Rigor mortis development at elevated temperatures induces pale exudative turkey meat characteristics

PROCESSING AND PRODUCTS Rigor Mortis Development at Elevated Temperatures Induces Pale Exudative Turkey Meat Characteristics S. R. MCKEE and A. R. SAMS1 Department of Poultry Science, Texas A&M University System, College Station, Texas 77843-2472 values, which persisted through 4 h. By 1 h, the 40 C treatment pH and glycogen levels were lower than the 0 C treatment; however, they did not differ from those of the 20 C treatment. Increased L* values indicated that color became more pale by 2 h post-mortem in the 40 C treatment when compared to the 20 and 0 C treatments. Drip loss, cook loss, and shear value were increased whereas sarcomere lengths were decreased as a result of the 40 C treatment. These findings suggested that elevated post-mortem temperatures during processing resulted in acceleration of rigor mortis and biochemical changes in the muscle that produced pale, exudative meat characteristics in turkey.

(Key words: pale, soft, exudative, turkey, rigor mortis) 1998 Poultry Science 77:169–174

Post-mortem temperature was noted to be the most important processing factor affecting rigor development and overall meat quality (Lee et al., 1979). de Femery and Pool (1960) have shown that birds exposed to postmortem temperatures of 37 to 41 C during processing exhibit a rapid rate of post-mortem glycolysis and an early onset of rigor mortis. The large size of tom turkey carcasses may impede the chilling process. Moreover, evidence from pork studies indicates that acceleration of the onset of rigor mortis in combination with high carcass temperatures is concomitant with development of PSE meat characteristics (Briskey, 1964). Specifically, Briskey (1964) postulated that the low pH combined with high carcass temperatures resulting from rapid early post-mortem metabolism caused extensive protein denaturation in the muscle. The loss of protein functionality due to extensive protein denaturation is considered to be the primary factor associated with the development of PSE meat characteristics (Warris and Brown 1987; Fernandez et al., 1994; Santos et al., 1994). In addition, Bendell and Wismer-Pedersen (1962) revealed that rigor development in pork muscles at elevated postmortem temperatures of 37 C always resulted in PSE meat characteristics.

INTRODUCTION Further processed or value-added products are the fastest growing area in the turkey industry. However, with the shift in the market from whole birds to further processed products, processors have observed an increased number of problems with meat quality (Ferket and Foegeding, 1994). These problems tend to be associated with the texture, cohesiveness, color, and water-holding properties of the turkey meat (Sosnicki and Wilson, 1991). Specifically, industry reports indicate an increased incidence of formed turkey breast loaves exhibiting poor texture and excessive exudative meat characteristics. These meat characteristics are similar to those quality defects found in pale, soft, exudative (PSE) pork, which is characterized by its pale color, soft texture, and poor water-holding capacity (Briskey, 1964). In processed turkey products, the exudate accumulates as excess purge in the packages upon cooking. This defect, combined with the reduced cohesiveness of the formed breast loaves, results in products that are unacceptable in appearance and in quality to both meat processors and consumers (Ferket and Foedgeding, 1994).

Received for publication November 12, 1996. Accepted for publication August 14, 1997. 1To whom correspondence should be [email protected]

addressed:

Abbreviation Key: ATP = adenosine triphosphate; PSE = pale, soft, and exudative; R=value = ratio of inosine:adenosine.

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ABSTRACT Development of rigor mortis at elevated post-mortem temperatures may contribute to turkey meat characteristics that are similar to those found in pale, soft, exudative pork. To evaluate this effect, 36 Nicholas tom turkeys were processed at 19 wk of age and placed in water at 40, 20, and 0 C immediately after evisceration. Pectoralis muscle samples were taken at 15 min, 30 min, 1 h, 2 h, and 4 h post-mortem and analyzed for R-value (an indirect measure of adenosine triphosphate), glycogen, pH, color, and sarcomere length. At 4 h, the remaining intact Pectoralis muscle was harvested, and aged on ice 23 h, and analyzed for drip loss, cook loss, shear values, and sarcomere length. By 15 min post-mortem, the 40 C treatment had higher R-

