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