Effect of refrigeration on texture of meat Whilst a number of characteristics affect the overall quality and acceptability of both fresh and frozen meats, tenderness is the major characteristic of eating quality because it determines the ease with which meat can be chewed and swallowed. The tenderness of meat is affected by both chilling/freezing and storage. Under the proper conditions, tenderness is well maintained throughout the chilled/frozen storage life, but improper chilling/freezing can produce severe toughening and meat of poor eating quality Some of the factors that influence the toughness of meat are inherent in the live animal. It is now well established that it is the properties of the con- nective tissue proteins, and not the total amount of collagen in meat, that largely determine whether meat is tough or tender( Church and Wood 1992). As the animal grows older the number of immature reducible cross- inks decreases. The mature cross-links result in a toughening of the colla gen and this in turn can produce tough meat. Increasing connective tissue toughness is probably not commercially signi ntil a beast is about four-years-old(Husband and Johnson, 1985) Although there is common belief that in beef, breed has a major effect CSIRO (1992) state although there are small differences in tenderness due to breed, they are slight and currently of no commercial significance to Australian consumers However. there are substantial differences in the proportion of acceptable tender meat and toughness between Bos indicus and Bos taurus cattle. The proportion of acceptable tender meat decreased from 100% in Hereford Angus crosses to 96% in Tarentaise, 93%in Pinzgauer, 86% in Brahman and only 80% in Tsahiwal(Koch et al., 1982) Toughness of meat increases as the proportion of Bos indicus increases Crouse et al., 1989)
3 Effect of refrigeration on texture of meat Whilst a number of characteristics affect the overall quality and acceptability of both fresh and frozen meats, tenderness is the major characteristic of eating quality because it determines the ease with which meat can be chewed and swallowed. The tenderness of meat is affected by both chilling/freezing and storage. Under the proper conditions, tenderness is well maintained throughout the chilled/frozen storage life, but improper chilling/freezing can produce severe toughening and meat of poor eating quality. Some of the factors that influence the toughness of meat are inherent in the live animal. It is now well established that it is the properties of the connective tissue proteins, and not the total amount of collagen in meat, that largely determine whether meat is tough or tender (Church and Wood, 1992). As the animal grows older the number of immature reducible crosslinks decreases. The mature cross-links result in a toughening of the collagen and this in turn can produce tough meat. Increasing connective tissue toughness is probably not commercially significant until a beast is about four-years-old (Husband and Johnson, 1985). Although there is common belief that in beef, breed has a major effect, CSIRO (1992) state ‘although there are small differences in tenderness due to breed, they are slight and currently of no commercial significance to Australian consumers.’ However, there are substantial differences in the proportion of acceptable tender meat and toughness between Bos indicus and Bos taurus cattle. The proportion of acceptable tender meat decreased from 100% in Hereford Angus crosses to 96% in Tarentaise, 93% in Pinzgauer, 86% in Brahman and only 80% in Tsahiwal (Koch et al., 1982). Toughness of meat increases as the proportion of Bos indicus increases (Crouse et al., 1989)
4 Meat refrigerati There can also be significant differences within a breed Longissimus dorsi shear force values for double muscled Belgium Blue bulls were sig- nificantly higher than those of the same breed with normal conformation (Uytterhaegen et al., 1994). Calpain I levels at 1h and 24 h post-mortem were also much lower. It was suggested that the lower background tough ness in the double muscled was compensated for by reduced post-mortem proteolytic tenderisation Again, castration appears to have little influence on tenderness. Huff and Parrish(1993)compared the tenderness of meat from 14-month-old bulls and steers Strip loins were removed from carcasses ca. 24 h post-mortem found between the tenderness of bulls and steel \s. no differences were vacuum packed and held at 2C for up to 28 day Experiments designed to determine the effect of treatments immediately before or at the point of slaughter appear to show that they have little effect on meat texture Exercising pigs before slaughter has been shown to have no effect on texture parameters, i.e. muscle shortening and shear force (Ivensen et aL., 1995). The use of different stunning methods(both electri- cal and carbon dioxide) does not seem to have a significant effect on the quality of pork( Garrido et aL., 1994) Consumers' surroundings influence their appreciation of tenderness (Miller et aL, 1995). Consumers were more critical of the tenderness of beef steaks cooked in the home than those cooked in restaurants the Warner-Bratzler force transition level for acceptable steak tenderness was between 4.6 and 5.0kg in the home and between 4.3 and 5.2 kg in restau rants. Warner-Bratzler tests are probably the most uniformly used method of texture measurement. However, there are many other methods of deter mining the mechanical properties of meat(Lepetit and Culioli, 1994). In cooked meat it is suggested that applying mechanical tests in different strain directions is likely to produce information that can be more readily related to perceived texture End-point temperature after cooking is crucial to tenderness. Davey and Gilbert (1974) showed that there was a three-to four-fold toughening occurring between 40 and 50C and a further doubling between 65 and Refrigeration has two critical roles in meat tenderness. One is in the prevention of muscle shortening in the period immediately following slaughter. The second is in the conditioning of the meat so that the desired degree of tenderness is obtained 3.1 Muscle shortening Chilling has serious effects on the texture of meat if it is carried out rapidly when the meat is still in the pre-rigor condition, that is, before the meat pH has fallen below about 6.2(Bendall, 1972). In this state the muscles contain
