Baixe Pou...nce - f 10 effects of stunning and slaughter methods on carcass an e outras Notas de estudo em PDF para zootecnia, somente na Docsity! 231 © CAB International 1999. Poultry Meat Science (eds R.I. Richardson and G.C. Mead) CHAPTER 10 Effects of stunning and slaughter methods on carcass and meat quality A.B.M. Raj Division of Food Animal Science, School of Veterinary Science, University of Bristol, Langford, Bristol BS40 5DU, UK PRODUCTION AND CONSUMPTION IN EUROPE Chicken and turkey meat production and consumption have been increasing steadily in Europe (Table 10.1). Poultry meat is leaner and is less expensive than red meats and carcass portioning and deboning have given the consumer more choice (white breast and darker thigh or drumstick meat) and opportunity to purchase preferred quantity. In addition, developments in further processing of poultry meat has given them varieties of products with different taste and texture (whole, flaked or ground meat). IMPORTANCE OF CARCASS AND MEAT QUALITY Consumer awareness of quality has also improved considerably over recent years and the appearance of carcass or meat has become more important in Table 10.1. Broiler chicken and turkey meat production and per capita consumption in European Union (12 Member States) (Sources: Foreign Agricultural Service, Commodity and Marketing Programs, Dairy, Livestock and Poultry Division). Year 1995 1996(p) 1997(f) Production (1000 metric tonnes in ready-to-cook equivalents) Broiler 5280 5460 5530 Turkey 1561 1625 1668 Per capita consumption (kg) Broiler 14.8 15.4 15.6 Turkey 3.6 3.7 3.8 p = preliminary; f = forecast. 232 A.B.M. Raj determining consumer acceptability and retail price. In general, carcass and meat quality problems can originate from various factors starting from catching on the farm to the evisceration process in the processing plant (Gregory and Wilkins, 1990a). However, immersion or convection chilling, in comparison with air chilling, can mask the appearance defects (Moran and Bilgili, 1995). In the USA, immersion chilling of broiler carcass is commonly used irrespective of whether the meat is sold fresh or frozen. By contrast, in Europe, all the fresh broiler meat should be air chilled and only frozen carcasses can be immersion chilled, and therefore, carcass appearance is more important in this continent. Turkey carcasses however can be immersion chilled in Europe. Some of the carcass and meat quality defects are listed in Table 10.2 and the commercial significance of these defects varies according to market outlet. However, the prevalence of appearance defects can lead to downgrading and loss of value of fresh carcasses. Other conditions, such as haemorrhagic wing veins, may warrant trimming and loss of valuable portions. The broken pectoral bones (mainly furculum) come away with the breast fillets when the carcasses are prepared as portions and haemorrhages appear as dark spots when the fillets are cooked. These are undesirable from the consumer’s point of view. Therefore, the fillets will have to be examined, bone fragments removed and haemorrhagic areas in the breast fillets trimmed prior to packaging. These procedures involve additional man-power, costs and loss in valuable meat. Broken coracoid bones frequently cause haemorrhaging in breast muscles due to either rupture of blood vessels adjacent to the bone or splinters damaging the muscle. Residual blood retained in the breast meat, occurring due to poor bleed out, might be squeezed out during retail packaging and this may appear either as a lump of clot or serum smear over the meat. In addition, engorged blood vessels may appear as dark streaks in cooked meats. These could be visually less appealing to the consumer. Table 10.2. Carcass and meat quality defects leading to downgrading or trimming. Carcass appearance defects: Red wingtips Red pygostyles Red feather tracts Engorged wing veins Haemorrhagic wing veins Haemorrhage in shoulders Claw damage and torn skin Haemorrhage in the muscles: Breast Leg Dislocated or broken bones: Pectoral bones Wing bones Leg bones Effects of stunning and slaughter methods 235 (Sparrey et al., 1993). This could account for the highly variable carcass and meat quality seen under commercial electrical waterbath stunning conditions. The Silsoe Research Institute in the UK has developed a constant current stunner which will control the current flow through individual birds in a waterbath stunner and, when used commercially, can improve the situation. STUNNING METHODS The conventional method of poultry processing involves uncrating and shackling live birds and then electrically stunning them using a multiple bird waterbath stunner prior to neck cutting. The concept, mode of operation and recent