Rigor Mortis Development in Turkey Breast Muscle and the Effect of Electrical Stunning

Rigor Mortis Development in Turkey Breast Muscle and the Effect of Electrical Stunning C. Z. Alvarado and A. R. Sams 1 Texas A&M University, Department of Poultry Science, College Station, Texas 77843-2472 ABSTRACT Rigor mortis development in turkey breast were no significant differences between the surface and muscle and the effect of electrical stunning on this process are not well characterized. Some electrical stunning procedures have been known to inhibit postmortem (PM) biochemical reactions, thereby delaying the onset of rigor mortis in broilers. Therefore, this study was designed to characterize rigor mortis development in stunned and unstunned turkeys. A total of 154 turkey toms in two trials were conventionally processed at 20 to 22 wk of age. Turkeys were either stunned with a pulsed direct current (500 Hz, 50% duty cycle) at 35 ma (40 V) in a saline bath for 12 seconds or left unstunned as controls. At 15 min and 1, 2, 4, 8, 12, and 24 h PM, pectoralis samples were collected to determine ph, R-value, L* value, sarcomere length, and shear value. In Trial 1, the samples obtained for ph, R-value, and sarcomere length were divided into surface and interior samples. There interior samples among any parameters measured. Muscle ph significantly decreased over time in stunned and unstunned birds through 2 h PM. The R-values increased to8hpminunstunned birds and 24 h PM in stunned birds. The L* values increased over time, with no significant differences after 1hPMforthecontrols and 2 h PM for the stunned birds. Sarcomere length increased through2hpminthecontrols and 12 h PM in the stunned fillets. Cooked meat shear values decreased through the 1 h PM deboning time in the control fillets and 2 h PM in the stunned fillets. These results suggest that stunning delayed the development of rigor mortis through 2 h PM, but had no significant effect on the measured parameters at later time points, and that deboning turkey breasts at 2 h PM or later will not significantly impair meat tenderness. (Key words: turkey, stunning, rigor mortis, tenderness, meat quality) 2000 Poultry Science 79:1694 1698 INTRODUCTION According to the 1958 Humane Methods of Slaughter Act, all processors of food animals must render USDAinspected animals insensible to pain prior to exsanguination. Even though this act excludes poultry, electrical stunning is still widely used in both turkey and broiler processing to comply with the Poultry Inspection Regulations of 1984. This regulation states that poultry will be slaughtered in a manner resulting in thorough bleeding of the carcass and ensuring that breathing has ceased prior to entrance of the carcasses into the scalder. The industry justifications for electrical stunning are rapid immobilization for more efficient automated killing, humane slaughter, uniform bleeding to prevent carcass quality defects, and reduction of struggle to reduce carcass damage. Electrical stunning is the most common method of immobilization, because it is inexpensive, convenient, and safe (Bilgili 1992; Fletcher, 1993). Received for publication March 6, 2000. Accepted for publication June 30, 2000. 1 To whom correspondence should be addressed: asams@poultry. tamu.edu. Previous reports have indicated that some electrical stunning procedures delayed postmortem (PM) glycolysis due to a decrease in struggling in broilers and turkeys (Lee et al., 1979; Thomson et al., 1986; Kim et al., 1988; Murphy et al., 1988). Fillets from stunned birds had higher ph, adenosine triphosphate (ATP), and creatine phosphate and lower lactate levels compared with fillets from unstunned controls. Stunning has been reported to minimize perimortem struggle that decreases the utilization of glycogen by 50%, resulting in a higher muscle ph early PM (de Fremery and Lineweaver, 1962). Papinaho et al. (1995) reported that electrical stunning does not directly affect PM biochemistry of breast muscle. Denervated breast muscles, which eliminated the effects of perimortem struggle, had significantly lower lactate concentrations and higher ph values early PM compared with sham-operated muscles. The denervation of the breast fillets, therefore, resulted in a delay of rigor mortis development. The time for rigor mortis completion in fillets from unstunned turkeys has been reported to be 143 min, compared with 314 min in stunned birds (Ma and Addis, 1973). Abbreviation Key: ATP = adenosine triphosphate; PM = postmortem. 1694

