Meat Quality During Processing

Meat Quality During Processing A. R. SAMS1 Department of Poultry Science, Texas A&M University System, College Station, Texas 77843-2472 ABSTRACT The study of growth and development of any food animal such as poultry needs to consider the effects of the muscle changes on the use of the muscle as meat. If a treatment could increase muscle growth but the increased meat was of poor quality, then the increase in production would be of little value. Muscle is of particular concern because not only is it the tissue of greatest value for food, but it also is an excitable tissue and responsive to its environment. Many of these responses can be quite deleterious to meat quality. The basis for the response of muscle to its environment is in postmortem metabolism and the simultaneous development of rigor mortis. Although the animal may die in a matter of minutes following the neck cut, its muscle cells continue to metabolize and respond for hours after respiratory cessation and brain death. During these

hours, the muscle has energy that fuels the responses to the environment, most commonly in terms of color and texture. Heat, transportation, and handling all contribute to the preslaughter stress that can alter color, texture, and related protein functionality. Stunning is another preslaughter factor that has a large effect on postmortem metabolism and meat quality. After death, chilling can toughen meat while it adds juiciness, and aging prevents the meat from toughening in response to deboning. Electrical stimulation is a recent beneficial innovation that reduces the need for aging by accelerating postmortem energy depletion and reducing the muscle’s ability to toughen during deboning. This paper reviews the responsiveness of the muscle and gives examples of how these responses can hurt or help meat quality.

(Key words: meat quality, muscle metabolism, processing, meat color, meat texture) 1999 Poultry Science 78:798–803

INTRODUCTION

THE CONVERSION OF MUSCLE TO MEAT

To fully understand the implications of improvements and problems in muscle growth and development, one must consider their impact on the ultimate application of the muscle as a human food. Many of the muscle’s physiological processes and abnormalities either impact meat quality directly, or impact it through the response of the muscle to the processing plant environment. The bird’s preslaughter environment, its inherent muscle physiology, and the processing plant activities all interact to determine the quality of the resulting meat. Therefore, it is important to remember that meat quality is an extremely complex subject that involves not only muscle anatomy and metabolism, but also engineering, psychology, and marketing, to name a few. Other presentations in this symposium address the direct impacts of muscle growth and development on meat quality through muscle mass and structure. This review will focus on the ways muscle responds to its processing plant environment to influence poultry meat quality.

Rigor Mortis Development — Death of the Muscle Cell When a bird is subjected to the activities of the processing plant, it initially responds at the level of the intact organism. However, as the animal dies, the individual tissues and cells continue to react to their environment. As the tissues and cells die, they lose their ability to react to their environment and muscle becomes meat. The process of rigor mortis development is central to the process of muscle death and to meat quality. For the purpose of understanding the death process and its effect on meat quality, the muscle can be thought of as an aggregate of individual muscle cells, with each of these cells undergoing its own response to the environment and death. As the animal dies due to loss of blood and the resulting anoxia, the muscle cells continue to respire, producing and consuming adenosine triphosphate (ATP), the primary currency of cellular energy. As cellular oxygen is depleted, the cell depends almost solely on anaerobic metabolism for the production

Received for publication August 1, 1998. Accepted for publication December 8, 1998. 1To whom correspondence should be addressed: asams@poultry. tamu.edu

Abbreviation Key: ATP = adenosine triphosphate; ES = electrical stimulation; PSE = pale, soft, and exudative;

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SYMPOSIUM: MUSCLE GROWTH AND DEVELOPMENT

