Meat quality during processing

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Received for publication August 1, 1998.Accepted for publication December 8, 1998.1To whom correspondence should be addressed: asams@poultry.

tamu.eduAbbreviation Key: ATP = adenosine triphosphate; ES = electrical

stimulation; PSE = pale, soft, and exudative;

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 ofany food animal such as poultry needs to consider theeffects of the muscle changes on the use of the muscle asmeat. If a treatment could increase muscle growth butthe increased meat was of poor quality, then the increasein production would be of little value. Muscle is ofparticular concern because not only is it the tissue ofgreatest value for food, but it also is an excitable tissueand responsive to its environment. Many of theseresponses can be quite deleterious to meat quality. Thebasis for the response of muscle to its environment is inpostmortem metabolism and the simultaneous develop-ment of rigor mortis. Although the animal may die in amatter of minutes following the neck cut, its muscle cellscontinue to metabolize and respond for hours afterrespiratory cessation and brain death. During these

hours, the muscle has energy that fuels the responses tothe environment, most commonly in terms of color andtexture.

Heat, transportation, and handling all contribute tothe preslaughter stress that can alter color, texture, andrelated protein functionality. Stunning is another pres-laughter factor that has a large effect on postmortemmetabolism and meat quality. After death, chilling cantoughen meat while it adds juiciness, and aging preventsthe meat from toughening in response to deboning.Electrical stimulation is a recent beneficial innovationthat reduces the need for aging by accelerating postmor-tem energy depletion and reducing the muscle’s abilityto toughen during deboning. This paper reviews theresponsiveness of the muscle and gives examples of howthese 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

To fully understand the implications of improve-ments and problems in muscle growth and develop-ment, one must consider their impact on the ultimateapplication of the muscle as a human food. Many of themuscle’s physiological processes and abnormalitieseither impact meat quality directly, or impact it throughthe response of the muscle to the processing plantenvironment. The bird’s preslaughter environment, itsinherent muscle physiology, and the processing plantactivities all interact to determine the quality of theresulting meat. Therefore, it is important to rememberthat meat quality is an extremely complex subject thatinvolves not only muscle anatomy and metabolism, butalso engineering, psychology, and marketing, to name afew. Other presentations in this symposium address thedirect impacts of muscle growth and development onmeat quality through muscle mass and structure. Thisreview will focus on the ways muscle responds to itsprocessing plant environment to influence poultry meatquality.

THE CONVERSION OF MUSCLE TO MEAT

Rigor Mortis Development —Death of the Muscle Cell

When a bird is subjected to the activities of theprocessing plant, it initially responds at the level of theintact organism. However, as the animal dies, theindividual tissues and cells continue to react to theirenvironment. As the tissues and cells die, they lose theirability to react to their environment and muscle becomesmeat. The process of rigor mortis development is centralto the process of muscle death and to meat quality.

For the purpose of understanding the death processand its effect on meat quality, the muscle can be thought ofas an aggregate of individual muscle cells, with each ofthese cells undergoing its own response to the environ-ment and death. As the animal dies due to loss of bloodand the resulting anoxia, the muscle cells continue torespire, producing and consuming adenosinetriphosphate (ATP), the primary currency of cellularenergy. As cellular oxygen is depleted, the cell dependsalmost solely on anaerobic metabolism for the production

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SYMPOSIUM: MUSCLE GROWTH AND DEVELOPMENT 799

of its needed ATP (Lawrie, 1991). As glycogen is depletedand 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 furtherglycolysis, and ATP production ceases. However, ATPconsumption continues, most importantly in the role ofATP as a plasticizer to dissociate actin and myosin,maintaining muscle extensibility. When the ATP concen-tration falls to a critical level [1 mM/g (Hamm, 1982)],there is insufficient ATP to dissociate all of the actin andmyosin. These proteins begin to remain complexed asactomyosin, and the onset phase of rigor mortis begins.These complexes continue to accumulate until the ATPconcentration reaches about 0.1 mM/g, at which time rigoris developed. Once rigor mortis has developed, the muscleis not extensible (cannot “relax”) and becomes stiff.Lawrie (1991) and Foegeding et al. (1996) have providedexcellent overviews of the entire rigor mortis process.