RIGOR DEVELOPMENT IN PECTORALIS FROM STUNNED AND UNSTUNNED TURKEYS 1695 Lee et al. (1979) also reported from shear data that electrically stunned broilers were 33% tougher than the nonstunned birds. Dickens and Lyon (1993) observed that even though shear values from stunned and unstunned birds differed when they were deboned at 2 h PM,both treatments resulted in acceptable tenderness levels by 4 h PM. However, Papinaho and Fletcher (1995) and Ma and Addis (1973) reported that stunning did not significantly affect shear values from broilers deboned early PM. Thomson et al. (1986) noted that PM aging prior to deboning fillets had a more pronounced effect on tenderness than did electrical stunning. Because rigor mortis development in turkey breast muscle and the effect of electrical stunning on this process are not well characterized, the objective of this study was to characterize rigor mortis development in turkey pectoralis muscles from stunned and unstunned turkeys and to determine biochemical and tenderness differences in stunned and unstunned turkey pectoralis muscles deboned at different PM times. MATERIALS AND METHODS One-hundred fifty-four turkey toms at 18 wk of age were obtained from a commercial processor and transported to the Texas A&M University poultry research farm. The birds were housed in litter-covered floor pens and provided feed ad libitum using a commercial feeding ration that met the nutritional requirements of the turkeys (NRC, 1994). At 20 wk (Trial 1) and 22 wk (Trial 2) of age, the birds were conventionally processed in two trials of 70 and 84 birds, respectively. Feed was removed from the birds 12 h prior to processing, but they were allowed access to water until 2 h prior to processing. In each trial, the birds were hung on a shackle line and either electrically stunned 2 with a pulsed direct current (500 Hz, 50% duty cycle) at 35 ma (40 V) in a 1% saline bath for 12 s or left unstunned as controls. The bird s head contacted the charged sodium chloride solution, and the foot contact with the shackle was the ground. The contact between the foot and shackle was sprayed with a 1% sodium chloride solution to increase conductivity. Immediately following stunning, the birds were killed by bleeding for approximately 3 min from a single unilateral neck cut severing the right carotid artery and jugular vein. After bleeding, the turkeys were scalded in an agitated water scalder 3 (61 C, 45 s) and defeathered in a rotary drum picker 4 (25 s). Following picking, the turkeys were placed in a two-stage chilling system consisting of a prechill for 15 min at 12 C and a manually-agitated ice water chill tank at 1 C for 45 min. Following chilling, the car- 2 Model SF-7000, Simmons Engineering Corp., Dallas, GA 30132. 3 Model SS-36-SS, Brower Corp., Houghton, IA 52631. 4 Model SP3055, Brower Corp., Haughton, IA 52631. 5 Minolta Chroma Meter Model CR-200, Minolta Corp., NJ 07446. 6 Blodgett Zephaire G-1 speed, Blodggett Oven Co., Burlington, VT 05402. 7 Instron Corp., Canton, MA 02021. casses were removed from the chill tank and placed on ice until the designated time for deboning. The deboning times were 15 min (following picking), 1 h (following chilling), and 2, 4, 8, and 24 h PM. Trial 1 had an added deboning time point of 12 h PM. At the time of harvesting, samples from the anterior portion of the pectoralis fillet were analyzed for ph using the iodoacetate method described by Jeacocke (1977) as modified by Sams and Janky (1986), and R-value using the methods of Khan and Frey (1971) and Honikel and Fischer (1977) as modified for poultry by Thompson et al. (1987). The samples were immediately frozen in liquid nitrogen and stored at 76 C until analysis (<1 mo). The L* value (lightness) was measured on the remainder of the fillet at the time of deboning with a colorimeter 5 by averaging three readings on the newly cut surface of the fillet. The remaining portion of the fillet was stored in a zip-sealed plastic bag and placed on ice in a cooler at 4 C for 24 h. Following 24 h aging of the remaining portion of the fillets, another anterior sample was excised (at least 30 mm from the previous biochemical analysis sample point), frozen in liquid nitrogen, and stored at 76 C for later analysis of