There can also be significant differences within a breed. Longissimus dorsi shear force values for double muscled Belgium Blue bulls were significantly higher than those of the same breed with normal conformation (Uytterhaegen et al., 1994). Calpain I levels at 1 h and 24h post-mortem were also much lower. It was suggested that the lower background toughness in the double muscled was compensated for by reduced post-mortem proteolytic tenderisation. Again, castration appears to have little influence on tenderness. Huff and Parrish (1993) compared the tenderness of meat from 14-month-old bulls and steers. Strip loins were removed from carcasses ca. 24 h post-mortem, vacuum packed and held at 2°C for up to 28 days. No differences were found between the tenderness of bulls and steers. Experiments designed to determine the effect of treatments immediately before or at the point of slaughter appear to show that they have little effect on meat texture. Exercising pigs before slaughter has been shown to have no effect on texture parameters, i.e. muscle shortening and shear force (Ivensen et al., 1995). The use of different stunning methods (both electrical and carbon dioxide) does not seem to have a significant effect on the quality of pork (Garrido et al., 1994). Consumers’ surroundings influence their appreciation of tenderness (Miller et al., 1995). Consumers were more critical of the tenderness of beef steaks cooked in the home than those cooked in restaurants. The Warner–Bratzler force transition level for acceptable steak tenderness was between 4.6 and 5.0 kg in the home and between 4.3 and 5.2 kg in restaurants. Warner–Bratzler tests are probably the most uniformly used method of texture measurement. However, there are many other methods of determining the mechanical properties of meat (Lepetit and Culioli, 1994). In cooked meat it is suggested that applying mechanical tests in different strain directions is likely to produce information that can be more readily related to perceived texture. End-point temperature after cooking is crucial to tenderness. Davey and Gilbert (1974) showed that there was a three- to four-fold toughening occurring between 40 and 50°C and a further doubling between 65 and 70 °C. Refrigeration has two critical roles in meat tenderness. One is in the prevention of muscle shortening in the period immediately following slaughter. The second is in the conditioning of the meat so that the desired degree of tenderness is obtained. 3.1 Muscle shortening Chilling has serious effects on the texture of meat if it is carried out rapidly when the meat is still in the pre-rigor condition, that is, before the meat pH has fallen below about 6.2 (Bendall, 1972). In this state the muscles contain 44 Meat refrigeration
Effect of refrigeration on texture of meat 45 sufficient amounts of the contractile fuel, adenosine triphosphate(ATP), for forcible shortening to set in as the temperature falls below 11C, the most severe effect occurring at about 3 C. Cold- shortening first became apparent in New Zealand, when tough lamb began to be produced routinely by the improved refrigeration techniques which were introduced after the Second World War(Locker, 1985). The shortening phenomenon was first observed scientifically by Locker and Hagyard (1963)and the resulting extremely tough meat after cooking by Marsh and Leet (1966).The mecha- nism of cold shortening has been well described by Bendall(1974)and Jeacocke(1986)and forms the basis of the next sections of this chapter. 3.1.1 Mechanism of shortening The characteristic pattern of post-mortem chemical change, found in all the skeletal muscles of the mammals so far investigated, is shown in Fig. 3.1 The figure has an arbitrary timescale, because although the pattern is irtually constant its duration is highly temperature dependent. Relative time scales can be interpolated from the temperature data in Fig. 3.2 It can be seen in Fig 3. 1 that the supply of contractile fuel (ATP)remains constant and high for some time. It is kept topped up by two resynthetic processes that counteract its slow wastage in the resting muscle. The first of 7.0 6.5 ig. 3.1 Biochemical changes during the course of rigor mortis. Arrow 1 indicates onset of rapid decline of ATP, and arrow 2 the time for half-change of ATP. The time scale is arbitrary and highly temperature dependent(see Fig 3.2)(source: Bendall
sufficient amounts of the contractile fuel, adenosine triphosphate (ATP), for forcible shortening to set in as the temperature falls below 11 °C, the most severe effect occurring at about 3 °C. ‘Cold-shortening’ first became apparent in New Zealand, when tough lamb began to be produced routinely by the improved refrigeration techniques which were introduced after the Second World War (Locker, 1985). The shortening phenomenon was first observed scientifically by Locker and Hagyard (1963) and the resulting extremely tough meat after cooking by Marsh and Leet (1966). The mechanism of cold shortening has been well described by Bendall (1974) and Jeacocke (1986) and forms the basis of the next sections of this chapter. 3.1.1 Mechanism of shortening The characteristic pattern of post-mortem chemical change, found in all the skeletal muscles of the mammals so far investigated, is shown in Fig. 3.1. The figure has an arbitrary timescale, because although the pattern is virtually constant its duration is highly temperature dependent. Relative time scales can be interpolated from the temperature data in Fig. 3.2. It can be seen in Fig. 3.1 that the supply of contractile fuel (ATP) remains constant and high for some time. It is kept topped up by two resynthetic processes that counteract its slow wastage in the resting muscle. The first of Effect of refrigeration on texture of meat 45 100 50 7.5 7.0 6.5 6.0 5.5 pH PC 1 2 0 3 4 5 6 7 8 9 Time (arbitrary) PC or ATP as % initial value pH ATP 1 2 Fig. 3.1 Biochemical changes during the course of rigor mortis. Arrow 1 indicates onset of rapid decline of ATP, and arrow 2 the time for half-change of ATP.The time scale is arbitrary and highly temperature dependent (see Fig. 3.2) (source: Bendall, 1974)