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MATERIALS AND METHODS A total of 36 live Nicholas toms (19 wk of age) were obtained from a commercial processing plant. Birds were transported (< 160 km) to the Texas A&M University Poultry Farm on two separate occasions. The turkeys were housed in litter-covered floor pens and fed a corn and soybean meal diet for 1 d. The experiment consisted of two replications with 18 turkeys per replicate. Following a 12-h feed withdrawal, 18 turkeys were hung on shackles and killed by bleeding for 90 s from a single cut severing the right carotid artery and jugular vein. Application of electrical stunning was not utilized prior to exsanguination because this type of stunning has been reported to retard rigor mortis development, and this study required rigor to develop without any conflicting factors (Papinaho and Fletcher, 1995). After bleeding, birds were sub-scalded2 at 63 C for 45 s, defeathered in a rotary drum picker3 for 35 s, and manually eviscerated. Immediately following evisceration, all birds were numbered and weighed, and a penetrating 47-cm stainless steel thermocouple4 was inserted from the anterior towards the middle of the left Pectoralis muscle of each carcass to monitor deep muscle temperature. Turkeys (n = 12 per treatment) were then placed into one of three post-mortem temperature treatments of tap water at 0 C (ice slush), 20 C, or 40 C. Water temperatures were monitored5 and varied by no

2Model SS-36-SS, Brower Corp., Houghton, IA 52631. 3Model SP3055, Brower Corp., Houghton, IA 52631. 4Cole-Palmer Instrument Co., Niles, IL 60714. 5Scanning thermocouple thermometer model 92800-00,

Branant Co., Barrington, IL 60010. 6Minolta Chroma Meter Model CR-200, Minolta Corp., NJ 07446. 7Binder clips, Acco USA Inc., Wheeling, IL 60090-6070. 8Sigma Chemical Co., St. Louis, MO 63178-9916. 9Blodggett Zepharie G-1 speed, Blodggett Oven Co., Burlington, VT 05402.

more than 2 C from the target temperature. Temperatures were adjusted by adding ice or hot water. All treatments were manually agitated to equilibrate temperatures of the water. Water and internal muscle temperatures of the turkeys were recorded every 5 min during the treatment application. A lengthwise incision was made in the skin covering the right breast muscle prior to the sampling of muscle tissue. Muscle tissue samples were cut (20 × 80 × 30 mm) parallel to the muscle fiber direction from the right Pectoralis at 15 min, 30 min, 1 h, 2 h, and 4 h postmortem (12 samples per treatment per post-mortem sampling time). Each sample was taken at least 30 mm from the previous sample area. The samples for biochemical analyses were bagged in labeled plastic bags, placed directly into liquid nitrogen, and stored at –75 C until analyzed (< 1 mo). Immediately after the samples were removed, lightness (L* value) was evaluated in triplicate6 on the cut surface (parallel with the fiber direction) of the remaining intact breast muscle. Following each sampling period, the skin covering the breast was pulled together and clamped7 so that water did not come into direct contact with the muscle. Location of sampling in the Pectoralis muscle was randomized to ensure that any variation within the muscle was randomly distributed. Tissue samples at 15 min, 30 min, 1 h, 2 h, and 4 h post-mortem were used for the biochemical measurements of R-value, pH, and glycogen. The ratio of inosine:adenosine (R-value) is an indication of adenosine triphosphate (ATP) depletion in the muscle and was determined using the method described by Thompson et al. (1987). The pH of samples was determined using the iodoacetate method of Sams and Janky (1986). Muscle glycogen was extracted using the procedures described by Lo et al. (1970) and measured using a glucose detection kit8 with results expressed as milligrams of glycogen per gram of muscle. To correct for muscle glucose levels, background glucose was measured before the muscle glycogen was hydrolyzed to glucose. Additional samples were taken from the anterior portion of the right breast muscle at 4 h post-mortem and placed in plastic bags, aged on ice for 24 h and then frozen at –20 C until analyzed (< 1 mo) for sarcomere length. Because muscle samples were harvested postrigor (24 h), freezing and thawing did not influence sarcomere length measurement (Sams et al., 1990). Sarcomere lengths (4 h post-mortem samples) were measured using the neon laser diffraction method of Cross et al. (1980), later modified by Sams and Janky (1986). After final sampling from the right breast muscle at 4 h post-mortem, the undisturbed left breast muscle was removed, weighed, and packaged into 8.46-L zip seal, plastic bags that had been perforated to allow for drainage and then stored on ice for an additional 20 h. Following the 20 h storage, all fillets were reweighed in order to determine drip loss and then all fillets were baked on racks in aluminum foil-covered steel baking pans (one breast fillet per pan) in an air convection over9

