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Fragmentation, Tenderness, and Post-Mortem Metabolism of Early-Harvested Broiler Breast Fillets from Carcasses Treated with Electrical Stimulation and Muscle Tensioning1
PROCESSING AND PRODUCTS Fragmentation, Tenderness, and Post-Mortem Metabolism of Early-Harvested Broiler Breast Fillets from Carcasses Treated with Electrical Stimulation and Muscle Tensioning1 S. G. BIRKHOLD and A. R. SAMS2 Department of Poultry Science, Texas A&M University System, College Station, Texas 77843-2472
1993 Poultry Science 72:577-582
INTRODUCTION Increased consumer willingness to pay a premium price for cut-up poultry has shifted the marketing focus of the broiler industry from fresh, whole carcasses to fresh, cut-up poultry and further processed products. Currently, broiler carcasses must be aged for a minimum of 4 h to avoid the toughening that accompanies prerigor harvesting of broiler breast meat (Stewart et at, 1984; Dawson et al., 1987; Lyon et al., 1989). Because holding carcasses for the aging period is expensive, post-mortem electrical stimulation (ES) has been examined as a means to eliminate the toughness associated with early harvesting (1 h post-mortem) of broiler breast fillets (Thompson et ah, 1987; Lyon et al, 1989; Sams et al, 1989). Research by Birkhold et al. (1992) indicated that a tandem treatment of high
Received for publication June 15, 1992. Accepted for publication November 12, 1992. 1 This research was supported, in part, by Grant 999902150 from the Texas Higher Education Coordinating Board Advanced Technology Program. 2 To whom correspondence should be addressed.
voltage (820 V) ES and prechill muscle tensioning (MT) would improve the tenderness of early-harvested broiler breast fillets. The authors found that improvements in tenderness from MT carcasses were not always attributable to increased sarcomere lengths and suggested that fragmentation of muscle fibers may play a role in tenderizing fillets harvested at 1 h post-mortem from carcasses treated with MT. In addition to preventing excessive sarcomere shortening, it has also been proposed that myofibrillar disruption (fragmentation) plays a role in the tenderization that accompanies post-mortem ES (Cross, 1979; Thompson et al, 1987). This fragmentation mechanism has not been investigated in poultry meat. Although Birkhold et al. (1992) used high voltage (820 V) ES to prevent toughness associated with harvesting of broiler breast meat at 1 h post-mortem, this high voltage raises concerns for the safety of plant personnel. It would be desirable to investigate the tenderizing effectiveness of ES and combination ES and MT treatments using a lower voltage. Therefore, the objectives of this research were to examine the effects of 440 V ES
577
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 5, 2015
ABSTRACT Two 72-bird trials were conducted to determine the effects of 15 s electrical stimulation (ES) (440 V, 2 s on and 1 s off) and prechill muscle tensioning (MT) on fragmentation, tenderness, and post-mortem metabolism of early-harvested (1 h post-mortem) broiler breast fillets. Compared with controls, all treatments increased sarcomere length and decreased shear value. Electrical stimulation reduced muscle pH values. Histological examination of samples from fillets harvested early and then aged 24 h revealed that all treatments increased fiber disruption compared with controls. Both fragmentation and excessive sarcomere shortening prevention were important to the improvement in fillet tenderness associated with the ES and MT treatments. (Key words: fragmentation, electrical stimulation, muscle tensioning, tenderness, broiler meat)
578
BIRKHOLD AND SAMS
and MT on tenderness, sarcomere length, and post-mortem pH decline of earlyharvested broiler breast fillets and to examine the effects of these treatments on muscle fragmentation using both a myofibrillar fragmentation index procedure and light microscopy. MATERIALS AND METHODS
3
Cervin Electrical Systems, Minneapolis, MN 55410. 4 Brower, Houghton, IA 52631. 5 Instron Corp., Canton, MA 02021.