Meat refrigeration 5 11 三a品 82-39兰 05101520253035 Temperature(C) Fig 3.2 Time for half-change of ATP during rigor in beef LD muscle, plotted against temperature. Initial pH=7.0 in all cases. Curve 1: times for an initial re- action. Curve 2: observed times Curve 3: work done during shortening(source hese processes is the creatine kinase reaction, which resynthesises ATP from its breakdown product, ADP, and phosphocreatine(PC). The second is the complex process of glycolysis in which the energy for resynthesis comes from the breakdown of glycogen to lactate, with concomitant acidi- fication and fall of muscle pH. The ATP supply remains constant only while PC is still available, but begins to fall as soon as glycolysis is left on its own as the sole source of resynthesis. This phase of declining ATP supply is known as the rapid phase of rigor, because it is then that the stiffening (rigor)of the muscle sets in. It is shown by the first arrow in Fig. 3.1 At temperatures above 12C the post-mortem muscle remains in a passive, relaxed state until the ATp supply begins to dwindle at the onset of the rapid phase of rigor. It then begins to shorten actively. At body tem- perature(38C)the shortening can reach 40% or more of the muscle length if unopposed by the force of a load. This so-called'rigor shortening can be overcome by quite small loads and is incapable of doing much work, even at 38C(see Fig 3. 2). The effect of temperature on the duration of the chemical changes during rigor is shown in Fig. 3.2, using the time for half-change of ATP as
these processes is the creatine kinase reaction, which resynthesises ATP from its breakdown product, ADP, and phosphocreatine (PC). The second is the complex process of glycolysis in which the energy for resynthesis comes from the breakdown of glycogen to lactate, with concomitant acidi- fication and fall of muscle pH. The ATP supply remains constant only while PC is still available, but begins to fall as soon as glycolysis is left on its own as the sole source of resynthesis. This phase of declining ATP supply is known as the rapid phase of rigor, because it is then that the stiffening (rigor) of the muscle sets in. It is shown by the first arrow in Fig. 3.1. At temperatures above 12 °C the post-mortem muscle remains in a passive, relaxed state until the ATP supply begins to dwindle at the onset of the rapid phase of rigor. It then begins to shorten actively. At body temperature (38 °C) the shortening can reach 40% or more of the muscle length if unopposed by the force of a load. This so-called ‘rigor shortening’ can be overcome by quite small loads and is incapable of doing much work, even at 38 °C (see Fig. 3.2). The effect of temperature on the duration of the chemical changes during rigor is shown in Fig. 3.2, using the time for half-change of ATP as 46 Meat refrigeration Work done (mJ g–1) during 'rigor' shortening 11 10 9 8 7 6 5 4 0 5 10 15 20 25 30 35 40 Temperature (°C) Time for half change of ATP (h) 1 2 3 5 4 3 2 1 0 Fig. 3.2 Time for half-change of ATP during rigor in beef LD muscle, plotted against temperature. Initial pH = 7.0 in all cases. Curve 1: times for an initial reaction. Curve 2: observed times. Curve 3: work done during shortening (source: Bendall, 1974)
Effect of refrigeration on texture of meat 47 the criterion(see second arrow in Fig 3. 1). From 38C down to 25C the duration increases in the manner for a normal chemical reaction(cf curves 1 and 2). Below this point, however, the experimental points diverge more and more from the predicted line; in other words, the processes take place more quickly. At about 10C the experimental curve actually inverts, so that the rate of chemical change at 2C is greater than at 15C. Such anomalous temperature dependence can only mean that new reactions are occurring with increasing intensity as the temperature is reduced The clue to the nature of the new reactions is given by curve 3, which represents the total work the muscle does during shortening From 16C ur to 38C the total work increases about 2.5-fold, but even so it is very small. By contrast, it increases by a similar amount by going only from 16 to 9C and eight-fold by going to 2C. Quite clearly, therefore, the new reactions intervening below 9-10C are somehow concerned with the increased muscle shortening The shortening occurring below 10C is usually described as"cold short ening or ' cold contracture. In some muscles, it can develop a force of between 1 and 2 Ncm-2. which is between 4 and 8% of the total force devel- oped in a fully stimulated contraction of living muscle. It is supposed to set n because the trigger for contraction is itself highly temperature sensitive and fires spontaneously to an increasing extent as the temperature is reduced below10° This trigger has been shown to be the release of calcium ions, Ca +,from the sarcoplasmic reticulum (Bendall, 1974; Jeacocke, 1986; Offer et al 1988). During use, muscle cells are triggered to contract by calcium ions (Ca)liberated from internal stores within the muscle cell. Although the early stages of activation in muscle contraction in life and cold shortening In a carcass liffer, the final stage, the release of calcium ions, is the same. In resting muscle, the intrafibrillar level of free Ca2* is very low. Most of the total store of intracellular calcium(about 10 Mole)is locked up in highly specialised structures which enwrap each of the 1000 or so fibrils within a muscle fibre(see Fig. 3.3). These structures which are part of the so-called sarcoplasmic reticulum(SR) have transverse connections(SR(T)) with the outer membrane or sarcolemma() of the fibre, so that when a nervous impulse from the motor nerve(MN) arrives at the motor end-plate (EP)it travels in both directions along the sarcolemma and invades the muscle fibre itself via the myriads of these transverse tubules. These tubules are in contact with the longitudinal elements (SR(L) of the SR which enwrap each fibril(see upper fibril in Fig. 3.3). The contact is made via the triad junctions(T)where two dense structures, the so-called lateral cis- terne, are closely opposed to the transverse tubules (Sr(T). It is thought that the lateral cisternae are the storehouse for Ca in the resting muscle In many muscles there are pairs of cisternae at the level of the Z-discs(z) of each sarcomere, so that in a fibril that is 10cm in length there are about 40000 transverse connections and pairs of cisternae