of its needed ATP (Lawrie, 1991). As glycogen is depleted and lactic acid, the end product of anaerobic metabolism, accumulates due to the lack of blood flow to remove it, sarcoplasmic pH decreases to a level that inhibits further glycolysis, and ATP production ceases. However, ATP consumption continues, most importantly in the role of ATP as a plasticizer to dissociate actin and myosin, maintaining muscle extensibility. When the ATP concentration falls to a critical level [1 mM/g (Hamm, 1982)], there is insufficient ATP to dissociate all of the actin and myosin. These proteins begin to remain complexed as actomyosin, and the onset phase of rigor mortis begins. These complexes continue to accumulate until the ATP concentration reaches about 0.1 mM/g, at which time rigor is developed. Once rigor mortis has developed, the muscle is not extensible (cannot “relax”) and becomes stiff. Lawrie (1991) and Foegeding et al. (1996) have provided excellent overviews of the entire rigor mortis process. The stiffness of a muscle in rigor mortis is a function of the extent of myofibrillar overlap of thick and thin filaments, which is determined by the strength of the opposing muscle groups (Cason et al., 1997), the presence of skeletal attachments (Stewart et al., 1984), the presence of external restraints (Papa and Fletcher, 1988), and temperature (Wood and Richards, 1974; Bilgili et al, 1989; Dunn et al., 1995). These factors serve to prevent or increase myofibrillar overlap, sarcomere shortening, and contraction that can occur during rigor mortis development. Opposing muscles, skeletal attachments, and external restraints are all various forms of resistance to filament overlap and sarcomere shortening. Although “heat shortening” is a possibility if prerigor meat is cooked, its effect on tenderness depends on the rate of heating and the condition of the meat before cooking (deFremery and Pool, 1960; Khan, 1971; Lawrie, 1991). In addition to the effect of sarcomere shortening on toughness via filament overlap, it also increases water loss, which can further increase toughness (Honikel et al., 1968; Dunn et al., 1993). The muscles are initially subject to physiological regulation, but eventually experience a variety of unregulated stimuli and respond to them. Gregory (1989) reported that the bird is dead between 1.5 and 6 min after neck cut, depending on the method of slaughter, and using brain failure as the definition of death. Regardless of the time to death, the bird is unresponsive long before the muscle cells become unresponsive through rigor mortis development (3 to 6 h). Furthermore, this time to “cell death” differs between red, aerobic and white, anaerobic muscles (Kijowski et al., 1982; Sams and Janky, 1990). This difference suggests that the muscle cell maintains the ability to metabolically respond to its environment, even beyond the development of rigor mortis. Finally, the individual protein molecules and aggregates can respond to factors in their chemical environment such as pH, temperature, and water activity to alter their solubility, water holding ability, binding ability, color, texture, etc. (Lawrie, 1991; Claus et al., 1994; Foegeding et al., 1996). Thus, the muscle or its components never really fully lose

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their ability to respond to the environment, and meat quality can be influenced until consumption.

PROCESSING TECHNIQUES AND MEAT QUALITY

Antemortem Factors Although the antemortem environment customarily refers to the farm, the 12 h prior to slaughter is a time of intense activity that can be extremely stressful to the bird. During this critical period, catching, transportation, unloading, and hanging can reduce quality and yield if performed improperly. The reduced quality and yield is in part due to the fact that these operations are still largely manual, and are performed outside, exposed to the weather. Preslaughter heat stress has been reported to accelerate rigor mortis development, reduce water holding ability, and increase paleness in poultry meat (Northcutt et al., 1994; McKee and Sams, 1997a,b). Chen et al. (1991) reported that preslaughter feed withdrawal and exercise reduced the postmortem pH decline, producing dark, firm, and dry meat in ducks. Transportation stress has also been reported to reduce tenderness and increase lightness of chicken meat (Ehinger, 1977; Cashman et al., 1989). Kannan et al. (1997) observed that crating broilers for 1 h produced lighter breast meat than when crating them for 3 h. These authors also reported that providing the birds a 4-h rest period between transport and slaughter reduced plasma levels of corticosterone. Moran and Bilgili (1995) reported that birds transported before slaughter had a lower incidence of carcass bruising, presumably because the level of physical activity is reduced during transportation. Stunning is another antemortem procedure that can have profound effects on meat quality. The purpose of stunning is to immobilize the bird for automated killing and to render it insensible to pain or stress. The most common stunning technique is to pass an electric current from a saline bath into the bird’s head, through its body, and out its feet to the shackle, which is grounded (Bilgili et al., 1989). The current causes generalized contractions that can obviously have a marked effect on muscle characteristics. Because of the forceful contractions it causes, electric stunning has the capability of inducing hemorrhages and broken bones if excessive current or low frequencies are used (Veercamp and de Vries, 1983; Gregory and Wilkins, 1989a,b; Rawles et al., 1995a,b). The death struggle accelerates rigor mortis development, whereas preventing this struggle with electric stunning retards rigor mortis development (Ma and Addis, 1973; Papinaho and Fletcher, 1995). Although Lee et al. (1979) reported that the rigor-slowing effect of electric stunning reduced rigor shortening and therefore improved tenderness of breast meat at 24 h postmortem, this represents little benefit for the processor wanting to debone the meat with minimal aging. The effect of electric stunning on meat quality and carcass damage depends largely on the electric conditions