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Although elevated post-mortem temperatures have been shown to increase post-mortem metabolism and the development of PSE meat in pork, a similar relationship has not been established in turkeys. A study confirming the relationship between post-mortem processing temperature and overall meat quality in poultry is needed because the differences in chemical composition of the muscle, fiber type distribution, and post-mortem metabolic rates between poultry and swine (Addis, 1986). Temperature treatments of 0, 20, and 40 C were chosen to determine the effect of rigor development on meat tenderness as well color and waterholding capacity in turkeys. Additionally, these temperatures are indicative of meat temperatures observed when exiting the chiller in some commercial plants. Temperatures ranging from 10 to 25 C have been found to have no affect on poultry meat tenderness; however, color and water-holding properties of the meat were not evaluated (Khan, 1971).

RIGOR MORTIS DEVELOPMENT AT ELEVATED TEMPERATURES

RESULTS AND DISCUSSION The R-value is an indirect measure of ATP depletion in the muscle. During rigor mortis development, ATP in the muscle is depleted and R-value increases as it represents the ratio of inosine:adenosine-containing compounds in the muscle (Calkins et al., 1982). The Rvalue data presented in Figure 1 indicates that from 15 min through 4 h post-mortem, the 40 C treatment had higher R-values than the 20 and 0 C post-mortem temperature treatments. These results indicated that elevated post-mortem temperatures accelerate ATP depletion in the muscle. These findings were consistent with Khan (1971), who reported that chicken Pectoralis muscles held at 30 to 37 C had increased rates of postmortem glycolysis and dephosphorylation of high energy compounds, namely ATP, that resulted in increased meat toughness, whereas post-mortem temperatures of 10, 15, or 25 C did not adversely affect meat tenderness. Muscle glycogen levels measured in this study also support the fact that elevated post-mortem temperatures increases the rate of post-mortem metabolism. Glycogen levels illustrated in Figure 2 reveal that the 40 C temperature treatment had lower glycogen values at 1 h post-mortem than the 0 C treatments; however, the 20 C treatment was not different from either the 40 or 0 C treatment groups. By 4 h post-mortem, the 40 C

10Digi-sense

type J thermocouple thermometer, Vernon Hills, IL

60061. 11Instron Corp., Canton, MA 02021.

FIGURE 1. The increase in R-value (an indirect measure of adenosine triphosphate, ratio of Abs250/Abs260) of Pectoralis muscle from turkey carcasses held at 0, 20, or 40 C during post-mortem aging. Means (n = 12) within level of post-mortem time with no common letters differ significantly (P < 0.05).

treatment muscle glycogen content remained lower than in the 0 C treatment; however, the 20 C glycogen levels were not significantly different from either treatment. Studies by de Femery and Pool (1960) indicated an increased rate of glycogen degradation in broiler muscles exposed to elevated post-mortem temperatures of 37 to 40 C. Data from the current study also indicated that elevated post-mortem temperatures of 40 C resulted in increased rates of glycogen degradation in turkey Pectoralis muscles. Marsh (1954) has previously reported a slower decline in muscle pH and delayed glycolysis at

FIGURE 2. The decline in glycogen content in turkey Pectoralis muscles held at 0, 20, or 40 C during post-mortem aging. Means (n = 12) within level of post-mortem time with no common letters differ significantly (P < 0.05).

