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Tenderness and R-Value Changes in Early Harvested Broiler Breast Tissue Following Post-Mortem Electrical Stimulation1
Tenderness and R-Value Changes in Early Harvested Broiler Breast Tissue Following Post-Mortem Electrical Stimulation1 A. R. SAMS,2 D. M. JANKY,3 and S. A. WOODWARD4 Institute of Food and Agricultural Sciences, Department of Poultry Science, University of Florida, Gainesville, Florida 32611 (Received for publication June 9, 1988)
1989 Poultry Science 68:1232-1235 INTRODUCTION
As poultry processors experience increasing demands for boneless broiler breast meat, efforts are made to decrease production costs in labor, energy, and storage space by harvesting breast meat as soon as possible after slaughter. Consumer response and published research, however, indicate that this practice often results in the production of meat with unacceptable tenderness (Goodwin, 1984; Shelton, 1985; Thompson et al, 1987). Several researchers (Dodge and Stadelman, 1959; de Fremery and Pool, 1960; Stewart et al., 1984; Lyon et al., 1985) have observed that acceptable tenderness levels result only if boning is delayed 3 to 6 h after death of the animal. Post-mortem electrical stimulation has been applied to beef carcasses (Pearson and Dutson, 1985) to accelerate rigor mortis development and allow hot boning. In the area of poultry meat processing, Dransfield et al. (1984)
'Florida Agricultural Experiment Stations Journal Series Number 9041. 2 Present address: Department of Poultry Science, Texas A&M University, College Station, TX 77843-2472. To whom correspondence should be addressed. 4 Present address: Sunny Fresh Foods, P.O. Box 428, Monticello, MN 55362.
reported no tenderization of turkey breast meat using a 94 V post-mortem stimulation; however, Maki and Froning (1987) reported a significant tenderization response in turkey breast muscle when carcasses were electrically stimulated with 820 V. Researchers at Campbell Institute for Research and Technology have developed and patented a process involving low voltage electrical stimulation coupled with high temperature conditioning to produce acceptable tenderness in broiler breast meat harvested as early as 24 min post-mortem (Amey, 1988). However, this process requires an additional 2,200 feet of shackle line in an existing 185 bird/min line to achieve results. Thompson et al. (1987) reported that high voltage (820 V), short time (15 s) electrical stimulation increased tenderness levels of breast meat harvested immediately postchill to acceptable levels. In this latter study, the authors found no relationship between changes in post-mortem muscle metabolism and tenderness changes in the cooked muscle due to postmortem electrical stimulation similar to the relationship reported for red meats (Pearson and Dutson, 1985). Also, no tenderness response was observed due to high voltage electrical stimulation in hot-boned (10-min post-mortem boning) or age-boned (48-h postmortem boning) meat. The purpose of the
1232
Downloaded from http://ps.oxfordjournals.org/ at University of New Orleans on May 27, 2015
ABSTRACT This study was designed to compare tenderness (shear force) and post-mortem metabolism at time of boning (R value) of broiler breast fillets harvested from a total of 200 electrically stimulated carcasses (840 V, 340 mA, pulsed 2 s on, 1 s off for 15 s) and 200 nonstimulated (control) carcasses at various times post-mortem up to 240 min. As on-carcass aging time was increased prior to fillet harvesting, shear force values decreased for fillets from both electrically stimulated and nonstimulated carcasses. However, regression analysis of the data indicated that fillets harvested 100 min post-mortem from electrically stimulated carcasses were as tender as fillets harvested at 240 min post-mortem from nonstimulated carcasses. For those processors currently holding broiler carcasses for 3 to 4 h after chilling prior to boning, electrical stimulation would provide a decrease of approximately 60% in the holding time to reach tenderness levels similar to those attained without electrical stimulation. Electrical stimulation did not have a significant effect on the rate of post-mortem metabolism as measured by R value. (Key words: electrical stimulation, boning time, tenderness, broiler, fillet)
TENDERNESS AND ELECTRICAL STIMULATION
present study was to monitor broiler breast meat tenderness development and post-mortem metabolism as affected by high voltage electrical stimulation and post-mortem boning time through the first 4 h following death of the animal. MATERIALS AND METHODS