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 5, 2015
In each of two 72-bird trials, male broilers were raised to 7 wk of age in litter-covered floor pens and fed a corn and soybean-based diet (20% CP; 3,234 kcal of ME/kg of feed). Eight to 12 h prior to processing, birds were cooped (six birds per coop) without access to feed and water. Twelve broilers were hung on the shackle line per replication, stunned using a Cervin3 Model S stunner on low (220 V, 1 A) for 5 s, and allowed to bleed through a neck cut for 90 s. Then three carcasses from each replicate were assigned to either no treatment, 15 s of ES, prechill MT, or the combination treatment of 15 s of ES and prechill MT. For the ES-treated carcasses (six per replicate), a 12-gauge multistrand copper wire attached to the shackles was clipped to the posterior end of the keel of each carcass with a copper clip. The shackle rail system functioned as the ground. Electrical stimulation (440 V, 1 A) was applied by placing the kill knife of the Cervin3 Model S stunner against the dry skin across the extreme anterior portion of both right and left Pectoralis muscles. Current was pulsed on for 2 s and off for 1 s for five pulses. After ES, all carcasses were subscalded together at 60 C for 45 s, defeathered in a Brower4 Model SP 30SS rotary drum picker, and manually eviscerated. Muscle tensioning was applied to all six birds in each replication that was to receive MT or the combination treatment immediately after evisceration and just prior to entering the prechill tank (approximately 15 min post-mortem). Muscle tensioning was
applied by binding the wings behind the back at the elbow with plastic cable ties. All 12 carcasses were chilled together using a two-stage chilling regimen of 15 min prechill in tap water at 20 C, followed by 30 min chill in 1 C ice slush. Pectoralis muscles were removed immediately following chilling (1 h postmortem) by severing the humeral-scapular joint and pulling firmly downward on the wing to strip the muscle from the carcass (Hamm, 1981). Wings and skin were removed from fillets immediately after harvesting. The left fillet from all carcasses was aged in crushed ice for 24 h, cooked to an internal temperature of 85 C, and sheared using an Allo-Kramer shear cell on an Instron Universal Testing Machine5 as described by Sams (1990). A 2.5-cm sample for sarcomere length determination was cut from the anterior portion of the right fillet from each carcass, placed into a plastic bag, and immediately frozen in liquid nitrogen. After freezing, these samples were stored at -76 C until sarcomere length determinations were made using the laser diffraction method of Cross et al. (1981). A second sample (10 to 15 g) for fragmentation index was cut from the right fillet just posterior to the first, placed in a plastic bag, aged on ice 24 h, and frozen in liquid nitrogen. After freezing, these samples were held at -76 C until fragmentation index was determined using the oven-dried, gravimetric method described by Sams et al. (1991). A third sample (10 to 15 g) was cut from the right fillet just posterior to the second, placed in a plastic bag, and immediately frozen in liquid nitrogen. After freezing, samples were held at -76 C until pH was determined using the iodoacetate procedure described by Sams and Janky (1986). Five additional replications of 12 broilers each were processed and the entire experiment was repeated (Trial 2). Shear value, sarcomere length, pH, and fragmentation index data were subjected to ANOVA and Duncan's multiple range test (SAS Institute, 1985). The residual mean square error was used as the error term and because no significant interaction between trial and treatment was detected, data from the two trials were pooled.
579
BROILER BREAST FILLETS 1
TABLE 1. Shear value, sarcomere length, pH, and fragmentation index means ± SD of Pectoralis harvested 1 h post-mortem from broiler carcasses that received 440 V electrical stimulation (ES) and muscle tensioning (MT) Treatment Control ES (15 s) MT (during chill) ES + MT
Shear value
Sarcomere length
(kg/g) 12.55 ± 5.61a 8.52 ± 2.26< 10.71 ± 4.34b 7.66 ± 1.81c
1.64 1.85 1.76 1.96
w ± .26<* ± .10" ± .27c ± .14'
Fragmentation index
pH 6.05 5.81 5.99 5.79
± .30a ± .18" ± .24a ± .12b
102.95 105.13 83.16 108.49
± 44.00a ± 46.39' ± 38.88b ± 42.83"
a_d
Means within a column with no common superscripts differ significantly (P £ .05). n = 36 per mean.