the criterion (see second arrow in Fig. 3.1). From 38°C down to 25°C the duration increases in the manner for a normal chemical reaction (cf. curves 1 and 2). Below this point, however, the experimental points diverge more and more from the predicted line; in other words, the processes take place more quickly.At about 10 °C the experimental curve actually inverts, so that the rate of chemical change at 2°C is greater than at 15°C. Such anomalous temperature dependence can only mean that new reactions are occurring with increasing intensity as the temperature is reduced. The clue to the nature of the new reactions is given by curve 3, which represents the total work the muscle does during shortening. From 16 °C up to 38 °C the total work increases about 2.5-fold, but even so it is very small. By contrast, it increases by a similar amount by going only from 16 to 9°C and eight-fold by going to 2 °C. Quite clearly, therefore, the new reactions intervening below 9–10 °C are somehow concerned with the increased muscle shortening. The shortening occurring below 10 °C is usually described as ‘cold shortening’ or ‘cold contracture’. In some muscles, it can develop a force of between 1 and 2 N cm-2 , which is between 4 and 8% of the total force developed in a fully stimulated contraction of living muscle. It is supposed to set in because the trigger for contraction is itself highly temperature sensitive and fires spontaneously to an increasing extent as the temperature is reduced below 10 °C. This trigger has been shown to be the release of calcium ions, Ca2+ , from the sarcoplasmic reticulum (Bendall, 1974; Jeacocke, 1986; Offer et al., 1988). During use, muscle cells are triggered to contract by calcium ions (Ca2+ ) liberated from internal stores within the muscle cell. Although the early stages of activation in muscle contraction in life and cold shortening in a carcass differ, the final stage, the release of calcium ions, is the same. In resting muscle, the intrafibrillar level of free Ca2+ is very low. Most of the total store of intracellular calcium (about 10-3 Mole) is locked up in highly specialised structures which enwrap each of the 1000 or so fibrils within a muscle fibre (see Fig. 3.3). These structures which are part of the so-called sarcoplasmic reticulum (SR) have transverse connections (SR(T)) with the outer membrane or sarcolemma (S) of the fibre, so that when a nervous impulse from the motor nerve (MN) arrives at the motor end-plate (EP) it travels in both directions along the sarcolemma and invades the muscle fibre itself via the myriads of these transverse tubules.These tubules are in contact with the longitudinal elements (SR(L)) of the SR which enwrap each fibril (see upper fibril in Fig. 3.3). The contact is made via the triad junctions (TJ) where two dense structures, the so-called lateral cisternae, are closely opposed to the transverse tubules (SR(T)). It is thought that the lateral cisternae are the storehouse for Ca2+ in the resting muscle. In many muscles there are pairs of cisternae at the level of the Z-discs (Z) of each sarcomere, so that in a fibril that is 10cm in length there are about 40 000 transverse connections and pairs of cisternae. Effect of refrigeration on texture of meat 47
Meat refrigerati Fig 3.3 Diagram of part of a muscle fibre in longitudinal section to demonstrate the effect of a nervous impulse. For abbreviations see text( source: Bendall, 1974). The effect of an impulse invading the muscle fibres is to cause release of 2+ from the cisternae of each fibril. The Ca+ then diffuses down its elec trochemical gradient, finally reaching the microfilaments of actin(thin )and myosin(thick) shown in the lower, stripped fibril in Fig. 3.3. Here the Ca2+ is temporarily absorbed and thereby triggers the contractile explosion. This consists first of the rapid splitting of ATP to ADP and Pi (inorganic phe phate) at active centres on the myosin filaments. Then there is transduction of some of the free energy released into relative movement of the two sorts of interdigitating filaments. The process is not unlike the explosion of the petrol/air mixture in a car cylinder, when the sparking plug fires. For a crude nalogy, the cylinder can be likened to the myosin filaments and the piston to the actin filaments. However, it is the upstroke which resembles a con- traction and not the downstroke (i.e. the piston is actively pulled or pushed cy Relaxation is the opposite process during which the status quo at the sarcolemma is restored, thereby enabling the lateral cisternae of the Sr to re-accumulate the Ca released during the contraction. They do this by an nctive pumping process, using the energy of ATP-splitting to push Ca up the now adverse electrochemical gradient. Meanwhile fresh ATP has