developments in electrical waterbath stunners have been described by Bilgili (1992) and Sparrey et al. (1992, 1993). It should be noted that the amount of current (amperage) received by individual birds, rather than the voltage supplied to a waterbath stunner, determines the effectiveness of the stun and the carcass and meat quality. In Europe, certain minimum RMS (root mean square) currents have been recommended by the Commission, however, they are yet to be made law (Table 10.3). Nonetheless, stunning poultry under commercial condition is a statutory requirement and the minimum current applied varies from 80 to 120 mA per broiler applied for a minimum of 4 s. In the USA, there is no statutory requirement to stun poultry; however, the majority of poultry processors voluntarily implement electrical stunning using 5—6 mA per broiler for a minimum of 10 s (Heath et al., 1994). It is very likely that various wave forms and frequencies of electric currents used commercially could have different effects on the birds. However, while using a 50 Hz sinusoidal alternating current in a waterbath stunner, these minimum currents will induce cardiac arrest in about 90% of birds (Gregory and Wilkins, 1989a, 1990b). There are two reasons for selecting cardiac arrest as a criterion for setting the minimum currents. First, it is the only method which produces rapid brain death following stunning, at least in chickens (Gregory and Wotton, 1986). Secondly, induction of cardiac arrest at stunning eliminates the chances of resumption of consciousness in birds due to delayed or inappropriate neck cutting. However, some scientists argue, on bird welfare grounds, that these current levels do not abolish brain responsiveness (somatosensory evoked potentials) immediately after stunning, and therefore, the recommended currents should be not less than 120 mA per Table 10.3. Minimum RMS (root mean square) electric current (per bird) recommended in Europe for stunning chickens and turkeys. Waterbath stunning Head-only stunning Chickens 100 mA 240 mA Turkeys 150 mA 400 mA Minimum duration 4 s 3 s 236 A.B.M. Raj chicken and 250 mA per turkey (Gregory and Wotton, 1990, 1991). Alternatively, novel methods of killing poultry using gas mixtures (controlled atmosphere stunning) have been approved in the UK, under The Welfare of Animals (during Slaughter or Killing) Regulations (1995), for killing chickens and turkeys in their transport containers (HMSO, 1995). They are: 1. anoxia induced with 90% argon or other inert gases in air (leaving 8% residual nitrogen and 2% residual oxygen from air); 2. a mixture of 30% carbon dioxide and 60% argon or other inert gases in air (leaving 8% residual nitrogen and 2% residual oxygen from air). There is no doubt that killing poultry in transport containers using gas mixtures will enable shackling to be performed on relaxed carcasses and this would eliminate the live bird handling at the processing plants. These two gas mixtures have been selected on the basis of the following. 1. The induction of unconsciousness with anoxia is smooth and the birds, given a free choice, spontaneously enter this atmosphere and are killed. 2. The induction of unconsciousness with the carbon dioxide—argon mixture is rapid and the majority of birds, given a free choice, enter this atmosphere and are killed. 3. Birds find an atmosphere containing a high concentration of carbon dioxide aversive, and given a free choice, the majority of birds avoid it (Raj et al., 1992a; Raj and Gregory, 1994; Raj, 1996). BLOOD LOSS It is known that, in comparison with mammals, poultry have unusually long blood clotting times. This is because they rely on tissue thromboplastin for homeostasis rather than on the generation of plasma thromboplastin as in mammals (Bigland, 1964). Therefore, any differences seen in the rate of bleed- out or total blood loss can be attributed to a combination of factors, such as, whether the birds have been killed or just stunned (resulting in relaxed carcasses with fibrillated hearts or birds with non-fibrillated hearts showing tremors during bleeding, respectively), the delay between killing and neck cutting, blood vessels cut and the bleed-out time permitted before scalding. A poor bleed-out can increase the prevalence of some of the carcass downgrading conditions. Nevertheless, in the UK, bleed out times of 90 s for chickens and 180 s for turkeys must be provided following electrical stunning. In the USA, a minimum of 90 s bleed out time has been recommended for chickens (Kuenzel et al., 1978). In Europe, it is a common practice to cut the necks of poultry at the back of their heads severing a vertebral artery, or cut unilaterally on the ventral side of the necks severing one carotid artery and one jugular