lation treatment (20 min post-mortem). Previously harvested right side fillets and remaining carcasses were chilled together in agitated tap water for 10 min at 21 C, followed by an additional 30-min chill at 1 C. Fillets were harvested as before from one carcass within each stimulation treatment after the 10min, 21-C prechill (30 min post-mortem) and after 10, 20, and 30 min of chilling at 1 C (40, 50, and 60 min post-mortem). Right side fillets were returned to the appropriate chilling solution and left side fillets were sampled for R value analysis as previously described. A chill water-to-carcass/fillet ratio of 3:1 (wt/wt) and an ice-to-water ratio (1 C chill solution only) of 1:9 (wt/wt) were maintained. Chill water agitation was achieved by manually moving carcass/fillet-filled wire baskets in an up-and-down motion in the chill tanks throughout the chilling period. Following the chill period, all right side fillets and the remaining eight carcasses per replication were packed in crushed ice and stored at 2 C. Fillet harvesting and R value sampling procedures were repeated on one carcass per stimulation treatment at 75, 90, 120, and 240 min post-mortem. At 24 h postmortem all fillets were removed from the crushed ice, reidentified with self-piercing wingbands attached at the caudal end of each fillet, separated from the wing, packaged in Cryovac® bags (W. R. Grace & Co., Apex, NC), frozen, and stored at -40 C until analyzed for shear force (less than 4 wk). Fillets were thawed (48 h, 2 C) and baked to an internal temperature of 82 C using procedures described by Thompson et al. (1987). After cooling to room temperature, cooked fillets were wrapped in aluminum foil and held at 2 C overnight for shear force analysis. Two samples (40 x 20 x 7 mm) from each fillet (cranial and medial portions) were sectioned and analyzed for shear force with a Food Technology Texture Test System® (Food Technology Corporation, Rockville, MD) using procedures described by Dukes and Janky (1984). The two shear values obtained for each fillet were averaged to produce a mean shear force per carcass measurement prior to statistical analysis of the data. These procedures resulted in a factorial arrangement of two stimulation treatments (control and stimulated) and ten boning times (10, 20, 30, 40, 50, 60, 75, 90, 120, and 240
Downloaded from http://ps.oxfordjournals.org/ at University of New Orleans on May 27, 2015
Cobb, feather-sexed, male broilers were reared in litter-covered floor pens and fed commercial-type starter, finisher, and withdrawal corn-soybean diets. At 49 days of age, 100 broilers within a weight range of 1,700 to 2,200 g were selected from the general population, cooped (10 birds/coop), and held for 16 h without access to food or water. Broilers were wingbanded and slaughtered in five replicate groups of 20 birds each using electrical stunning with a Cervin Model FS stunner (Cervin Electrical Systems, Inc., Minneapolis, MN) (setting No. 4) followed by exsanguination. Following a 90-s bleeding period, half of the carcasses were electrically stimulated using the Cervin Model FS stunner equipped with a rheostat to maintain a constant voltage of 820 V (340 mA). Current was pulsed (2 s on, 1 s off for 15 s) from the kill knife (positive electrode), placed on the dorsal surface of the last cervical vertebra, through the carcass to the shackle line (negative electrode). Both control (nonstimulated) and electrically stimulated carcasses were then subscalded (60 C, 45 s) and picked in a rotary drum picker (25 s). Using the stripping technique described by Hamm (1981), both breast fillets were harvested from one control and one electrically stimulated carcass per replication at 10 min post-mortem. A 2.54-cm long section of the anterior portion of each left-side fillet was sampled, frozen in liquid nitrogen, and stored at -40 C for later R value analysis using the procedure described by Thompson et al. (1987). The R value, the ratio of the concentration of various adenine phosphatidyl compounds (ATP, ADP, AMP, and others) to that of a major breakdown product, inosine monophosphate (IMP), was calculated as the ratio of absorbance at 250 nm (IMP) divided by absorbance at 260 nm (ATP). The R value serves as a good monitor of ATP depletion. Remaining picked carcasses were eviscerated and fillets harvested and sampled as previously described from one carcass within each stimu-
1233
1234
SAMS ET AL. RESULTS AND DISCUSSION
E 10
Control y= .000138 x 2 -.048x +12.136 r ! = .85 ^ J * range of x = 10 - 240
Electrically stimulated y=.000117x2-.60x +12.631 r!=93 range of x= 10-240
4 OT60 120 180 Post-mortem boning time fmin)
240
min post-mortem) within each of the five replicate groups per trial. The entire experiment was repeated with an additional 100-bird trial. Two additional 100-bird trials were conducted as previously described except that analysis for R value was omitted. Data were subjected to analysis of variance and regression procedures using computer programs available in the Statistical Analysis System (SAS, 1985). Because no significant trial x treatment or boning time interactions were observed, shear force data from all four trials (n = 400) and the R value data from two trials (n = 200) were combined. A regression curve for each stimulation treatment was constructed for both shear force and R value as a function of boning time to evaluate the effect of electrical stimulation on post-mortem changes in these parameters. Maximizing the r^ was used to evaluate the regression models for closeness of fit. The significance of improvements in closeness of regression model fit obtained by using second-order regressions instead of linear models was tested with the F test described by Ott (1984). The distances between regression curves within each parameter were compared at 10-min intervals using the least significant difference (LSD) procedure presented in the Statistical Analysis System (SAS, 1985) to estimate the postmortem time at which the curves became significantly different (P<.05).