The effect of an impulse invading the muscle fibres is to cause release of Ca2+ from the cisternae of each fibril. The Ca2+ then diffuses down its electrochemical gradient, finally reaching the microfilaments of actin (thin) and myosin (thick) shown in the lower, stripped fibril in Fig. 3.3. Here the Ca2+ is temporarily absorbed and thereby triggers the contractile explosion. This consists first of the rapid splitting of ATP to ADP and Pi (inorganic phosphate) at active centres on the myosin filaments. Then there is transduction of some of the free energy released into relative movement of the two sorts of interdigitating filaments. The process is not unlike the explosion of the petrol/air mixture in a car cylinder, when the sparking plug fires. For a crude analogy, the cylinder can be likened to the myosin filaments and the piston to the actin filaments. However, it is the upstroke which resembles a contraction and not the downstroke (i.e. the piston is actively pulled or pushed into the cylinder). Relaxation is the opposite process during which the status quo at the sarcolemma is restored, thereby enabling the lateral cisternae of the SR to re-accumulate the Ca2+ released during the contraction. They do this by an active pumping process, using the energy of ATP-splitting to push Ca2+ up the now adverse electrochemical gradient. Meanwhile fresh ATP has 48 Meat refrigeration EP MN S SR (T) SR (L) TJ Fibril Fibril (stripped) ZZZZZ Fig. 3.3 Diagram of part of a muscle fibre in longitudinal section to demonstrate the effect of a nervous impulse. For abbreviations see text (source: Bendall, 1974)
Effect of refrigeration on texture of meat 49 flooded the microfilaments, thus separating them from each other once more and enabling them to slide freely over each other in response to any externally applied force Two features of the calcium-pumping mechanism are of special impor- tance in the present context. First, it is likely that the calcium storage vesi- cles are somewhat leaky, even in resting muscle, so that the calcium pump has to operate continuously, albeit slowly, to keep the intrafibrillar Ca concentration at its low resting level. Second, the calcium pump has an extremely high temperature coefficient, so that at 10"C it works at 1/200th and at 2C at only 1/1000th of the rate at the body temperature of about 38C (Bendall, 1974). Passive diffusion(leakage) out of the pump would only be reduced at 10C to about half the value at 38C. Thus, there is ncreasing chance of net Ca- leakage into the myofilaments as the tem perature falls, the effect becoming dramatic below 10C. Such leakage stimulates the contractile ATP-ase, bringing about the shortening charac- teristic of cold shortening and increasing the production of ADP. The latter in its turn would then stimulate the reactions of ATP synthesis mentioned earlier, so that the timescale in Fig 3. 1 would become shorter and shorter the lower the temperature. This explains the anomalous temperature dependence of the time for half change of ATP, shown in Fig. 3.2 The contracture, which occurs when a rapidly frozen muscle is thawed resembles cold contracture in that it sets in while the level of contractile fuel(ATP) is still high. However, it differs because the amount of work done and force developed are much higher. Withthaw shortening the tem- perature is raised through the calcium release'danger zone from 0 to 10C, whereas in cold shortening it is reduced through this zone. The rate of contracture depends entirely on the rate of thawing. Rapid thawing of a reely suspended, unloaded muscle strip causes very dramatic shortening. often to less than 40% of the frozen'length 3.1.2 Preventing shortening Rapid chilling has many practical advantages but increases the danger of cold shortening. As discussed in Chapter 2 the breakdown of glycogen lactic acid occurs at different speeds in different species. In lamb and beef, the rate is low and the pH falls slowly. Hence, it is only too easy to cool car- casses of these animals, at least on the surface, below 10C when the pH is above 6.2 and such carcasses are extremely vulnerable to cold shortening In pork, the rate of breakdown of glycogen is more rapid and under noderate chilling regimes, cold shortening will not occur. However, pig muscle can cold shorten, and with fast chilling, for example using sub-zero air temperatures, cold shortening has been clearly demonstrated. Another point that should be made is that at an early stage, the surface of the carcass will reach the same temperature as that of the air. Since the air temperature used in chilling is commonly below 10C, there exists the
flooded the microfilaments, thus separating them from each other once more and enabling them to slide freely over each other in response to any externally applied force. Two features of the calcium-pumping mechanism are of special importance in the present context. First, it is likely that the calcium storage vesicles are somewhat leaky, even in resting muscle, so that the calcium pump has to operate continuously, albeit slowly, to keep the intrafibrillar Ca2+ concentration at its low resting level. Second, the calcium pump has an extremely high temperature coefficient, so that at 10 °C it works at 1/200th and at 2 °C at only 1/1000th of the rate at the body temperature of about 38 °C (Bendall, 1974). Passive diffusion (leakage) out of the pump would only be reduced at 10 °C to about half the value at 38°C. Thus, there is an increasing chance of net Ca2+ leakage into the myofilaments as the temperature falls, the effect becoming dramatic below 10 °C. Such leakage stimulates the contractile ATP-ase, bringing about the shortening characteristic of cold shortening and increasing the production of ADP. The latter in its turn would then stimulate the reactions of ATP synthesis mentioned earlier, so that the timescale in Fig. 3.1 would become shorter and shorter the lower the temperature. This explains the anomalous temperature dependence of the time for half change of ATP, shown in Fig. 3.2. The contracture, which occurs when a rapidly frozen muscle is thawed, resembles cold contracture in that it sets in while the level of contractile fuel (ATP) is still high. However, it differs because the amount of work done and force developed are much higher.With ‘thaw shortening’ the temperature is raised through the ‘calcium release’ danger zone from 0 to 10 °C, whereas in cold shortening it is reduced through this zone. The rate of contracture depends entirely on the rate of thawing. Rapid thawing of a freely suspended, unloaded muscle strip causes very dramatic shortening, often to less than 40% of the ‘frozen’ length. 