sarcomere length by the laser diffraction method of Cross et al. (1980), which was later modified by Sams and Janky (1986). In Trial 1, ph, R-value, and sarcomere length samples were collected from the surface and center (interior) of the muscle to determine any biochemical differences due to location or chilling rate differences. At 24 h PM, L* values were again determined on the newly cut surface of the fillet. The remaining anterior portion of the fillet was weighed and cooked at 24 h PM for shear value determination. Because the skin and underlying fascia on the second fillet were undisturbed at the keel bone, the remaining pectoralis fillet was harvested at the next time point. For the tenderness determinations using shear values, fillets were baked on racks in aluminum foil-lined, covered pans in an air convection oven 6 to an internal temperature of 75 C as described by Sams (1990). Cooked fillets were individually wrapped in foil and held overnight in a 4 C cooler prior to shearing. Shear values (kg/g of meat) were determined on interior and surface samples (40 20 7 mm) in triplicate for each fillet. Shear value was measured using an Instron Universal Testing Machine 7 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 cross-head speed of 500 mm/min. The data were subjected to analysis of variance using the general linear model procedure of SAS, and the means were separated using Duncan s multiple range test with a significance level of P < 0.05 (SAS Institute Inc., 1990). The effect of PM time and treatment was analyzed to determine the development of rigor mortis over time in muscles from stunned and control carcasses. Also, to determine differences due to stunning, the data were blocked by treatment and time. Data from Trial 1 were also analyzed to determine significant differences between interior and surface sampling. The location data from Trial 1 were pooled, because there were no signifi-

1696 ALVARADO AND SAMS cant differences in stunned or unstunned birds between interior and surface sampling sites of the fillet in any parameter measured. The data from the two trials were pooled, because no trial-by-treatment interactions were noted. RESULTS AND DISCUSSION Postmortem metabolism was monitored in this study by measuring muscle ph and R-value. The effects of stunning on muscle ph and R-value are shown in Table 1. Postmortem ph decreases as lactic acid increases in the muscle due to the glycolytic breakdown of glucose during rigor mortis development (Lawrie, 1991). Muscle ph significantly decreased over time in fillets from stunned and unstunned turkeys through2hpm;however, there were no significant differences after 2 h PM in either treatment group. Stunning delayed rigor development, as indicated by the stunned birds having a significantly higher ph compared with the unstunned birds through 2 h PM. This decreased rate of ph decline in the stunned fillets was probably due to a decrease in ante-mortem struggling and, therefore, decreased glycogen utilization due to stunning. These results are consistent with previous reports that indicated that unrestrained free struggle resulted in accelerated postmortem metabolism, resulting in faster rigor development (Ma and Addis, 1973; Thomson et al., 1986; Papinaho et al., 1995). The R-value is an indirect measurement of the ATP content within the muscle, and measures the ratio of inosine nucleotides (ATP degradation products) to adenosine nucleotides (ATP components). Therefore, R-value increases as ATP becomes depleted (Calkins et al., 1982). The R-values in control turkey pectoralis muscle increased over time, indicating a decrease in ATP until an ultimate value was reached at 8 h PM (Table 1). However, R-values in fillets from stunned turkeys declined significantly over time through 24 h PM. Control turkeys had significantly higher R-values (decreased ATP) than stunned turkeys through 2 h PM, with no significant dif- ferences between the two groups thereafter. This finding indicates that stunning delayed rigor development early PM, possibly due to a decrease in struggle in the stunned birds. These results are consistent with previous studies that indicate that electrical stunning does not directly affect PM biochemistry of the breast muscle, but does delay PM metabolism due to the elimination of perimortem struggle (Ma and Addis, 1973; Papinaho et al., 1995). Color was measured