Downloaded from http://ps.oxfordjournals.org/ at University of New Orleans on May 27, 2015
FIGURE 1. Regression lines representing changes in shear force of cooked broiler breast fillets from broiler carcasses electrically stimulated with 820 V, 340 mA, pulsed 2 s on, 1 s off for 15 s and nonstimulated (control) carcasses as a function of post-mortem boning time.
Cooked meat shear force values decreased as post-mortem time interval prior to fillet harvesting was increased for meat from both control and electrically stimulated carcasses (Figure 1). This pattern of shear force decrease with increasing post-mortem interval prior to boning has been well documented in broiler breast meat from both nonstimulated (de Fremery and Poole, 1960; Stewart et al., 1984; Sams and Janky, 1986) and electrically stimulated (Thompson et al., 1987) carcasses. Simpson and Goodwin (1974) found that cooked broiler light meat with a shear force value of 8 kg force/g or less would be considered tender by sensory panelists. Regression of shear force for cooked meat from control carcasses as a function of post-mortem boning time indicated that shear force reached a minimum level of approximately 8 kg force/ g at approximately 180 min post-mortem and maintained this level through 240 min (Figure 1). However, regression of shear force for cooked meat from electrically stimulated carcasses as a function of post-mortem boning time indicated a more rapid decline in shear force, reaching a level of 8 kg force/g after 100 min post-mortem and continuing to decline to a minimum value of approximately 5 kg force/ g at 240 min. This minimum shear force value would be similar to that observed for meat from carcasses that had been aged for 24 h prior to boning (Sams and Janky, 1986). A statistical comparison of the two regression lines indicated that electrical stimulation of carcasses produced a significantly (P<.05) lower shear force in cooked meat than in cooked meat from control carcasses when postmortem boning times were equal to or greater than 100 min. For those processors currently holding broiler carcasses for 3 to 4 h after chilling prior to boning, electrical stimulation would provide a decrease of approximately 60% in the holding time to reach tenderness levels similar to those attained without electrical stimulation. Raw meat R values (ratio of absorbance at 250 nm to absorbance at 260 nm), even though low, increased as post-mortem boning time was increased, regardless of the presence or lack of the electrical stimulation treatment (Figure 2), indicating that R value did in fact increase as rigor mortis developed. However, regression analysis of the data indicated that electrical stimulation did not produce a signifi
TENDERNESS AND ELECTRICAL STIMULATION
1235
REFERENCES y
y /
Control y = .000226x +.847
/
y
r 2 =.89
T -87
y /
^^y y
O
^
y^^
60 Post-mortem
'
/ ^ ^-"^
^
^
^ - ^ ^
^^*
^^"^
S s > ^ • ^
/
%
range of x = 10-240 y y
Electrically stimulated y = . 000143 x + . 852 range of x = 10 - 240 r2 = .64
120 180 boning time (min)
240
cant increase in the rate of R value change over mat associated with control samples. Calkins et al. (1982) reported that post-mortem increases in R value were a direct result of the normal catabolism of adenine nucleotides associated with post-mortem metabolism leading to the development of rigor mortis. Carse (1973) attributed the increased rate of tenderness development in electrically stimulated red meat carcasses to an acceleration of postmortem metabolism that would result in earlier onset of rigor mortis, thus negating the adverse effects of early boning procedures on the degree of rigor mortis development. In the present study, however, the relationship of increased tenderness development due to electrical stimulation could not be explained by post-mortem metabolism changes as measured by R value. Thompson et al. (1987), in work with chicken muscle tissue, also were unable to show a relationship between post-mortem metabolism changes and increased levels of tenderness attributed to high voltage (820 V) electrical stimulation. Those authors concluded that tenderness improvement was a result of increased physical disruption of the myofibrils and increased sarcomere lengths observed in electrically stimulated muscle tissue.