3.1.2 Preventing shortening Rapid chilling has many practical advantages but increases the danger of cold shortening. As discussed in Chapter 2 the breakdown of glycogen to lactic acid occurs at different speeds in different species. In lamb and beef, the rate is low and the pH falls slowly. Hence, it is only too easy to cool carcasses of these animals, at least on the surface, below 10 °C when the pH is above 6.2 and such carcasses are extremely vulnerable to cold shortening. In pork, the rate of breakdown of glycogen is more rapid and under moderate chilling regimes, cold shortening will not occur. However, pig muscle can cold shorten, and with fast chilling, for example using sub-zero air temperatures, cold shortening has been clearly demonstrated. Another point that should be made is that at an early stage, the surface of the carcass will reach the same temperature as that of the air. Since the air temperature used in chilling is commonly below 10°C, there exists the Effect of refrigeration on texture of meat 49
50 Meat refrigerati possibility that cold shortening may occur at the surface, even if it does not occur in the bulk of the meat. Whether or not cold shortening occurs on the surface will often depend on the amount of fat cover over the carcass. This leads to the question of whether shortening can be eliminated whilst retaining high cooling rates? This can be done in two ways: (1)by prevent- ing the underlying cold contraction or(2) by restraining the muscle suffi ciently to prevent the deleterious shortening. The second solution has been veloped with considerable success and has generally involved adopting novel methods of hanging the carcass, such as from the hip(Taylor, 1996) The alternative avenue, of prevention, has found favour with the wide spread application of electrical stimulation(ES)of the carcass immediately after death. This procedure greatly accelerates post-mortem metabolism by stimulating the muscles to contract and relax at a very fast rate, which quickly depletes glycogen and ATP and thus accelerates rigor. Es of the carcass after slaughter can allow rapid chilling without much of the toughening effect of cold shortening. Taylor (1987) and Taylor (1996) provide details of optimum ES treatments. Es has also been shown to be effective in reducing cold shortening in deer meat( Chrystall and Devine 1983; Drew et al-,1988) Although chilling or freezing pre-rigor produces tough meat caused by old shortening or thaw rigor it still has good functional properties(Xiong and Blanchard, 1993 ). It is therefore feasible to manufacture good quality comminuted meat products from hot boned pre-rigor refrigerated beef. Abu-Bakar et al.(1989) found no differences in eating quality between Wieners manufactured from either hot boned beef chilled rapidly using CO2 or brine, or conventionally chilled cold boned beef. As arule of thumb, cooling to temperatures not below 10"C in 10h for beef and lamb(Offer et al, 1988)and in 5h for pork(Honikel, 1986)can avoid cold shortening 3.2 Development of conditioning(ageing) The terms'conditioning,, ageing,, ripening,, maturing'and"the resolution of rigor have all been applied to the practice of storing meat for periods beyond the normal time taken for cooling and setting, to improve its tenderness after cooking. Conditioning imposes a severe limitation on processing conditions because it is a slow process. The deficiencies in the commercial conditioning of meat were clearly lustrated by replies to a questionnaire to sections of the trade in the UK in 1977/8(Dransfield, 1986). At the time a period of storage for wholesale meat was often not specified by retailers. When specified the duration of ad much to do with distribution and turnover of meat and could often be shortened by commercial pressures. At retail, beef was kept for 1-4 days and most beef was sold 3-6 days after slaughter(Palmer, 1978)
possibility that cold shortening may occur at the surface, even if it does not occur in the bulk of the meat. Whether or not cold shortening occurs on the surface will often depend on the amount of fat cover over the carcass. This leads to the question of whether shortening can be eliminated whilst retaining high cooling rates? This can be done in two ways: (1) by preventing the underlying cold contraction or (2) by restraining the muscle suffi- ciently to prevent the deleterious shortening. The second solution has been developed with considerable success and has generally involved adopting novel methods of hanging the carcass, such as from the hip (Taylor, 1996). The alternative avenue, of prevention, has found favour with the widespread application of electrical stimulation (ES) of the carcass immediately after death. This procedure greatly accelerates post-mortem metabolism by stimulating the muscles to contract and relax at a very fast rate, which quickly depletes glycogen and ATP and thus accelerates rigor. ES of the carcass after slaughter can allow rapid chilling without much of the toughening effect of cold shortening. Taylor (1987) and Taylor (1996) provide details of optimum ES treatments. ES has also been shown to be effective in reducing cold shortening in deer meat (Chrystall and Devine, 1983; Drew et al., 1988). Although chilling or freezing pre-rigor produces tough meat caused by cold shortening or thaw rigor it still has good functional properties (Xiong and Blanchard, 1993). It is therefore feasible to manufacture good quality comminuted meat products from hot boned pre-rigor refrigerated beef. Abu-Bakar et al. (1989) found no differences in eating quality between Wieners manufactured from either hot boned beef chilled rapidly using CO2 or brine, or conventionally chilled cold boned beef. As a ‘rule of thumb’, cooling to temperatures not below 10 °C in 10h for beef and lamb (Offer et al., 1988) and in 5 h for pork (Honikel, 1986) can avoid cold shortening. 3.2 Development of conditioning (ageing) The terms ‘conditioning’, ‘ageing’, ‘ripening’, ‘maturing’ and ‘the resolution of rigor’ have all been applied to the practice of storing meat for periods beyond the normal time taken for cooling and setting, to improve its tenderness after cooking. Conditioning imposes a severe limitation on processing conditions because it is a slow process. The deficiencies in the commercial conditioning of meat were clearly illustrated by replies to a questionnaire to sections of the trade in the UK in 1977/8 (Dransfield, 1986). At the time a period of storage for wholesale meat was often not specified by retailers. When specified the duration of storage had much to do with distribution and turnover of meat and could often be shortened by commercial pressures. At retail, beef was kept for 1–4 days and most beef was sold 3–6 days after slaughter (Palmer, 1978). 50 Meat refrigeration