The stiffness of a muscle in rigor mortis is a function ofthe extent of myofibrillar overlap of thick and thinfilaments, which is determined by the strength of theopposing muscle groups (Cason et al., 1997), the presenceof skeletal attachments (Stewart et al., 1984), the presenceof external restraints (Papa and Fletcher, 1988), andtemperature (Wood and Richards, 1974; Bilgili et al, 1989;Dunn et al., 1995). These factors serve to prevent orincrease myofibrillar overlap, sarcomere shortening, andcontraction that can occur during rigor mortis develop-ment. Opposing muscles, skeletal attachments, and exter-nal restraints are all various forms of resistance to filamentoverlap and sarcomere shortening. Although “heat short-ening” is a possibility if prerigor meat is cooked, its effecton tenderness depends on the rate of heating and thecondition of the meat before cooking (deFremery andPool, 1960; Khan, 1971; Lawrie, 1991). In addition to theeffect of sarcomere shortening on toughness via filamentoverlap, it also increases water loss, which can furtherincrease toughness (Honikel et al., 1968; Dunn et al., 1993).

The muscles are initially subject to physiologicalregulation, but eventually experience a variety of unregu-lated stimuli and respond to them. Gregory (1989)reported that the bird is dead between 1.5 and 6 min afterneck cut, depending on the method of slaughter, andusing brain failure as the definition of death. Regardless ofthe time to death, the bird is unresponsive long before themuscle cells become unresponsive through rigor mortisdevelopment (3 to 6 h). Furthermore, this time to “celldeath” differs between red, aerobic and white, anaerobicmuscles (Kijowski et al., 1982; Sams and Janky, 1990). Thisdifference suggests that the muscle cell maintains theability to metabolically respond to its environment, evenbeyond the development of rigor mortis. Finally, theindividual protein molecules and aggregates can respondto 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

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

PROCESSING TECHNIQUESAND MEAT QUALITY

Antemortem Factors

Although the antemortem environment customarilyrefers to the farm, the 12 h prior to slaughter is a time ofintense activity that can be extremely stressful to the bird.During this critical period, catching, transportation,unloading, and hanging can reduce quality and yield ifperformed improperly. The reduced quality and yield is inpart due to the fact that these operations are still largelymanual, and are performed outside, exposed to theweather. Preslaughter heat stress has been reported toaccelerate rigor mortis development, reduce water hold-ing ability, and increase paleness in poultry meat(Northcutt et al., 1994; McKee and Sams, 1997a,b). Chen etal. (1991) reported that preslaughter feed withdrawal andexercise reduced the postmortem pH decline, producingdark, firm, and dry meat in ducks. Transportation stresshas also been reported to reduce tenderness and increaselightness of chicken meat (Ehinger, 1977; Cashman et al.,1989). Kannan et al. (1997) observed that crating broilersfor 1 h produced lighter breast meat than when cratingthem for 3 h. These authors also reported that providingthe birds a 4-h rest period between transport andslaughter reduced plasma levels of corticosterone. Moranand Bilgili (1995) reported that birds transported beforeslaughter had a lower incidence of carcass bruising,presumably because the level of physical activity isreduced during transportation.

Stunning is another antemortem procedure that canhave profound effects on meat quality. The purpose ofstunning is to immobilize the bird for automated killingand to render it insensible to pain or stress. The mostcommon stunning technique is to pass an electric currentfrom a saline bath into the bird’s head, through its body,and out its feet to the shackle, which is grounded (Bilgili etal., 1989). The current causes generalized contractions thatcan obviously have a marked effect on muscle characteris-tics. Because of the forceful contractions it causes, electricstunning has the capability of inducing hemorrhages andbroken bones if excessive current or low frequencies areused (Veercamp and de Vries, 1983; Gregory and Wilkins,1989a,b; Rawles et al., 1995a,b). The death struggleaccelerates rigor mortis development, whereas preventingthis struggle with electric stunning retards rigor mortisdevelopment (Ma and Addis, 1973; Papinaho andFletcher, 1995). Although Lee et al. (1979) reported that therigor-slowing effect of electric stunning reduced rigorshortening and therefore improved tenderness of breastmeat at 24 h postmortem, this represents little benefit forthe processor wanting to debone the meat with minimalaging. The effect of electric stunning on meat quality andcarcass damage depends largely on the electric conditions

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used. The most prominent factor is stunning with highvoltages (150 V, 125 mA per bird), which produces morecarcass damage and a greater rigor-slowing effect thanlower voltages (15 to 60 V, 10 to 45 mA per bird) (Bilgili etal., 1989; Papinaho and Fletcher, 1995). Other factorsaffecting carcass damage include stunning duration(Young and Buhr, 1997; Papinaho and Fletcher, 1995),wave form and frequency (Gregory, 1989; Kettlewell andHallworth; 1990), and bird uniformity (Rawles et al.,1995a,b).