Downloaded from http://ps.oxfordjournals.org/ at University of New Orleans on May 27, 2015
FIGURE 2. Regression lines representing changes in R value (ratio of absorbance at 250 run to absorbance at 260 nm) of broiler breast fillets from electrically stimulated (820 V, 340 mA, pulsed 2 s on, 1 s off for 15 s) and nonstimulated (control) broiler carcasses as a function of post-mortem boning time.
Amey, D., 1988. Minimum time process system-tender meat in 24 minutes. Broiler Industry 51(2):22-24, 26, 54-56. Calkins, C. R., T. R. Dutson, G. C. Smith, and Z. L. Carpenter, 1982. Concentration of creatine phosphate, adenine nucleotides and their derivatives in electrically stimulated and nonstimulated beef muscle. J. Food Sci. 47:1350-1353. Carse, W. A., 1973. Meat quality and the acceleration of post mortem glycolysis by electrical stimulation. J. Food Technol. 8:163-166. de Fremery, D., and M. F. Pool, 1960. Biochemistry of chicken muscle as related to rigor mortis and tenderization. Food Res. 25:73-87. Dodge, J. W., and W. J. Stadelman, 1959. Post mortem aging of poultry meat and its effect on the tenderness of breast muscles. Food Technol. 13:81-84. Dransfield, E., A. A. Down, A. A. Taylor, and P. K. Locker, 1984. Influence of electrical stimulation and slow chilling on the texture of turkey breast muscle. Proc. Eur. Meeting of Meat Res. Workers No. 30,4:10,180 in: Food Sci. Technol. Abstr. 17(8):8S138. (Abstr.) Dukes, M G., and D. M. Janky, 1984. Physical characteristics of pectoralis superficialis from broiler carcasses chilled in either water or sodium chloride solutions under commercial conditions. J. Food Sci. 49: 849-851, 858. Goodwin, T. L., 1984. It takes tough discipline to make tender chicken! Broiler Ind. 47(9):43-44. Hamm, D., 1981. Unconventional meat harvesting. Poultry Sci. 60:1666. (Abstr.) Lyon, C. E., D. Hamm, and J. E. Thomson, 1985. pH and tenderness of broiler breast meat deboned various times after chilling. Poultry Sci. 64:307-310. Maki, A., and G. W. Froning, 1987. Effect of post-mortem electrical stimulation on quality of turkey meat. Poultry Sci. 66:1155-1157. Ott, L., 1984. An Introduction to Statistical Methods and Data Analysis. 2nd ed. Duxbury Press, Boston, MA. Pearson, A. M., and T. R. Dutson, 1985. Scientific basis for electrical stimulation. Pages 185-218 in: Advances in Meat Research. Vol. 1, Electrical Stimulation. AVI Publishing Co., Westport, CT. Sams, A. R., and D. M. Janky, 1986. The influence of brine chilling on tenderness of hot-boned, chill-boned, and age-boned broiler breast fillets. Poultry Sci. 65: 1316-1321. SAS, 1985. SAS/STAT Guide for Personal Computers. 6th ed. SAS Inst. Inc., Cary, NC. Shelton, T., 1985. Broiler industry in the year 2000. Broiler Industry 48(11):36, 38, 40-42, 44. Simpson, M. D., and T. L. Goodwin, 1974. Comparison between shear values and taste panel scores for predicting tenderness of broilers. Poultry Sci. 53: 2042-2046. Stewart, M K., D. L. Fletcher, D. Hamm, and J. E. Thomson, 1984. The influence of hot boning broiler breast muscle on pH decline and toughening. Poultry Sci. 63: 1935-1939. Thompson, L. D., D. M. Janky, and S. A. Woodward, 1987. Tenderness and physical characteristics of broiler breast fillets harvested at various times from postmortem electrically stimulated carcasses. Poultry Sci. 66:1158-1167.