x
RESULTS AND DISCUSSION Both the ES and MT treatments significantly reduced shear values when compared with controls (Table 1), but the combined ES and MT treatment yielded fillets that would be considered tender by consumers. This conclusion was based on the observation that their average shear value was below the 8 kg/g tough-tender threshold of Simpson and Goodwin (1974), within the 8.8 to 6.0 kg/g "moderately tender" to "slightly tender" range of
Lyon and Lyon (1990), and below the 8.1 kg/g threshold for "good" to "very good" overall texture acceptability (Lyon and Lyon, 1990). This trend was similar to that reported by Birkhold et al. (1992) when ES was applied at a higher voltage (820 V), suggesting that this lower voltage would be effective for tenderizing earlyharvested broiler breast fillets. The ES treatments significantly reduced pH values at 1 h post-mortem when compared with unstimulated carcasses (Table 1). This suggested an acceleration in the rate of rigor mortis development, which has been associated with an accelerated depletion of adenosine triphosphate (ATP) via the conversion of adenosinecontaining nucleotides to inosinecontaining nucleotides (Thompson et al., 1987; Lyon et al, 1989). Sarcomere length is a measure of the state of muscle contraction and has been associated with tenderness (Locker, 1960). Average sarcomere lengths were shortest in the control birds (Table 1). Control carcasses had significantly shorter sarcomeres than MT carcasses, which had significantly shorter sarcomeres than ES carcasses (Table 1). The carcasses receiving the combined ES and MT treatment had sarcomere lengths that were significantly longer than the average for either treatment alone. This pattern also followed the reduction in shear values, indicating that the prevention of excessive sarcomere shortening is an important factor in improving the tenderness of early-harvested fillets from treated carcasses. Average fragmentation index values were not significantly different for con-
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 5, 2015
An additional replication was processed as previously described. The left fillets were removed at 1 h post-mortem and aged in crushed ice for 24 h. From each left fillet, one sample for light microscopy was dissected from the anterior portion of the aged Pectoralis in approximately 10 mm x 10 mm x 30 mm strips, with the long axis running parallel to the fibers. The Pectoralis strips were then fixed in 10% formalin, imbedded in paraffin, and cut with a microtome into longitudinal sections of 5 to 7 /xm that were stained with eosin and hematoxylin as described by Dougherty (1981) and photographed at 200x magnification. Upon visual examination of the entire area of each section, selected fields were photographed to represent the subjective determination by three meat scientists of distinct and consistent differences in fragmentation due to treatments. Because of the subjective nature of these micrographs, only general, qualitative conclusions can be obtained from their presentation.
580
BIRKHOLD AND SAMS
Light microscopy revealed no fragmentation in control fibers (Figure 1). Electrical stimulation caused longitudinal breaks in the fibers, but they were not accompanied by wide gaps between the fiber ends (Figure 2). Muscle tensioning caused a wavy appearance in muscle from these carcasses (Figure 3A). Some of the wrinkling may be the result of "passive contraction" as described by Voyle (1969) in which the broken uncontracted fiber is
FIGURE 1. Photomicrograph of longitudinal sections of Pectoralis from control carcasses. Magnification 200x, bar indicates 25 [im.
crumpled by the contraction of the adjacent fibers to which it is attached. Wrinkling similar to that observed in fibers from MT carcasses (Figure 3A) was described by Rowe (1982) as "macroscopic wrinkling," which occurred when raw bovine muscles entered rigor restrained, later shortening when freed due to the elastic nature of the perimysium. Muscle tensioning also caused longitudinal breaks in the fibers accompanied by wide gaps between the fiber ends (Figure 3B). Electrical stimulation combined with MT resulted in the combination of wrinkled, wavy fibers and longitudinal fragmentation with and without gap formation (Figure 4). These differing types of myofibrillar fragmentation may not all be
FIGURE 2. Photomicrograph of longitudinal sections of Pectoralis from carcasses treated with electrical stimulation. Arrows indicate points of longitudinal fragmentation without large gap formation. Magnification 200x, bar indicates 25 /jm.