vein. By contrast, in the USA, a complete ventral neck cut, severing both carotid arteries and both jugular veins in the neck, is commonly practised. The bleed-out times are generally based on the fact that, if the birds were just stunned and had their necks cut, brain death should occur through Effects of stunning and slaughter methods 237 exsanguination before they enter the scald tank. Therefore, duration of unconsciousness with a stunning procedure must be longer than the sum of the time taken to perform neck cutting and the time to onset of brain death through blood loss (Gregory and Wotton, 1986). Prevalence of red skin conditions has been reported to be due to an acute inflammatory reaction occurring in live birds entering the scald tank (Griffiths, 1985). In the past, at least in Europe, poultry have been stunned using 50—60 Hz sine wave alternating current. Alternating current frequencies of up to 100 Hz can be more effective in inducing fibrillation of hearts (cardiac arrest) than higher frequencies of AC or DC. However, as described earlier, stunning with low frequency currents can lead to muscle contractions, and consequently, haemorrhaging in breast muscles. Therefore, the poultry industry in Europe, as in the USA, has opted to use high frequency (> 300 Hz) AC or DC. Reports concerning blood loss in broilers following electrical stunning are conflicting. For example, Griffiths et al. (1985) found that neither bleed-out nor the residual blood in the carcass meat varied significantly between the birds which were stunned or killed with an electric current. Gregory and Wilkins (1989b) examined blood loss using seven different neck cutting methods in broilers and found that the total bleed-out after 2.25 min of neck cutting was similar in all the treatments. Dickens and Lyon (1993) also found that the total blood loss was very similar in chickens stunned with either 50 V AC or 200 V AC. By contrast, Veerkamp and de Vries (1983) found that blood loss at 285 s after neck cutting decreased as the stunning voltage increased from 75 to 200 V. Craig and Fletcher (1997) reported that, after 150 s bleed-out time, broilers stunned with 125 mA (50 Hz sine wave AC) lost on average 0.3% less blood than those stunned with 5—6 mA (500 Hz square wave pulsed DC). It is possible that, in birds which suffer cardiac arrest at stunning, the wings begin to relax very soon and hang lower than the shoulder joints which can result in pooling of blood in the wing veins. On the other hand, in birds which have non-fibrillated hearts, the wings are held stiffly against the breast muscle, and this, in association with the tremors can improve the drainage from the wing veins. These differences are apparent in the initial rate of bleed-out, however, the total blood loss is very similar (Raj and Johnson, 1997). In the case of turkeys, when compared with 50 Hz AC, high frequency electrical stunning results in a faster rate of bleed-out and higher total blood loss (Mouchoniere et al., 1998). Nevertheless, irrespective of the stunning current parameters, cutting all the major blood vessels in the neck should become mandatory under commercial conditions. Under the gas or controlled atmosphere killing of poultry in their transport containers, birds would leave the unit in large numbers. These crates will have to be emptied and carcasses shackled prior to neck cutting. Therefore, it is very likely that the time between end of killing and neck cutting is to be longer than the corresponding time under the electrical stunning systems. In addition to this delay in neck cutting, physiological effects of gases can also affect the bleed-out. For example, Kotula et al. (1957) and Kotula and Helbacka (1966) found that the birds stunned with carbon dioxide in air retained a higher percentage of blood in the internal organs. It has been 240 A.B.M. Raj reported that the duration of 5, 10, 20 or 40 s of electrical stunning with 100 V AC had no effect on the broiler breast muscle pH at 15 min post-mortem. By contrast, Young and Buhr (1997) stunned broilers with 50 V AC for either 0, 2, 4, 6, 8 or 10 s and found that the breast muscle pH measured after 1 h post- mortem increased with the duration of electrical stunning. However, it should be noted that, in these studies, there are differences in live bird handling, application of stunning, carcass chilling and pH measurement. Papinaho et al. (1995) reported that, when broilers were restrained during electrical stunning or had their breast muscles denervated surgically prior to stunning, there was no significant effect of current (50 or 125 mA per bird) on the mean pH at 15 min post-mortem. Therefore, these authors concluded that the muscular activity occurring peri-mortem rather than