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at 177 C to an internal temperature of 76 C.10 Cooked fillets were reweighed for cook loss determination, individually wrapped in foil, held overnight in a 4 C cooler, and were then subjected to textural analysis. Shear value (kilograms of shear force per gram of meat) was determined on duplicate 40 × 20 × 7 mm samples from the interior of the fillets that were cut parallel with the muscle fiber at the cranial and medial sections of each fillet. Samples were weighed and sheared using an Instron Universal Testing Machine11 equipped with a 10-blade Allo-Kramer shear compression cell using a 500-kg load cell with a load range of 200 kg and a crosshead speed of 500 mm/min. Data were classed by treatment and replication and all parameter were subjected to ANOVA in this completely randomized block design (SAS Institute, 1985). Because a significant interaction was detected as expected between treatments and time, data were analyzed by time. Because no interaction was detected between treatment and replication, the data from both replicates were pooled. The residual mean square was used as the error term and means were compared using Duncan’s multiple range test (SAS Institute, 1985). Significance was determined using P < 0.05.

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FIGURE 3. The pH decline in Pectoralis muscles from turkey carcasses held at 0, 20, or 40 C during post-mortem aging. Means (n = 12) within level of post-mortem time with no common letters differ significantly (P < 0.05).

post-mortem temperatures of 27, 17, and 7 C compared to elevated post-mortem temperatures of 37 and 43 C. The decline in pH is related to the accumulation of lactic acid in the muscle during post-mortem glycolysis (Lawrie, 1991). Results in Figure 3 illustrate that as early as 15 min post-mortem, the 40 C treatment turkey Pectoralis muscles exhibit a lower pH than in the 0 C treatment. However, the muscles from carcasses held at 20 C did not differ from either 0 or 40 C treatments (Figure 3). Moreover, by 4 h post-mortem, the pH in the 20 and 40 C treatments was lower than the pH of the muscles held at 0 C. Although Fernandez et al. (1994) found that ultimate pH had a significant effect on many traits associated with development of PSE meat characteristics in pork, Warris and Brown (1987) found that the early postmortem pH decline from 30 min to 1 h was determined to be the most important factor in determining the degree of exudation in pork. It is early post-mortem when extensive protein denaturation occurs as a result of low pH combined with high carcass temperatures (Warris and Brown, 1987). Once extensive protein denaturation has occurred, it is irreversible and protein functionality for water-holding properties, cohesiveness, and color of loaf meat products is reduced. Early studies by Bendell and Wismer-Pedersen (1962) revealed that rigor development in pork muscles at elevated postmortem temperatures of 37 C always resulted in PSE meat characteristics. Although pH is apparently a prominent factor in determining the severity of the problems associated with PSE meat, the interrelationship between temperature and pH in the development of PSE meat characteristics is also important and well established (Swatland, 1993; Fernandez et al., 1994). Low pH

at high body temperature causes more severe damage than the same pH under cooler temperatures. Therefore, poor chilling conditions may also lead to the development of PSE meat characteristics in normal glycolyzing muscles (Offer, 1991). As a result, the lower pH in the turkey breast muscles in the current study combined with the elevated postmortem temperature of the muscle in the 40 C treatment (Figure 4), may have contributed more to meat quality defects than Pectoralis muscles with the same pH but at lower muscle temperatures of 20 or 0 C. Therefore, in turkeys, slow chilling may represent a substantial contributor in the development of PSE-like meat characteristics. The difference between bird temperature and water temperature (0 C) is large in magnitude and results in a rapid chilling rate (Figure 4) of carcasses. However, the carcass temperatures of the turkeys in the 20 and 40 C post-mortem temperature treatments were slower to decline, which could have resulted in some loss of protein functionality at the lower pH observed for these warmer treatments (Figure 3). Color is one of the main indicators of PSE meat and is directly affected by pH and protein denaturation in the muscle (Bendell and Wismer-Pedersen, 1962). The paleness of PSE meat can be attributed to the denaturation of sarcoplasmic proteins, which increases light scattering in the muscle (Lawrie, 1991; Swatland, 1993). Data in Figure 5 indicated that by 2 h post-mortem, the muscles of both the 40 and 20 C treatments developed a lighter color than the 0 C muscles as indicated by the increase in L* value. However, by 4 h post-mortem, the 40 C treatment muscles had greater L* values than the 0 and 20 C treatment muscles. These results suggested that the biochemical changes in the muscle occurring early postmortem had a cumulative effect and were manifested as color changes in the muscle by 2 h post-mortem. Data in Table 1 illustrate the extent to which elevated post-mortem temperatures at or near body temperature

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FIGURE 4. Pectoralis muscle temperatures of turkey carcasses held at 0, 20, or 40 C during post-mortem aging.

