《肉制品冷冻技术》（英文版） Part 3 Effect of refrigeration on texture of meat

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 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 phos￾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 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 con￾traction 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 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 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 tem￾perature falls, the effect becoming dramatic below 10 °C. 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.With ‘thaw shortening’ the tem￾perature 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 car￾casses 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 prevent￾ing 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 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 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 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 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 develop￾ment 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