the stunning current per se is responsible for any differences seen in the rate of rigor development. However, it should be noted that, in the denervated breast muscles, the effect of stimulation of the central nervous system and the spinal motor neurones has been removed, and therefore, the effect of electrical stunning on the denervated muscles would have been a consequence of direct stimulation of muscles only. Wing flapping occurring during gas or controlled atmosphere stunning can accelerate the rate of pH fall early post-mortem depending on the physiological properties of the gas used in the stunning atmosphere. For example, in comparison with 45% carbon dioxide in air, stunning of broiler chickens with 90% argon in air resulted in significantly lower breast muscle pH at 20 min post-mortem (6.37 vs. 5.91; Raj et al., 1990b). This is possibly due to: 1. argon induced anoxia at the cellular level triggering the break down of ATP, and thus, anaerobic glycolysis; 2. glycolysis being accelerated by the wing flapping occurring under the anoxic conditions. Together, these effects appear to result in rigor development (pH < 6.0) within 20 min post-mortem in broilers (Raj et al., 1990b, 1991) and turkeys (Raj, 1994a). In these studies, anoxia was induced in chickens and turkeys by exposing them very rapidly (less than 20 s) to < 2% oxygen in argon, whereas, Poole and Fletcher (1995) did not find rapid rigor development in broilers stunned with argon or nitrogen when anoxia was induced gradually (in about 1 min). In that study, although broilers exposed to anoxia showed severe wing flapping, the times to onset and duration of convulsions were found to be different. This implies that the rate of induction of anoxia at the cellular level may also have an effect on the rate of rigor development. In contrast with the effect of true anoxia induced with argon, addition of carbon dioxide to argon (hypercapnic anoxia) appears to retard the rate of rigor development. For example, when 10%, 20% or 30% carbon dioxide was added to argon, leaving 5% residual oxygen in all the mixtures, it was found that the mean pH values in the breast muscles at 20 min post-mortem were 6.38, 6.60 and 6.61, respectively (Raj et al., 1992b). A more recent study showed that stunning of broilers with a mixture of 30% carbon dioxide and 60% argon, Effects of stunning and slaughter methods 241 leaving a residual oxygen of 2%, resulted in a mean breast muscle pH of 6.4 and 6.1 at 2 and 4 h post-mortem, respectively (Raj et al., 1997). Poole and Fletcher (Georgia, 1997, personal communication) also found that broilers stunned with 30% carbon dioxide and 60% argon reached a pH of below 6.0 at 5 h post-mortem. These results show that, in comparison with the pure anoxia induced by argon, carbon dioxide gas inhibits the rate of rigor development. Sams and Dzuik (1995) reported that electrical stimulation (applied after bleed-out) of broilers killed with carbon dioxide (> 70%) failed to breakdown ATP or glycogen, as determined using R-value (adenosine to inosine com- pounds ratio) and pH at 1 h post-mortem. Based on these, the authors concluded that carbon dioxide killing negated the acceleration of rigor development induced by the electrical stimulation treatment. The exact mechanism by which carbon dioxide retards the rate of rigor development in poultry is not known. However, there is some evidence to suggest that the acidosis induced by carbon dioxide gas reduced the twitch force and the ATP cost per twitch by 50% in skinned cat skeletal muscle fibres when 10 s trains of twitches (2 s at 1 Hz) were induced (Harkema et al., 1997). In addition, carbon dioxide acidosis can cause: 1. activation of potassium channels; 2. inhibition of calcium channels; 3. increases in intramitochondrial potential for ATP synthesis; 4. inhibition of the release of glycolytic enzymes either directly or through inactivation of the sarcoplasmic reticulum. Theoretically, points 1 and 2 will induce muscle relaxation, and 3 and 4 will sustain ATP and glycogen reserves in muscles. Further research should clarify these effects in poultry. However, the relaxation of frog skeletal muscle was slower when intracellular pH was lowered, independent of other metabolites, with carbon dioxide, and the relaxation was prolonged when the tetanus duration was longer (Curtin, 1988). On the other hand, at low temperatures (12—20°C), a decreased tetanus tension and enhanced tension relaxation of rat skeletal muscle have been reported during carbon dioxide-induced acidosis (Ranatunga, 1987). CARCASS APPEARANCE DEFECTS The inevitable live bird handling is a universal welfare problem associated with the conventional electrical stunning systems. Wing flapping prior to stunning can lead