in this study because it is an important meat quality attribute. The L* value is a measurement of lightness of the meat, with the lighter colors corresponding to higher L* values (Barbut, 1993). Denaturation of sarcoplasmic proteins and shrinkage of myofibrils that occur early PM can cause increased scattering of light and lighter meat (Bendall, 1973; Offer et al., 1989; Swatland, 1993). The L* values increased over time, reaching an ultimate value at 1 h PM for the controls and 2 h PM for the fillets from stunned turkeys (Table 1), which indicates a delay in the rate of color development due to stunning. There were no significant differences between the control and stunned turkeys following the 1 h PM deboning time point. Because L* value is negatively correlated with ph, a higher muscle ph is associated with lower L*values (van Hoof, 1979; Barbut, 1993; Owens et al., 1998). This relationship was proposed to result from the partial denaturation of proteins caused by reduced ph earlier PM. Therefore, the lower L* values of the fillets from the stunned turkeys at1hpmcompared with those of the control fillets may be associated with a delay in rigor development, as indicated by higher ph levels early PM. Sarcomere length was measured in this study as an indicator of the contractile state of the muscle and as a contributor to toughness. Shorter sarcomeres are strongly correlated with toughness (Herring et al., 1967). Sarcomere length significantly increased over time, up to 2 h PM, in fillets from the control and stunned turkeys (Table 2). There were no significant differences between fillets from control and stunned turkeys at any time PM. The previously observed differences in ph and R-value may TABLE 1. Means and SEM for ph, R-value, and L* value in pectoralis muscle from control (NS) and stunned turkeys at different deboning times 1 Parameter ph R-value L* value Item NS Stunned NS Stunned NS Stunned Postmortem deboning time (h) 0.25 6.16 a, * 6.33 a, * 1.02 c, * 0.94 d, * 47.30 b 46.47 c 1 6.04 ab, * 6.17 b, * 1.19 b, * 1.02 c, * 49.99 a, * 48.14 b, * 2 6.00 b 6.01 c 1.23 ab, * 1.01 c, * 50.30 a 50.28 a 4 5.97 b 6.00 c 1.23 ab 1.20 b 50.00 a 49.95 a 8 5.95 b 5.98 c 1.30 a 1.24 b 50.36 a 50.01 a 12 5.95 b 5.96 c 1.30 a 1.23 b 24 5.91 b 5.91 c 1.30 a 1.33 a 49.54 a 49.88 a Pooled SEM 0.01 0.01 0.18 a d Means within a column with no common superscript differ significantly (P < 0.05). *Means within a row and within the same parameter differ significantly (P < 0.05). 1 For each mean, n = 24; for L* value, n = 15; for 12 and 24 h deboning, n = 4.

RIGOR DEVELOPMENT IN PECTORALIS FROM STUNNED AND UNSTUNNED TURKEYS 1697 TABLE 2. Means and SEM for sarcomere length and shear value of pectoralis from control (NS) and stunned turkeys at different deboning times 1 Sarcomere length (µm) Parameter Shear value (kg/g) Item NS Stunned NS Stunned Postmortem deboning time (h) 0.25 1.65 c 1.63 d 13.61 a, * 16.22 a, * 1 1.73 b 1.72 c 8.86 b, * 10.43 b, * 2 1.83 a 1.79 b 8.00 b 8.60 c 4 1.88 a 1.84 ab 7.91 b 8.17 c 8 1.89 a 1.85 ab 7.93 b 7.62 c 12 1.90 a 1.90 a 8.04 b 8.06 c 24 1.91 a 1.89 a 7.75 b 7.19 c Pooled SEM 0.01 0.21 a c Means within a column with no common superscript differ significantly (P < 0.05). *Means within a row and within the same parameter differ significantly (P < 0.05). 1 For each mean, n = 24; for 12 and 24 h deboning, n = 4. not have been sufficient to induce a detectable difference in sarcomere length. Shear values of meat deboned at various PM times and then cooked at 24 h PM were also measured in this study as a measurement of tenderness. As shear values decrease, toughness of the meat also decreases (Herring et al., 1967). Shear values decreased significantly until the 1 h PM deboning time in unstunned turkeys, and the 2 h PM deboning time in stunned turkeys (Table 2). When comparing fillets from control versus stunned turkeys, the stunned treatment fillets were significantly less tender than the controls until 1 h PM. In both treatments, the shear values from fillets deboned after 2 h PM were low enough to be considered acceptably tender by consumers (Lyon and Lyon, 1990). These results suggest that the electrical stunning procedure used in this study delays the development of rigor mortis in turkey pectoralis, as indicated by lower ph and greater R-values, through 2 h PM. 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