An alternate form of stunning that has receivedattention in recent years is gas stunning. Birds are eitherexposed to the anesthetic gas carbon dioxide at levels anddurations sufficient to induce sedation (Mohan Raj et al.,1990; Poole and Fletcher, 1995; Kang and Sams, 1999) or toa mixture of carbon dioxide and argon at levels anddurations to deprive the bird of oxygen and asphyxiatethem while they are unconscious (Mohan Raj andGregory, 1990; Mohan Raj et al., 1992; Raj, 1994). Althoughstudies have reported that gas stunning causes less carcassdamage than electrical stunning, this difference seems toresult 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 highlevels of carcass damage (broken bones and hemorrhages)from the electric stun and a subsequent lower incidencewith gas stunning. In contrast, studies using loweramperages commonly found in the U.S. (12 mA per bird)report lower rates of electric stunner-induced damage andno improvement from gas stunning. The studies that haveused intermediate amperages (35 to 50 mA per bird)produced intermediate results. These trends suggest thatEuropean processors required to stun with higher amper-ages would benefit from gas stunning, whereas U.S.processors generally would not.

Because of the reduced oxygen conditions used to stunbirds with gas, there is the potential to influence thedevelopment of rigor mortis and the need for aging.However, the effect seems to depend on the gas used andon the electric stun to which it is compared. Poole andFletcher (1995) and Kang and Sams (1999) observed thatstunning with carbon dioxide slowed rigor mortisdevelopment compared to no stunning, or to a35-mA electric stun. This slowing was thought to possiblyresult from the anesthetic effect of the carbon dioxide. Incontrast, Raj et al. (1990) reported that stunning withcarbon dioxide accelerated rigor mortis developmentcompared to a 107 mA electric stun. Using a gas mixture of30 to 40% carbon dioxide and 50 to 60% argon (to achieve aresidual oxygen concentration of < 2%), Raj (1994), Raj etal. (1997), and Poole and Fletcher (1998) reported that gasstunning accelerated rigor mortis and reduced thenecessary aging time before deboning, but only whencompared to a high amperage (80 to 125 mA) electric stun.Poole and Fletcher (1998) observed that there was no rigormortis acceleration when gas stunning was compared to alow amperage stun (10 to 15 mA).

Chilling

Broiler carcasses are chilled to below 4 C within 1.5 h ofdeath with water immersion chilling or 2.5 h of death withair chilling. Because of their larger size and more commonuse in restructured products, turkeys reach this targettemperature much later, at 3 to 6 h after death. Rapidchilling of poultry mainly serves to reduce microbialgrowth, but also serves to increase the firmness of themuscle and stiffness of the skeleton to facilitate automaticportioning and deboning. Obviously, the degree ofstiffness of rigor mortis is a factor in this facilitation aswell.

Exposure to low temperatures when ATP is still presentin the muscle cell, such as prior to rigor mortisdevelopment, has been demonstrated to toughen the meatthrough a process termed “cold shortening” (Hamm,1982). At low temperatures, the sarcoplasmic reticularmembranes become less efficient at sequestering Ca2+ andallow it to “leak” into the myofibrillar space. If ATP ispresent, the Ca2+ initiates the contraction cycle and causesthe sarcomere to shorten. Although broiler breast muscleis primarily composed of aW (white) fibers (Sams andJanky, 1991; Smith et al., 1993), which are less prone to coldshortening than red fibers (Bendall, 1975), broiler breastmuscle has still been shown to experience cold shortening(Wood and Richards, 1974; Bilgili et al., 1989).

Although genetics and the antemortem environmentboth contribute to the development of pale, soft, andexudative (PSE) meat, slow chilling rates also contribute tothis abnormal meat. Bendall and Wismer-Pederson (1962)and McKee and Sams (1997a,b) demonstrated that rigordevelopment at elevated temperatures, as with slowchilling, resulted in meat with pale color and poor water-holding properties similar to PSE in pork and turkey,respectively. Because genetically abnormal animals maydevelop PSE meat with ideal chilling rates, poor chillingmay represent a greater problem, by inducing PSE meat inotherwise normal-glycolyzing animals (Offer, 1991).