to a higher prevalence of red wingtips in carcasses (Gregory et al., 1989). Tipping of live birds from transport modules, hanging on tight shackles, and prestun electric shocks received at the entrance to waterbath stunners can cause wing flapping. The effects of stunning current using 50 Hz sine wave AC on the prevalence of carcass and meat quality defects have been reported by Gregory and Wilkins (1989a; 1990b) for broilers and turkeys (Tables 10.5 and 10.6). These studies have shown that the prevalence of carcass downgrading 242 A.B.M. Raj conditions tend to be relatively high when a stunning current of 121—161 mA per broiler is applied. In the case of turkeys, the prevalence of haemorrhagic wings alone increased when the stunning current was raised from 75 to 250 mA per bird. The prevalence of carcass downgrading conditions was similar in broilers when the frequency of current was varied from 50, 250 or 350 Hz delivered with a pulsed square wave DC (Table 10.7). Gas or controlled atmosphere killing, in comparison with the waterbath electrical stunning, reduced the prevalence of some of the carcass downgrading conditions in broilers (Table 10.8) and turkeys (Table 10.9). However, Raj et al. (1990b) showed that the prevalence of carcass downgrading conditions are low in manually plucked broiler carcasses which implies that the severity of plucking could contribute to the occurrence of carcass downgrading conditions. For example, any residual blood in the superficial vessels will be massaged by the plucker fingers and exacerbate the conditions. Table 10.5. Effect of stunning current (50 Hz sine wave AC) on the prevalence of carcass and meat quality defects in broilers (laboratory study) (after Gregory and Wilkins, 1989a). Average stunning current (mA) 45 85 121 141 161 181 220 Percentage of carcasses with: Appearance defects: Red wingtips 7 8 15 16 8 9 9 Haemorrhagic wing veins 4 7 11 16 12 8 9 Haemorrhagic shoulder 12 — 22 23 14 18 13 Haemorrhage in breast muscle 15 10 17 25 23 28 19 Table 10.6. Effect of stunning current (50 Hz sine wave AC) on the prevalence of carcass and meat quality defects in turkeys (laboratory study) (after Gregory and Wilkins, 1990b). Average stunning current (mA) 75 150 250 Percentage of carcasses with: Appearance defects: Red wingtips 0 1 1 Haemorrhagic wings 3 5 8 Shoulder haemorrhage 6 9 5 Haemorrhage in breast muscle 6 8 20 Broken furculum 6 4 5 Broken coracoid 10 3 6 Effects of stunning and slaughter methods 245 Hillebrand et al. (1996) found that the overall prevalence of haemorrhaging in breast muscles was very similar in broilers stunned on shackles using either head-only using tongs (25 V, 200 Hz) or whole-body using a waterbath stunner (100 V, 200 Hz). However, waterbath stunning resulted in haemorrhages in the middle of pectoralis major and minor muscles, whereas head-only stunning resulted in haemorrhages in the distal end of the muscles and in the shoulder joints. The shoulder haemorrhages were attributed to the wing flapping which occurred during the epileptic phase following head-only electrical stunning while the birds were hanging on shackles. Wing flapping occurring while the birds are in transport containers during exposure to gas mixtures does not appear to result in haemorrhaging either in the shoulder or in the breast muscles of broiler chickens (Table 10.8) and turkeys (Table 10.9). It appears that wing flapping occurring on shackles is potentially more damaging to the breast muscles than that occurring in transport containers. However, in contrast with the tetanic muscle contractions occurring during waterbath electrical stunning, the clonic convul- sions (wing flapping) occurring during gas or controlled atmosphere stunning are twitch contractions and they appear to be less detrimental to carcass and meat quality. This is probably because wing flapping occurring during controlled atmosphere stunning is very similar to that of flight and the breast muscles are capable of coping with it, irrespective of the severity, over a short duration. Although haemorrhages occurring in leg muscles of birds stunned using waterbath electrical stunners are mainly attributed to the shackling procedure (Wilson and Brunson, 1968), electrical stunning per se can increase the prevalence of haemorrhaging in leg muscles (Walker et al., 1993). Head-only, in comparison with waterbath, electrical stunning on shackles results in lower incidence of this condition (Hillebrand et al., 1996). Broken Bones The prevalence of birds with one or more broken pectoral bones increased from 25% to 39% as the stunning current was raised from 74 to 269 mA per broiler using a 50 Hz sine wave AC (Gregory and Wilkins, 1989a). However, in the case of turkeys, the prevalence of broken pectoral bones is not found to be related to the amount of stunning current, and between 75 and 250 mA per turkey, the incidence appears to be very similar (Gregory and Wilkins, 1990b). The number of broken bones occurring in electrically stunned birds which are processed under commercial conditions (Table 10.8) appears to be significantly more due to the variations in the current flow through the individual bird. Stunning current frequencies of 50, 250 or 350 Hz applied with a pulsed square wave DC resulted in a similar prevalence of broken bones in broilers when the average current was 109—112 mA per bird (Gregory et al., 1991). 