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used. The most prominent factor is stunning with high voltages (150 V, 125 mA per bird), which produces more carcass damage and a greater rigor-slowing effect than lower voltages (15 to 60 V, 10 to 45 mA per bird) (Bilgili et al., 1989; Papinaho and Fletcher, 1995). Other factors affecting carcass damage include stunning duration (Young and Buhr, 1997; Papinaho and Fletcher, 1995), wave form and frequency (Gregory, 1989; Kettlewell and Hallworth; 1990), and bird uniformity (Rawles et al., 1995a,b). An alternate form of stunning that has received attention in recent years is gas stunning. Birds are either exposed to the anesthetic gas carbon dioxide at levels and durations sufficient to induce sedation (Mohan Raj et al., 1990; Poole and Fletcher, 1995; Kang and Sams, 1999) or to a mixture of carbon dioxide and argon at levels and durations to deprive the bird of oxygen and asphyxiate them while they are unconscious (Mohan Raj and Gregory, 1990; Mohan Raj et al., 1992; Raj, 1994). Although studies have reported that gas stunning causes less carcass damage than electrical stunning, this difference seems to result from the type of electric stun used for comparison. Studies that have used high amperages (100 mA per bird), consistent with European guidelines, have observed high levels of carcass damage (broken bones and hemorrhages) from the electric stun and a subsequent lower incidence with gas stunning. In contrast, studies using lower amperages commonly found in the U.S. (12 mA per bird) report lower rates of electric stunner-induced damage and no improvement from gas stunning. The studies that have used intermediate amperages (35 to 50 mA per bird) produced intermediate results. These trends suggest that European processors required to stun with higher amperages would benefit from gas stunning, whereas U.S. processors generally would not. Because of the reduced oxygen conditions used to stun birds with gas, there is the potential to influence the development of rigor mortis and the need for aging. However, the effect seems to depend on the gas used and on the electric stun to which it is compared. Poole and Fletcher (1995) and Kang and Sams (1999) observed that stunning with carbon dioxide slowed rigor mortis development compared to no stunning, or to a 35-mA electric stun. This slowing was thought to possibly result from the anesthetic effect of the carbon dioxide. In contrast, Raj et al. (1990) reported that stunning with carbon dioxide accelerated rigor mortis development compared to a 107 mA electric stun. Using a gas mixture of 30 to 40% carbon dioxide and 50 to 60% argon (to achieve a residual oxygen concentration of < 2%), Raj (1994), Raj et al. (1997), and Poole and Fletcher (1998) reported that gas stunning accelerated rigor mortis and reduced the necessary aging time before deboning, but only when compared to a high amperage (80 to 125 mA) electric stun. Poole and Fletcher (1998) observed that there was no rigor mortis acceleration when gas stunning was compared to a low amperage stun (10 to 15 mA).

Chilling Broiler carcasses are chilled to below 4 C within 1.5 h of death with water immersion chilling or 2.5 h of death with air chilling. Because of their larger size and more common use in restructured products, turkeys reach this target temperature much later, at 3 to 6 h after death. Rapid chilling of poultry mainly serves to reduce microbial growth, but also serves to increase the firmness of the muscle and stiffness of the skeleton to facilitate automatic portioning and deboning. Obviously, the degree of stiffness of rigor mortis is a factor in this facilitation as well. Exposure to low temperatures when ATP is still present in the muscle cell, such as prior to rigor mortis development, has been demonstrated to toughen the meat through a process termed “cold shortening” (Hamm, 1982). At low temperatures, the sarcoplasmic reticular membranes become less efficient at sequestering Ca2+ and allow it to “leak” into the myofibrillar space. If ATP is present, the Ca2+ initiates the contraction cycle and causes the sarcomere to shorten. Although broiler breast muscle is primarily composed of aW (white) fibers (Sams and Janky, 1991; Smith et al., 1993), which are less prone to cold shortening than red fibers (Bendall, 1975), broiler breast muscle has still been shown to experience cold shortening (Wood and Richards, 1974; Bilgili et al., 1989). Although genetics and the antemortem environment both contribute to the development of pale, soft, and exudative (PSE) meat, slow chilling rates also contribute to this abnormal meat. Bendall and Wismer-Pederson (1962) and McKee and Sams (1997a,b) demonstrated that rigor development at elevated temperatures, as with slow chilling, resulted in meat with pale color and poor waterholding properties similar to PSE in pork and turkey, respectively. Because genetically abnormal animals may develop PSE meat with ideal chilling rates, poor chilling may represent a greater problem, by inducing PSE meat in otherwise normal-glycolyzing animals (Offer, 1991).

