimulation, by the low temperature, of massive release of calcium ions from the sarcoplasmic reticulum without subsequent sequestration. This is because the calcium pump of the sarcoplasmic reticulum does not appear to function very well at low temperatures. The calcium ions activate the actomyosin ATP-ase and lead to muscle contraction. ‘Red’
(oxidative) muscles tend to have a less well developed sarcoplasmic reticulum than ‘white’ (glycolytic) muscles and, in the pig at least, appear to be more prone to cold shortening, perhaps because of the consequent potential reduced ability to sequester the calcium. This calcium ion sequestering ability also seems to be less reduced at low temperatures, particularly below about 10°C, in ‘white’ fibres. An exception is the pig m. longissimus dorsi,which does seem to be prone to cold shortening despite being composed largely of these glycolytic fibres. Mitochondria also sequester calcium ions. However, under anaerobic conditions at low temperature this ability is reduced. ‘Red’
muscle fibres have more mitochondria. In redder muscles, the mitochondria release calcium ions that are not sequestered, so also promoting contraction. The relative involvement of the sarcoplasmic reticulum and mitochondria in cold shortening is not completely clear.
The question has been addressed by Cornforth et al.(1980).
Contraction of muscle that is not followed by relaxation produces shorter sarcomeres and tougher meat (Marsh and Leet, 1966). The shorter sarcomeres reflect greater overlap of thin (actin) and thick (myosin) filaments. Under normal conditions, muscles cooked pre- rigor are tender, become tough as rigor develops in the first 24 h post mortem and then progressively tenderize with longer storage times post mortem (Wheeler and Koohmaraie, 1994). If muscles are physically prevented from shortening after slaughter until they have entered rigor, both sarcomere length and texture after cooking (shear force) remain constant (Koohmaraie et al., 1996). This illustrates the fundamental importance of sarcomere length to meat texture. The relationship between the temperature at which muscles are held pre- rigor,the degree of their shortening, and the relation of this shortening to meat texture after cooking, was studied by Locker and Hagyard (1963). The effects are illustrated in Fig. 8.3. The apparent decrease in toughness at very high degrees of muscle shortening is thought to be because the overlap of thick and thin filaments in each sarcomere is so great under these conditions that some cross-bridges are no longer able to function. In practice, muscles that cold shorten show uneven
contraction with localized areas of shortening interspersed with adjacent areas of non-shortened or stretched muscle. This results in cold-shortened meat having more variable texture as well as being tougher overall. The extent of tenderization through conditioning decreases as muscles shorten more in relation to their resting length.
Therefore, meat with shorter sarcomeres tends to tenderize least.
Normally therefore, cold-shortened meat appears to undergo little or no tenderization during ageing (Davey et al.,1967; Locker and Wild, 1984) but for reasons that are not completely clear. The effects of cold shortening cannot then be overcome or ameliorated by long condition- ing times. However, Locker and Daines (1975) recorded an interesting phenomenon. They cold-shortened excised beef sternomandibularis muscles by holding them at 2°C. This resulted in a 33% shortening of the muscle length. However, if they subsequently held the muscles at 37°C until rigor was completed they did not become tough, the texture being equivalent to that of control muscles held at 15°C and not cold shortened. The effect could not be attributed to ageing effects at the high temperature since Z discs remained intact. Instead, they suggested that the temperature of the muscle at the time rigor mortis developed was the important factor and in some way affected the molecular bonding between myosin and actin, reducing its strength.
In normal beef, cold shortening can be prevented by ensuring that muscle temperature does not fall below 10°C until the muscle pH has reached 6.1, which would normally take about 10 h. By this time, the progress of rigor will be sufficiently advanced to prevent cold contrac- ture. In pork it is now relatively common commercial practice in some
Fig. 8.3. The relationship between temperature at which muscles are held pre- rigor, degree of shortening and toughness (based on Locker and Hagyard, 1963).
countries to chill carcasses very rapidly using blast chilling with air at temperatures of !20°C or lower. There is evidence that this may induce some cold shortening in muscles such as the m. longissimus dorsi but the effect on the texture of the cooked meat is seemingly small and possibly of little practical consequence. The subject of cold- induced toughening of meat was reviewed by Locker (1985).
Heat ring
The phenomenon of so-called ‘heat ring’ sometimes occurs in beef subjected to relatively fast chilling. In particular, the part of the m.
longissimus dorsi that is nearer the outside of the carcass cools more quickly than the inside so the rate of pH fall is reduced. This leads to a darker band of muscle forming. This has an unattractive appearance.
Very fast chilling
Recently, based on earlier observations that, under some conditions, expected cold shortening did not occur (Sheridan, 1990) so-called very fast chilling (VFC) of meat has been investigated more fully (Joseph, 1996). In this procedure the rate of chilling is extreme, with the carcasses held in intensely cold air (!70 to !20°C) moving at high speeds. Despite initial concerns, this can lead to tender meat in some circumstances, rather than the toughening associated with cold shortening. The reasons for this are unclear but several possibilities have been put forward. A hard crust formed by the frozen surface of the meat could prevent shortening of the underlying muscle by virtue of the physical restraint. The differential freezing might cause fractures or breaks in the meat structure, so tenderizing the meat, or the massive release of calcium ions caused by the low temperature might promote proteolysis by stimulating the calpain system.
Thaw rigor
If the rate of cooling of the carcass is sufficiently high and the meat freezes before the onset of rigor then, on thawing, the muscle shortens severely (to up to 50% of its length if unrestrained) and becomes very tough after cooking. Additionally, very large amounts of drip or exudate (30% of muscle weight) are lost during thawing. This is known as thaw rigor. It is thought that, on thawing, glycolysis is completed very rapidly and ATP breakdown is extremely rapid. This rapid metabolism is accompanied by a very strong contraction – unlike the
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.