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 5, 2015
trols, ES, or combined ES and MT carcasses. Carcasses receiving only MT had significantly more fragmentation, as indicated by lower index values (Table 1). Excessively shortened sarcomeres in control fillets (Table 1) may have provided protection to muscle fibers from the endogenous proteases that are responsible for increased tenderness during aging (Pearson and Young, 1989). This would be consistent with what Davey et al. (1967) noted as the inability of cold-shortened beef to successfully undergo aging. Additionally, because samples from carcasses receiving only MT had the most fragmentation and longer sarcomeres than controls (Table 1), it suggested that the amount of fiber overlap is important for allowing post-mortem proteolysis. Fiber overlap does not, however, explain why there was no increased fragmentation measured for ES-treated carcasses. It is possible that the type of fragmentation supposedly induced by ES is not detectable with this fragmentation index procedure. In support of this theory, the ES and the combined ES and MT treatments had significantly longer sarcomeres but significantly less fragmentation than the MT carcasses. Thus, the state of muscle contraction was not the only factor important for successful post-mortem aging. The lower pH of the ES-treated muscles may have inhibited myofibrillar fragmentation, despite the longer sarcomeres of these muscles compared with the controls. Sams et al. (1991) reported that ES and high temperature conditioning prevented post-mortem myofibrillar fragmentation. The authors postulated that because these post-mortem treatments accelerated the normal decline in intracellular pH, the activity of the calciumactivated, neutral protease was reduced.
581
BROILER BREAST FILLETS
W • N
equally detected by the fragmentation index procedure used in this experiment. This conclusion was based on the observation that the ES-containing treatments tenderized early-harvested fillets, at least partially by causing myofibrillar fragmentation that was observable with a light microscope but not detected by the gravimetric fragmentation index procedure used in this experiment (Table 1). ACKNOWLEDGMENT
REFERENCES
FIGURE 3. Photomicrographs of longitudinal sections of Pectoralis from carcasses treated with muscle tensioning. A) The difference between wavy fibers (W) and normal fibers (N). B) Presence of longitudinal fragmentation accompanied by large gaps. Magnification 200x, bar indicates 25 jtm.
W
w
FIGURE 4. Photomicrograph of longitudinal sections of Pectoralis from carcasses treated with electrical stimulation and muscle tensioning. Arrows indicate longitudinal fragmentation without large gap formation; G = longitudinal fragmentation with large gap formation, and W = wavy fibers. Magnification 200x, bar indicates 25 /im.
Birkhold, S. G., D. M. Janky, and A. R. Sams, 1992. Tenderization of early-harvested broiler breast fillets by high-voltage post-mortem electrical stimulation and muscle tensioning. Poultry Sci. 71:2106-2112. Cross, H. R., 1979. Effects of electrical stimulation on meat tissue and muscle properties—a review. J. Food Sci. 44:509-524. Cross, H. R., R. L. West, and T. R. Dutson, 1981. Comparison of methods for measuring sarcomere length in beef Semitendinosus muscle. Meat Sci. 5:261-266. Davey, C. L., H. Kuttel, and K. V. Gilbert, 1967. Shortening as a factor in meat aging. J. Food Technol. 2:53-56. Dawson, P. L., D. M. Janky, M. G. Dukes, L. D. Thompson, and S. A. Woodward, 1987. Effect of post-mortem boning time during simulated commercial processing on the tenderness of broiler breast meat. Poultry Sci. 66:1331-1333. Dougherty, W. I., 1981. Preparation of semi-thin sections of tissues embedded in water-soluble methacrylate for light microscopy. Pages 27-38 in: Staining Procedures. 4th ed. G. Clark, ed. Williams and Wilkins, Baltimore, MD. Hamm, D., 1981. Unconventional meat harvesting. Poultry Sci. 60:1666.(Abstr.) Locker, R. H., 1960. Degree of muscular contraction as a factor in tenderness of beef. Food Res. 25: 304-307. Lyon, C. E., C. E. Davis, J. A. Dickens, C. M. Papa, and J. O. Reagan, 1989. Effects of electrical stimulation on the post-mortem biochemical changes and texture of broiler Pectoralis muscle. Poultry Sci. 68:249-257. Lyon, C. E., and B. G. Lyon, 1990. The relationship of objective shear values and sensory tests to changes in tenderness of broiler breast meat. Poultry Sci. 69:1420-1427. Pearson, A. M., and R. B. Young, 1989. Postmortem changes during conversion of muscle to meat. Pages 391-444 in: Muscle and Meat Biochemistry. A. M. Pearson and R. B. Young, ed. Academic Press, Inc., San Diego, CA.