Aging Aging, or maturation, is the procedure of storing intact carcasses or breast halves for several hours at refrigerated temperatures before deboning to allow for the development of rigor mortis. Lowe (1948) and deFremery and Pool (1960) provided early reports that poultry meat harvested before the development of rigor mortis was objectionably tough. Reducing the need for aging would greatly expedite boneless meat production. deFremery and Pool (1960) measured the time course of rigor mortis development according to biochemical changes and the loss of extensibility and determined that the onset of rigor mortis occurred between 2.5 and 4 h postmortem. Kijowski et al. (1982) reported that chicken breast muscle ATP concentration declined to its minimum value by 2 h postmortem, whereas lactic acid levels required between 4 and 8 h postmortem to reach their ultimate plateau.

SYMPOSIUM: MUSCLE GROWTH AND DEVELOPMENT

Several studies have attempted to determine the minimum amount of aging needed before deboning of broiler breast meat. Stewart et al. (1984), Lyon et al. (1985), and Dawson et al. (1987) all reported that some time between 2 and 4 h postmortem was the critical period after which deboning did not cause toughening. This time was taken as a working indication of ATP depletion and rigor mortis development, and resulted in recommendations to store intact carcasses at refrigerated temperatures (< 4 C) for at least 4 h prior to deboning. It should be noted that this minimum aging time evolved as the time needed to prevent any statistically detectable change in shear value. This is not to say that it is the minimum time needed to produce meat that would be considered “tender” to consumers, because there are many degrees of tenderness (Lyon and Lyon, 1990a,b, 1991) and the definition of tender meat varies among cultures and geographic regions. Despite the initial toughening effect of prerigor deboning, McKee et al. (1997) observed that after 3 d of additional refrigerated storage, the meat tenderness improved sufficiently to be considered tender by consumers. The authors provided evidence that the normal degradative processes involved in rigor resolution were responsible for the tenderization. These prolonged changes in the muscle are further evidence of the dynamic nature of muscle and its ability to remain responsive to its environment. However, the tenderness did not return to the same level as that of meat deboned after rigor mortis had developed. Also, such a storage period may not be feasible for processors who freeze, cook, or ship their product immediately.

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stimulation systems that use “low” amperages of 0 to 200 mA per bird to induce contractions, exercise the muscle, and accelerate rigor mortis development. Although rigor is accelerated and the toughening of the resulting meat is significantly reduced, it is not reduced to a sufficient degree to allow the elimination of aging. 2) Electrical stimulation systems using “high” amperages of 350 to 500 mA per bird induce such forceful contractions that the muscle not only exercises to accelerate ATP depletion, but tears itself. Birkhold and Sams (1995) presented transmission electron micrographs of ES-treated muscle in which the myofilaments were torn and contracture bands had formed. This physical disruption tenderizes the meat and the acceleration of rigor mortis from the exercise prevents toughening. The combination of these two mechanisms has generally made high amperage ES more effective than low amperage ES in reducing the need for an aging period. An additional advantage of high amperage ES is that it requires only 15 s of kill line space, whereas the low amperage system requires from 1 to 15 min of kill line space. Both of these general types of ES systems are in commercial use.

CONCLUSIONS Muscle is a dynamic tissue and it responds to its environment before, during, and after the death of the animal. This responsiveness can have a large impact, positive or negative, on the quality and value of the resulting meat. Efforts to improve muscle growth and development should consider the effects of any improvements on the muscle as meat and on the behavior of this tissue during processing.

Electrical Stimulation Postmortem electrical stimulation (ES) of meat carcasses was first developed in the 1950s and became widely used by the red meat industry in the 1970s (Chrystall and Devine, 1985). Electrical stimulation pulses electricity through a carcass immediately after death, causing generalized muscle contractions throughout the carcass. These contractions serve to exercise the muscles and can therefore affect rigor mortis and toughness development. Cross (1979) reviewed three theories by which postmortem ES may tenderize meat. First, ES accelerates ATP depletion, resulting in the prevention of cold shortening. Secondly, ES hastens the decline of post-mortem pH while muscle temperatures are still high, possibly enhancing the action of endogenous proteases responsible for tenderization during the aging process. Finally, ES can tenderize meat by inducing physical disruption of muscle fibers. Electrical stimulation of poultry has been the subject of three recent reviews that provide in-depth analyses of the mechanisms, equipment, and implications of this technology (Li et al., 1993; Sams, 1999). There have been many methods of using ES with poultry, with varying degrees of effectiveness, reported in the literature. These systems can be grouped into two general categories. 1) Electical

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