1989 Poultry Science 68:1232-1235 INTRODUCTION
As poultry processors experience increasing demands for boneless broiler breast meat, efforts are made to decrease production costs in labor, energy, and storage space by harvesting breast meat as soon as possible after slaughter. Consumer response and published research, however, indicate that this practice often results in the production of meat with unacceptable tenderness (Goodwin, 1984; Shelton, 1985; Thompson et al, 1987). Several researchers (Dodge and Stadelman, 1959; de Fremery and Pool, 1960; Stewart et al., 1984; Lyon et al., 1985) have observed that acceptable tenderness levels result only if boning is delayed 3 to 6 h after death of the animal. Post-mortem electrical stimulation has been applied to beef carcasses (Pearson and Dutson, 1985) to accelerate rigor mortis development and allow hot boning. In the area of poultry meat processing, Dransfield et al. (1984)
'Florida Agricultural Experiment Stations Journal Series Number 9041. 2 Present address: Department of Poultry Science, Texas A&M University, College Station, TX 77843-2472. To whom correspondence should be addressed. 4 Present address: Sunny Fresh Foods, P.O. Box 428, Monticello, MN 55362.
reported no tenderization of turkey breast meat using a 94 V post-mortem stimulation; however, Maki and Froning (1987) reported a significant tenderization response in turkey breast muscle when carcasses were electrically stimulated with 820 V. Researchers at Campbell Institute for Research and Technology have developed and patented a process involving low voltage electrical stimulation coupled with high temperature conditioning to produce acceptable tenderness in broiler breast meat harvested as early as 24 min post-mortem (Amey, 1988). However, this process requires an additional 2,200 feet of shackle line in an existing 185 bird/min line to achieve results. Thompson et al. (1987) reported that high voltage (820 V), short time (15 s) electrical stimulation increased tenderness levels of breast meat harvested immediately postchill to acceptable levels. In this latter study, the authors found no relationship between changes in post-mortem muscle metabolism and tenderness changes in the cooked muscle due to postmortem electrical stimulation similar to the relationship reported for red meats (Pearson and Dutson, 1985). Also, no tenderness response was observed due to high voltage electrical stimulation in hot-boned (10-min post-mortem boning) or age-boned (48-h postmortem boning) meat. The purpose of the
1232
Downloaded from http://ps.oxfordjournals.org/ at University of New Orleans on May 27, 2015
ABSTRACT This study was designed to compare tenderness (shear force) and post-mortem metabolism at time of boning (R value) of broiler breast fillets harvested from a total of 200 electrically stimulated carcasses (840 V, 340 mA, pulsed 2 s on, 1 s off for 15 s) and 200 nonstimulated (control) carcasses at various times post-mortem up to 240 min. As on-carcass aging time was increased prior to fillet harvesting, shear force values decreased for fillets from both electrically stimulated and nonstimulated carcasses. However, regression analysis of the data indicated that fillets harvested 100 min post-mortem from electrically stimulated carcasses were as tender as fillets harvested at 240 min post-mortem from nonstimulated carcasses. For those processors currently holding broiler carcasses for 3 to 4 h after chilling prior to boning, electrical stimulation would provide a decrease of approximately 60% in the holding time to reach tenderness levels similar to those attained without electrical stimulation. Electrical stimulation did not have a significant effect on the rate of post-mortem metabolism as measured by R value. (Key words: electrical stimulation, boning time, tenderness, broiler, fillet)
TENDERNESS AND ELECTRICAL STIMULATION
present study was to monitor broiler breast meat tenderness development and post-mortem metabolism as affected by high voltage electrical stimulation and post-mortem boning time through the first 4 h following death of the animal. MATERIALS AND METHODS
lation treatment (20 min post-mortem). Previously harvested right side fillets and remaining carcasses were chilled together in agitated tap water for 10 min at 21 C, followed by an additional 30-min chill at 1 C. Fillets were harvested as before from one carcass within each stimulation treatment after the 10min, 21-C prechill (30 min post-mortem) and after 10, 20, and 30 min of chilling at 1 C (40, 50, and 60 min post-mortem). Right side fillets were returned to the appropriate chilling solution and left side fillets were sampled for R value analysis as previously described. A chill water-to-carcass/fillet ratio of 3:1 (wt/wt) and an ice-to-water ratio (1 C chill solution only) of 1:9 (wt/wt) were maintained. Chill water agitation was achieved by manually moving carcass/fillet-filled wire baskets in an up-and-down motion in the chill tanks throughout the chilling period. Following the chill period, all right side fillets and the remaining eight