246 A.B.M. Raj By contrast, gas or controlled-atmosphere stunning of broilers in transport containers results in a smaller number of carcasses with broken pectoral bones and reduces the number of broken pectoral bones per carcass (Raj et al., 1990a). It is likely that, under the batch stunning of chickens in transport containers using gas mixtures, the cushioning effect provided by the birds against each other reduces the incidence of broken bones which can otherwise occur due to wing flapping. Therefore, the stocking density in a crate could determine the incidence of dislocated or broken wing bones in gas stunned birds. BREAST MEAT COLOUR Colour of poultry meat, including the effect of electrical and carbon dioxide stunning methods, has been recently reviewed by Froning (1995). The effects of stunning broilers with either argon or carbon dioxide—argon mixture, in comparison with electrical stunning, is presented in Table 10.10. It appears that, excluding haemorrhagic areas, stunning methods do not adversely affect the colour of breast meat. COOKING LOSS Stunning methods do not appear to have a significant effect on cooking loss. For example, Raj et al. (1990b) found that stunning broilers with either an electric current, 90% argon in air, or 45% carbon dioxide in air did not significantly affect percentage cooking loss measured in the whole carcasses (on average 260, 261 and 262 g kg−1, respectively). In the same study, it was also found that hand plucked carcasses lost significantly less weight during cooking than those that were mechanically plucked (249 and 272 g kg−1). In another study, it was found that stunning broilers with either a mixture of 30% carbon dioxide and 60% argon in air or an electric current did not affect the cooking loss, however, regardless of the stunning method, breast muscles filleted early post-mortem (2, 3, 4 or 5 h) lost more weight during cooking than those filleted at 24 h post- Table 10.10. Effect of stunning methods on the colour of broiler breast meat (after Raj et al., 1990b, 1997). Laboratory study Commercial study Anoxia 30% CO2 and CIE colour parameters (90% argon in air) Electricala 60% argon in air Electricala L* 52.16 54.02 54.3 55.5 a* 2.00 1.80 3.4 2.9 b* –0.63 –0.23 4.0 4.0 a Laboratory study: 120 mA per bird, 50 Hz sine wave AC; commercial study: 80 mA per bird, 50 Hz clipped sine wave AC. Effects of stunning and slaughter methods 247 mortem (16 vs. 14%; Raj et al., 1997). Therefore, it appears that the severity of plucking and filleting time, rather than stunning method, can significantly increase the cooking loss in broilers. However, this does not agree with the results of Papinaho et al. (1996) who reported that, in electrically stunned broilers, the percentage cooking loss was not significantly affected by the breast muscle filleting time (on average 20%). The difference between these two studies could be attributed to the differences in cooking methods. Electrical stunning of turkeys resulted in slightly higher cooking loss than stunning with either 90% argon in air or a mixture of 30% carbon dioxide and 60% argon in air (Raj, 1994a). However, in that study, the effect of bird type was more significant than the effect of stunning methods. For example, large toms had a significantly higher cooking loss (37%) than the medium size toms (32%) and hens (30%). In the same study, interactions were found between bird type and filleting time, stunning method and filleting time, and bird type, stunning method and filleting time. However, the differences in the percentage cooking loss due to these interactions were found to be smaller than the effects of bird type. OBJECTIVE TEXTURE Poultry breast meat texture is an important eating quality criterion, though consumers’ preferences vary widely. The major factors affecting texture are: 1. rate of rigor development; 2. rate of carcass chilling; 3. filleting time. Stunning methods can affect texture mainly by altering the time course of rigor development and the extent of muscle contraction and relaxation occurring during early post-mortem. The rate of chilling can change the