Effect of refrigeration on texture of meat 5 The majority of beef the error had been only partially tenderness would have been improved if the beef had been stored for a further week. Many retailers nowadays condition beef for longer periods, but economic factors often still dictate the time of conditioning. Mechanism of ageing mue major change, which takes place in meat during ageing, occurs in the uscle fibre. Little or no change which can be related to tenderness improvement takes place in the structures which hold the fibres together (the connective tissue, collagen)(Herring et aL, 1967) Conditioning is caused by the presence of proteolytic enzymes in the muscle which slowly catalyse the breakdown of some of the muscle pro- teins. This causes weakening of the muscle so that the meat is more readily pulled apart in the mouth and is therefore tenderer. Two groups of enzymes are thought mainly responsible: calpains, which are active at neutral pH shortly after slaughter, and cathepsins, which are active at acid pH after rigor(Offer et aL, 1988) Dransfield(1994)states that it is generally accepted that tenderisation results from proteolysis by endogenous enzymes. The major problem in identifying the specific enzymes has been that the enzyme activities cannot be measured in meat since they depend on local in situ concentrations of cofactors and inhibitors. However, modelling the activation of calpains shows how tenderness develops and points to methods of optimising its and then calpain II is activated as the concentration of calcium ions ee y development. Calpain I is activated first, at low calcium ion concentratio further. There are enough free calcium ions to activate all of calpain I but only about 30% of calpain II. Tenderisation therefore begins when calpain I starts to be activated, normally at about pH 6.3 or about 6h after slaughter in beef, and rapidly increases as more calpain is activated After about 16h in beef, calpain II becomes activated and causes a further tenderization The calpain-tenderness model shows that in beef longissimus dorsi, most of the tenderisation is caused by calpain I. Approximately 50% of the tenderisation occurs in the first 24h, after which the rate is exponential. The model clearly shows that the ultimate tenderness of the meat will depend on(1)the tenderisation that occurs during chilling and(2)further tenderisation during storage. In extreme cases, for example dark, firm and Iry (dfd) beef, all the tenderisation will occur in stage 1 and none during ageing. The incidence of DFd beef is markedly dependent on the sex of the animal. It occurs in about 1-5% of steers and heifers. 6-10% of cows and 11-15% of young bulls (Tarrant and Sherington, 1981). Rigor develop- ment is very rapid in DFD beef and during normal cooling to an ultimate oH of 7.0, all of the tenderisation occurs before 24 h and no ageing occurs (Dransfield, 1994
The majority of beef therefore had been only partially conditioned and tenderness would have been improved if the beef had been stored for a further week. Many retailers nowadays condition beef for longer periods, but economic factors often still dictate the time of conditioning. 3.2.1 Mechanism of ageing The major change, which takes place in meat during ageing, occurs in the muscle fibre. Little or no change which can be related to tenderness improvement takes place in the structures which hold the fibres together (the connective tissue, collagen) (Herring et al., 1967). Conditioning is caused by the presence of proteolytic enzymes in the muscle which slowly catalyse the breakdown of some of the muscle proteins. This causes weakening of the muscle so that the meat is more readily pulled apart in the mouth and is therefore tenderer.Two groups of enzymes are thought mainly responsible: calpains, which are active at neutral pH shortly after slaughter, and cathepsins, which are active at acid pH after rigor (Offer et al., 1988). Dransfield (1994) states that it is generally accepted that tenderisation results from proteolysis by endogenous enzymes. The major problem in identifying the specific enzymes has been that the enzyme activities cannot be measured in meat since they depend on local in situ concentrations of cofactors and inhibitors. However, modelling the activation of calpains shows how tenderness develops and points to methods of optimising its development. Calpain I is activated first, at low calcium ion concentrations, and then calpain II is activated as the concentration of calcium ions rises further. There are enough free calcium ions to activate all of calpain I but only about 30% of calpain II. Tenderisation therefore begins when calpain I starts to be activated, normally at about pH 6.3 or about 6 h after slaughter in beef, and rapidly increases as more calpain is activated. After about 16 h in beef, calpain II becomes activated and causes a further tenderisation. The calpain-tenderness model shows that in beef longissimus dorsi, most of the tenderisation is caused by calpain I. Approximately 50% of the tenderisation occurs in the first 24h, after which the rate is exponential. The model clearly shows that the ultimate tenderness of the meat will depend on (1) the tenderisation that occurs during chilling and (2) further tenderisation during storage. In extreme cases, for example dark, firm and dry (DFD) beef, all the tenderisation will occur in stage 1 and none during ageing. The incidence of DFD beef is markedly dependent on the sex of the animal. It occurs in about 1–5% of steers and heifers, 6–10% of cows and 11–15% of young bulls (Tarrant and Sherington, 1981). Rigor development is very rapid in DFD beef and during normal cooling to an ultimate pH of 7.0, all of the tenderisation occurs before 24 h and no ageing occurs (Dransfield, 1994). Effect of refrigeration on texture of meat 51