carcasses per replication were packed in crushed ice and stored at 2 C. Fillet harvesting and R value sampling procedures were repeated on one carcass per stimulation treatment at 75, 90, 120, and 240 min post-mortem. At 24 h postmortem all fillets were removed from the crushed ice, reidentified with self-piercing wingbands attached at the caudal end of each fillet, separated from the wing, packaged in Cryovac® bags (W. R. Grace & Co., Apex, NC), frozen, and stored at -40 C until analyzed for shear force (less than 4 wk). Fillets were thawed (48 h, 2 C) and baked to an internal temperature of 82 C using procedures described by Thompson et al. (1987). After cooling to room temperature, cooked fillets were wrapped in aluminum foil and held at 2 C overnight for shear force analysis. Two samples (40 x 20 x 7 mm) from each fillet (cranial and medial portions) were sectioned and analyzed for shear force with a Food Technology Texture Test System® (Food Technology Corporation, Rockville, MD) using procedures described by Dukes and Janky (1984). The two shear values obtained for each fillet were averaged to produce a mean shear force per carcass measurement prior to statistical analysis of the data. These procedures resulted in a factorial arrangement of two stimulation treatments (control and stimulated) and ten boning times (10, 20, 30, 40, 50, 60, 75, 90, 120, and 240
Downloaded from http://ps.oxfordjournals.org/ at University of New Orleans on May 27, 2015
Cobb, feather-sexed, male broilers were reared in litter-covered floor pens and fed commercial-type starter, finisher, and withdrawal corn-soybean diets. At 49 days of age, 100 broilers within a weight range of 1,700 to 2,200 g were selected from the general population, cooped (10 birds/coop), and held for 16 h without access to food or water. Broilers were wingbanded and slaughtered in five replicate groups of 20 birds each using electrical stunning with a Cervin Model FS stunner (Cervin Electrical Systems, Inc., Minneapolis, MN) (setting No. 4) followed by exsanguination. Following a 90-s bleeding period, half of the carcasses were electrically stimulated using the Cervin Model FS stunner equipped with a rheostat to maintain a constant voltage of 820 V (340 mA). Current was pulsed (2 s on, 1 s off for 15 s) from the kill knife (positive electrode), placed on the dorsal surface of the last cervical vertebra, through the carcass to the shackle line (negative electrode). Both control (nonstimulated) and electrically stimulated carcasses were then subscalded (60 C, 45 s) and picked in a rotary drum picker (25 s). Using the stripping technique described by Hamm (1981), both breast fillets were harvested from one control and one electrically stimulated carcass per replication at 10 min post-mortem. A 2.54-cm long section of the anterior portion of each left-side fillet was sampled, frozen in liquid nitrogen, and stored at -40 C for later R value analysis using the procedure described by Thompson et al. (1987). The R value, the ratio of the concentration of various adenine phosphatidyl compounds (ATP, ADP, AMP, and others) to that of a major breakdown product, inosine monophosphate (IMP), was calculated as the ratio of absorbance at 250 nm (IMP) divided by absorbance at 260 nm (ATP). The R value serves as a good monitor of ATP depletion. Remaining picked carcasses were eviscerated and fillets harvested and sampled as previously described from one carcass within each stimu-
1233
1234
SAMS ET AL. RESULTS AND DISCUSSION
E 10
Control y= .000138 x 2 -.048x +12.136 r ! = .85 ^ J * range of x = 10 - 240
Electrically stimulated y=.000117x2-.60x +12.631 r!=93 range of x= 10-240
4 OT60 120 180 Post-mortem boning time fmin)
240
min post-mortem) within each of the five replicate groups per trial. The entire experiment was repeated with an additional 100-bird trial. Two additional 100-bird trials were conducted as previously described except that analysis for R value was omitted. Data were subjected to analysis of variance and regression procedures using computer programs available in the Statistical Analysis System (SAS, 1985). Because no significant trial x treatment or boning time interactions were observed, shear force data from all four trials (n = 400) and the R value data from two trials (n = 200) were combined. A regression curve for each stimulation treatment was constructed for both shear force and R value as a function of boning time to evaluate the effect of electrical stimulation on post-mortem changes in these parameters. Maximizing the r^ was used to evaluate the regression models for closeness of fit. The significance of improvements in closeness of regression model fit obtained by using second-order regressions instead of linear models was tested with the F test described by Ott (1984). The distances between regression curves within each parameter were compared at 10-min intervals using the least significant difference (LSD) procedure presented in the Statistical Analysis System (SAS, 1985) to estimate the postmortem time at which the curves became significantly different (P<.05).