severity of muscle tension development and the time course of relaxation early post-mortem. Separation of breast muscles from carcasses, before the completion of rigor, would remove the restraint offered by the skeleton and this could result in irreversible rigor and/or cold shortening to produce tough meat. In addition, since pectoralis major and p. minor muscles have opposite functions, cutting of supracoracoideus tendon at the humeral insertion soon after evisceration results in contraction in both the muscles and toughening of cooked meat (Cason et al., 1997). It has been reported that meat obtained from electrically stunned birds was more tender than that obtained from birds which were not stunned (Klose et al., 1972). Within electrical stunning, stunning broilers with either 50 V AC when compared with 200 V AC and filleting breast muscles at either 1, 2 or 4 h post-mortem, resulted in mean texture (Warner-Bratzler shear values) values of 10 vs. 11, 7 vs. 10, and 5 vs. 5, respectively (Dickens and Lyon, 1993). These results show that, irrespective of the stunning voltage, early filleting resulted in tougher meat and that the high voltage stunning produced tougher meat than the low voltage stunning when filleting was performed at 1 and 2 h 250 A.B.M. Raj electrically stunned broilers filleted at the corresponding times post-mortem. It would appear that, to produce a tender breast meat, filleting of broilers stunned with this gas mixture should be performed at 4 h post-mortem. By contrast, Poole and Fletcher (Georgia, 1997, personal communication) found that breast meat from broilers stunned with this gas mixture and filleted up to 5 h post-mortem produced tougher meat than those filleted at 24 h post-mortem. In the case of turkeys, both argon and carbon dioxide—argon mixture stunning resulted in tender meat when filleting was performed at 2 h post-mortem (Table 10.12). Further research should be carried out under similar stunning, carcass processing, chilling and cooking conditions to clarify the reasons for this difference between chickens and turkeys. SENSORY PROPERTIES OF MEAT Eating quality of breast meat evaluated using a trained sensory panel can yield more information than the instrumentally measured texture values. In general, the eating quality of breast meat improves considerably when aged on the carcass. It is known that calcium activated neutral proteinases (calpains) and lysosomal acidic proteinases (cathepsins) degrade myofibrillar and cytoskeletal proteins, and hydrolyse myofibrils, respectively. Liu et al. (1994) showed that structures of chicken semitendinosus muscle endomysium and perimysium disintegrate into thin sheets within 12 h post-mortem, and as a result, gaps appear in the endomysium and perimysium when examined under a light microscope. Under a scanning electron microscope, it was found that, in chicken semitendinosus muscle aged for 12 h at 4°C, the endomysium resolved into individual collagen fibrils and the perimysial sheets separated into collagen fibres (Liu et al., 1995). These structural changes in the connective tissue were minimal until 6 h post-mortem. As mentioned earlier, stunning methods, particularly anoxia, can result in the development of tenderness early post-mortem. In addition, Fagan et al. (1992) found that when isolated chick skeletal muscle was subjected to anoxia the ATP content fell to undetectable concentrations and the muscle suffered a faster rate (35% to 124%) of proteolysis (breakdown of total protein as well as myofibrillar proteins). These scientists have suggested that the calcium- activated proteases (calpains) were involved in the enhanced degradation of total muscle proteins in ATP-depleted tissue. In the same study, it was found that the proteolysis occurring in the ATP-depleted muscles was also mediated through an ATP-independent non-lysosomal process that required calcium. It is likely that, under an argon-stunning situation, anoxia induced at the cellular level results in an efflux of calcium from the sarcoplasmic reticulum and mitochondria and this in association with high carcass temperature during processing enhances proteolysis. It has been shown that, in comparison with electrical stimulation of broiler carcasses soon after plucking, stunning broilers with anoxia under commercial conditions produced tender breast meat when filleted at 2 h post-mortem (Raj et al., 1992c). Sensory profiling of breast meat from turkeys showed that Effects of stunning and slaughter methods 251 stunning with argon-induced anoxia resulted in more tender meat with powdery residue, whereas, electrical stunning produced significantly less tender meat with pulpy residue (Raj and Nute, 1995). 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