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RIGOR MORTIS DEVELOPMENT AT ELEVATED TEMPERATURES TABLE 1. Physical parameters of Pectoralis muscles from turkey carcasses held at 0, 20, and 40 C for 4 h post-mortem Post-mortem temperature treatments

Physical parameters

Pooled SEM

0 C

20 C

40 C

Drip loss, %1 Cook loss, % Shear value, kg/g Sarcomere length2

0.18 0.71 0.30 0.20

0.01b 24.05b 7.31b 1.89a

0.05b 26.99a,b 7.43b 1.87a

1.88a 28.86a 9.69a 1.79b

a,bMeans

(n = 12 per mean) within each row with no common superscript differ significantly (P < 0.05). h post-mortem. 24 h post-mortem. 124

FIGURE 5. The change in L* value of turkey Pectoralis muscles held at 0, 20, or 40 C during post-mortem aging. Means (n = 12) within level of post-mortem time with no common letters differ significantly (P < 0.05).

sarcomeres due to elevated post-mortem temperatures of 35 C. In contrast, the present study revealed shortened sarcomeres in muscle as a result of the 40 C post-mortem temperature treatments (Table 1). Differences between the studies could be attributed to the large variability in initial temperature, glycogen levels, and the adrenaline injections given to the gilts in the Fernandez et al. (1994) study. Shortened sarcomeres and increased toughness due to rigor development at elevated temperatures or “heat rigor” has been previously reported (Lawrie, 1991). Kahn (1971) also reported increased meat toughness due to rigor development at temperatures from 30 to 37 C. Shear values presented in Table 1 indicate that rigor development at 40 C increased turkey meat toughness compared to the 0 and 20 C treatments. Bilgili et al. (1989) also found that intact broiler Pectoralis muscles held at 0 C had lower shear values than muscles from carcasses held at 41 C during post-mortem aging. Studies by Welbourn et al. (1968) and Jungk and Marion (1970) concluded that there was little or no cold shortening in turkey Pectoralis muscles. Differences between broiler and turkey muscle in response to postmortem temperature treatments may be due to the size difference between broiler and turkey carcasses and the effect this size difference has on chilling rate. Earlier studies found a relationship between meat toughness and rapid pH decline at elevated temperatures (Khan, 1971). The resulting toughness in the current study appeared to be related to pH decline, shortened sarcomere lengths, and increased moisture loss. The meat from the 40 C treatment also appeared to have a stringy visual texture compared to the other treatments, a factor that may also have affected resulting meat toughness. In conclusion, elevated post-mortem temperatures of 40 C contribute to the pale, exudative meat characteristics of turkey breast fillet by accelerating the development of rigor mortis. The meat in the 40 C treatment appeared to have a stringy texture and was tougher than the other treatments. These results emphasize the importance of proper chilling regimens because increased post-mortem temperatures resulted in lighter, tougher meat with increased drip loss and cook loss.

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during rigor development affect drip loss, cook loss, shear value, and sarcomere length. The 40 C postmortem temperature treatment increased drip loss of turkey breast fillets compared to the 20 and 0 C temperature treatments. Cook loss was also increased in the 40 C treatment compared to the 0 C treatment; however, the 20 C treatment was not different from either the 0 or 40 C treatment. Santos et al. (1994) stated that the early development of rigor mortis in PSE pork combined with high carcass temperatures caused the denaturation of muscle sarcoplasmic and contractile proteins, which resulted in meat with poor waterholding capacity, which was reflected by higher drip loss and cooking losses. Fernandez et al. (1994) noted that water-holding properties of meat may not only be a result of denatured proteins, but may also be a result of shortened sarcomeres due to rigor development at elevated post-mortem temperatures. However, the results in the study by Fernandez (1994) indicated that in pork muscle, there was no apparent shortening of

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Ultimately, increased drip loss and cook loss would result in lower yields and profit for the producer.

ACKNOWLEDGMENT The turkeys used in this study were generously donated by Plantation Foods, Waco, TX 76705.

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