Downloaded from http://ps.oxfordjournals.org/ at University of New Orleans on May 27, 2015
FIGURE 1. Regression lines representing changes in shear force of cooked broiler breast fillets from broiler carcasses electrically stimulated with 820 V, 340 mA, pulsed 2 s on, 1 s off for 15 s and nonstimulated (control) carcasses as a function of post-mortem boning time.
Cooked meat shear force values decreased as post-mortem time interval prior to fillet harvesting was increased for meat from both control and electrically stimulated carcasses (Figure 1). This pattern of shear force decrease with increasing post-mortem interval prior to boning has been well documented in broiler breast meat from both nonstimulated (de Fremery and Poole, 1960; Stewart et al., 1984; Sams and Janky, 1986) and electrically stimulated (Thompson et al., 1987) carcasses. Simpson and Goodwin (1974) found that cooked broiler light meat with a shear force value of 8 kg force/g or less would be considered tender by sensory panelists. Regression of shear force for cooked meat from control carcasses as a function of post-mortem boning time indicated that shear force reached a minimum level of approximately 8 kg force/ g at approximately 180 min post-mortem and maintained this level through 240 min (Figure 1). However, regression of shear force for cooked meat from electrically stimulated carcasses as a function of post-mortem boning time indicated a more rapid decline in shear force, reaching a level of 8 kg force/g after 100 min post-mortem and continuing to decline to a minimum value of approximately 5 kg force/ g at 240 min. This minimum shear force value would be similar to that observed for meat from carcasses that had been aged for 24 h prior to boning (Sams and Janky, 1986). A statistical comparison of the two regression lines indicated that electrical stimulation of carcasses produced a significantly (P<.05) lower shear force in cooked meat than in cooked meat from control carcasses when postmortem boning times were equal to or greater than 100 min. For those processors currently holding broiler carcasses for 3 to 4 h after chilling prior to boning, electrical stimulation would provide a decrease of approximately 60% in the holding time to reach tenderness levels similar to those attained without electrical stimulation. Raw meat R values (ratio of absorbance at 250 nm to absorbance at 260 nm), even though low, increased as post-mortem boning time was increased, regardless of the presence or lack of the electrical stimulation treatment (Figure 2), indicating that R value did in fact increase as rigor mortis developed. However, regression analysis of the data indicated that electrical stimulation did not produce a signifi
TENDERNESS AND ELECTRICAL STIMULATION
1235
REFERENCES y
y /
Control y = .000226x +.847
/
y
r 2 =.89
T -87
y /
^^y y
O
^
y^^
60 Post-mortem
'
/ ^ ^-"^
^
^
^ - ^ ^
^^*
^^"^
S s > ^ • ^
/
%
range of x = 10-240 y y
Electrically stimulated y = . 000143 x + . 852 range of x = 10 - 240 r2 = .64
120 180 boning time (min)
240
cant increase in the rate of R value change over mat associated with control samples. Calkins et al. (1982) reported that post-mortem increases in R value were a direct result of the normal catabolism of adenine nucleotides associated with post-mortem metabolism leading to the development of rigor mortis. Carse (1973) attributed the increased rate of tenderness development in electrically stimulated red meat carcasses to an acceleration of postmortem metabolism that would result in earlier onset of rigor mortis, thus negating the adverse effects of early boning procedures on the degree of rigor mortis development. In the present study, however, the relationship of increased tenderness development due to electrical stimulation could not be explained by post-mortem metabolism changes as measured by R value. Thompson et al. (1987), in work with chicken muscle tissue, also were unable to show a relationship between post-mortem metabolism changes and increased levels of tenderness attributed to high voltage (820 V) electrical stimulation. Those authors concluded that tenderness improvement was a result of increased physical disruption of the myofibrils and increased sarcomere lengths observed in electrically stimulated muscle tissue.
Downloaded from http://ps.oxfordjournals.org/ at University of New Orleans on May 27, 2015
FIGURE 2. Regression lines representing changes in R value (ratio of absorbance at 250 run to absorbance at 260 nm) of broiler breast fillets from electrically stimulated (820 V, 340 mA, pulsed 2 s on, 1 s off for 15 s) and nonstimulated (control) broiler carcasses as a function of post-mortem boning time.
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