Aging

Aging, or maturation, is the procedure of storing intactcarcasses or breast halves for several hours at refrigeratedtemperatures before deboning to allow for the develop-ment of rigor mortis. Lowe (1948) and deFremery andPool (1960) provided early reports that poultry meatharvested before the development of rigor mortis wasobjectionably tough. Reducing the need for aging wouldgreatly expedite boneless meat production. deFremeryand Pool (1960) measured the time course of rigor mortisdevelopment according to biochemical changes and theloss of extensibility and determined that the onset of rigormortis occurred between 2.5 and 4 h postmortem.Kijowski et al. (1982) reported that chicken breast muscleATP concentration declined to its minimum value by 2 hpostmortem, whereas lactic acid levels required between 4and 8 h postmortem to reach their ultimate plateau.

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Several studies have attempted to determine theminimum amount of aging needed before deboning ofbroiler breast meat. Stewart et al. (1984), Lyon et al. (1985),and Dawson et al. (1987) all reported that some timebetween 2 and 4 h postmortem was the critical period afterwhich deboning did not cause toughening. This time wastaken as a working indication of ATP depletion and rigormortis development, and resulted in recommendations tostore intact carcasses at refrigerated temperatures (< 4 C)for at least 4 h prior to deboning. It should be noted thatthis minimum aging time evolved as the time needed toprevent any statistically detectable change in shear value.This is not to say that it is the minimum time needed toproduce meat that would be considered “tender” toconsumers, because there are many degrees of tenderness(Lyon and Lyon, 1990a,b, 1991) and the definition oftender meat varies among cultures and geographicregions.

Despite the initial toughening effect of prerigordeboning, McKee et al. (1997) observed that after 3 d ofadditional refrigerated storage, the meat tendernessimproved sufficiently to be considered tender by con-sumers. The authors provided evidence that the normaldegradative processes involved in rigor resolution wereresponsible for the tenderization. These prolongedchanges in the muscle are further evidence of the dynamicnature of muscle and its ability to remain responsive to itsenvironment. However, the tenderness did not return tothe same level as that of meat deboned after rigor mortishad developed. Also, such a storage period may not befeasible for processors who freeze, cook, or ship theirproduct immediately.

Electrical Stimulation

Postmortem electrical stimulation (ES) of meat car-casses was first developed in the 1950s and became widelyused by the red meat industry in the 1970s (Chrystall andDevine, 1985). Electrical stimulation pulses electricitythrough a carcass immediately after death, causinggeneralized muscle contractions throughout the carcass.These contractions serve to exercise the muscles and cantherefore affect rigor mortis and toughness development.Cross (1979) reviewed three theories by which postmor-tem ES may tenderize meat. First, ES accelerates ATPdepletion, resulting in the prevention of cold shortening.Secondly, ES hastens the decline of post-mortem pH whilemuscle temperatures are still high, possibly enhancing theaction of endogenous proteases responsible for tenderiza-tion during the aging process. Finally, ES can tenderizemeat by inducing physical disruption of muscle fibers.

Electrical stimulation of poultry has been the subject ofthree recent reviews that provide in-depth analyses of themechanisms, equipment, and implications of this technol-ogy (Li et al., 1993; Sams, 1999). There have been manymethods of using ES with poultry, with varying degrees ofeffectiveness, reported in the literature. These systems canbe grouped into two general categories. 1) Electical

stimulation systems that use “low” amperages of 0 to 200mA per bird to induce contractions, exercise the muscle,and accelerate rigor mortis development. Although rigoris accelerated and the toughening of the resulting meat issignificantly reduced, it is not reduced to a sufficientdegree to allow the elimination of aging. 2) Electricalstimulation systems using “high” amperages of 350 to 500mA per bird induce such forceful contractions that themuscle not only exercises to accelerate ATP depletion, buttears itself. Birkhold and Sams (1995) presented transmis-sion electron micrographs of ES-treated muscle in whichthe myofilaments were torn and contracture bands hadformed. This physical disruption tenderizes the meat andthe acceleration of rigor mortis from the exercise preventstoughening. The combination of these two mechanismshas generally made high amperage ES more effective thanlow amperage ES in reducing the need for an agingperiod. An additional advantage of high amperage ES isthat it requires only 15 s of kill line space, whereas the lowamperage system requires from 1 to 15 min of kill linespace. Both of these general types of ES systems are incommercial use.

CONCLUSIONS

Muscle is a dynamic tissue and it responds to itsenvironment before, during, and after the death of theanimal. This responsiveness can have a large impact,positive or negative, on the quality and value of theresulting meat. Efforts to improve muscle growth anddevelopment should consider the effects of any im-provements on the muscle as meat and on the behaviorof this tissue during processing.

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Young, L. L., and R. J. Buhr, 1997. Effects of stunning durationon quality characteristics of early deboned chicken fillets.Poultry Sci. 76:1052–1055.

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Meat quality during processing