52 Meat refrigeration 3.2.2 Prediction of tenderness There is great interest in the development of any measurement method that can be applied soon after slaughter, which will predict the tenderness of meat. Many laboratory techniques(Dransfield, 1986; Dransfield, 1996) have been used to detect changes in the muscle down to the molecular level. However there is no routine test which can indicate how much of the ten derising has occurred or, more usefully, how much longer a piece of meat must be stored In 1988 Marsh et al. proposed that the pH in the longissimus dorsi at 3 h post-mortem may have a use as a predictor of tenderness. However, sub equent studies involving large numbers of animals(Marshall and Tatum 1991; Shackelford et al., 1994) found that it was not highly correlated with tenderness. Thus it is not a reliable method of identifying potentially tough or tender meat Cross and belk reviewed. in 1994. all the non-invasive technologies capable of objectively determining yield or the eating quality of the lean meat in live animals or carcasses in commercial situations. Technologies included X-ray, nuclear magnetic resonance, electrical conductivity analy sis, near-infrared reflectance, video image analysis, optical fat/lean probes, optical connective tissue probes, bioelectrical impedance analysis, velocity of sound and elastography Elastography, which measures the internal dis- placement of small tissue elements in response to externally applied stress using ultrasonic pulses, was thought to have the best potential in the future. It may be capable of depicting muscle structure at the muscle bundle level, and of detecting differences in elasticity of muscle bundles, connective tissue amounts and the quality of intramuscular fat 3.2.3 Consumer appreciation of ageing Consumer assessments of unaged beef are variable, ranging from 'moder- ately tough'to'moderately tender' whilst beef conditioned for 9 days at 1C receives largely favourable reactions, being scored"'moderatelyto very' tender(Dransfield, 1985). Consumer panels, however, have rarely been used to assess the factors affecting conditioning. They have usually been measured by laboratory taste panels and mechanical tests. A type of shear'test is frequently used on cooked meat and the measurements are usually well related to sensory assessments(Dransfield, 1986) The results of Dransfield (1986) illustrate the effect of conditioning on a taste panel's assessment of texture of 3 beef joints. Tenderness increased in all 3 joints (Table 3. 1) and these changes were reflected in increases in the overall acceptability In further work roasted sirloin joints from a six-year-old cow were com- ared with an 18-month-old heifer at storage times of 2-15 days at 2C. By 3.2).By 15 days both joints showed significant vement in tenderness(Table
3.2.2 Prediction of tenderness There is great interest in the development of any measurement method that can be applied soon after slaughter, which will predict the tenderness of meat. Many laboratory techniques (Dransfield, 1986; Dransfield, 1996) have been used to detect changes in the muscle down to the molecular level. However, there is no routine test which can indicate how much of the tenderising has occurred or, more usefully, how much longer a piece of meat must be stored. In 1988 Marsh et al. proposed that the pH in the longissimus dorsi at 3 h post-mortem may have a use as a predictor of tenderness. However, subsequent studies involving large numbers of animals (Marshall and Tatum, 1991; Shackelford et al., 1994) found that it was not highly correlated with tenderness. Thus it is not a reliable method of identifying potentially tough or tender meat. Cross and Belk reviewed, in 1994, all the non-invasive technologies capable of objectively determining yield or the eating quality of the lean meat in live animals or carcasses in commercial situations. Technologies included X-ray, nuclear magnetic resonance, electrical conductivity analysis, near-infrared reflectance, video image analysis, optical fat/lean probes, optical connective tissue probes, bioelectrical impedance analysis, velocity of sound and elastography. Elastography, which measures the internal displacement of small tissue elements in response to externally applied stress using ultrasonic pulses, was thought to have the best potential in the future. It may be capable of depicting muscle structure at the muscle bundle level, and of detecting differences in elasticity of muscle bundles, connective tissue amounts and the quality of intramuscular fat. 3.2.3 Consumer appreciation of ageing Consumer assessments of unaged beef are variable, ranging from ‘moderately tough’ to ‘moderately tender’ whilst beef conditioned for 9 days at 1 °C receives largely favourable reactions, being scored ‘moderately’ to ‘very’ tender (Dransfield, 1985). Consumer panels, however, have rarely been used to assess the factors affecting conditioning. They have usually been measured by laboratory taste panels and mechanical tests. A type of ‘shear’ test is frequently used on cooked meat and the measurements are usually well related to sensory assessments (Dransfield, 1986). The results of Dransfield (1986) illustrate the effect of conditioning on a taste panel’s assessment of texture of 3 beef joints. Tenderness increased in all 3 joints (Table 3.1) and these changes were reflected in increases in the overall acceptability. In further work roasted sirloin joints from a six-year-old cow were compared with an 18-month-old heifer at storage times of 2–15 days at 2°C. By 8 days, the heifer joint showed significant improvement in tenderness (Table 3.2). By 15 days both joints showed significant improvements. 52 Meat refrigeration