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 5, 2015
The authors are grateful for the assistance of B. M. Hargis in the preparation and evaluation of microscopy samples.
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BIRKHOLD AND SAMS SAS Institute, 1985. SAS® User's Guide: Statistics. Version 5 Edition. SAS Institute Inc., Cary, NC. 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 post-mortem electrically stimulated carcasses. Poultry Sci. 66:1158-1167. Voyle, C. A., 1969. Some observations on the histology of cold-shortened muscle. J. Food Technol. 4:275-281.
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 5, 2015
Rowe, R.W.D., 1982. Muscle fibre deformation: rigor extensibility of raw bovine muscle. Meat Sci. 6: 199-209. Sams, A. R., 1990. Electrical stimulation and high temperature conditioning of broiler carcasses. Poultry Sci. 69:1781-1786. Sams, A. R., and D. M. Janky, 1986. The influence of brine chilling on tenderness of hot-boned, chillboned, and age-boned broiler breast fillets. Poultry Sci. 65:1316-1321. Sams, A. R., D. M. Janky, and S. A. Woodward, 1989. Tenderness and R-value changes in early harvested broiler breast tissue following postmortem electrical stimulation. Poultry Sci. 68: 1232-1235. Sams, A. R., S. G. Birkhold, and K. A. Mills, 1991. Fragmentation and tenderness in breast muscle from broiler carcasses treated with electrical stimulation and high-temperature conditioning. Poultry Sci. 70:1430-1433.
1993 Poultry Science 72:577-582
INTRODUCTION Increased consumer willingness to pay a premium price for cut-up poultry has shifted the marketing focus of the broiler industry from fresh, whole carcasses to fresh, cut-up poultry and further processed products. Currently, broiler carcasses must be aged for a minimum of 4 h to avoid the toughening that accompanies prerigor harvesting of broiler breast meat (Stewart et at, 1984; Dawson et al., 1987; Lyon et al., 1989). Because holding carcasses for the aging period is expensive, post-mortem electrical stimulation (ES) has been examined as a means to eliminate the toughness associated with early harvesting (1 h post-mortem) of broiler breast fillets (Thompson et ah, 1987; Lyon et al, 1989; Sams et al, 1989). Research by Birkhold et al. (1992) indicated that a tandem treatment of high
Received for publication June 15, 1992. Accepted for publication November 12, 1992. 1 This research was supported, in part, by Grant 999902150 from the Texas Higher Education Coordinating Board Advanced Technology Program. 2 To whom correspondence should be addressed.
voltage (820 V) ES and prechill muscle tensioning (MT) would improve the tenderness of early-harvested broiler breast fillets. The authors found that improvements in tenderness from MT carcasses were not always attributable to increased sarcomere lengths and suggested that fragmentation of muscle fibers may play a role in tenderizing fillets harvested at 1 h post-mortem from carcasses treated with MT. In addition to preventing excessive sarcomere shortening, it has also been proposed that myofibrillar disruption (fragmentation) plays a role in the tenderization that accompanies post-mortem ES (Cross, 1979; Thompson et al, 1987). This fragmentation mechanism has not been investigated in poultry meat. Although Birkhold et al. (1992) used high voltage (820 V) ES to prevent toughness associated with harvesting of broiler breast meat at 1 h post-mortem, this high voltage raises concerns for the safety of plant personnel. It would be desirable to investigate the tenderizing effectiveness of ES and combination ES and MT treatments using a lower voltage. Therefore, the objectives of this research were to examine the effects of 440 V ES
577
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 5, 2015
ABSTRACT Two 72-bird trials were conducted to determine the effects of 15 s electrical stimulation (ES) (440 V, 2 s on and 1 s off) and prechill muscle tensioning (MT) on fragmentation, tenderness, and post-mortem metabolism of early-harvested (1 h post-mortem) broiler breast fillets. Compared with controls, all treatments increased sarcomere length and decreased shear value. Electrical stimulation reduced muscle pH values. Histological examination of samples from fillets harvested early and then aged 24 h revealed that all treatments increased fiber disruption compared with controls. Both fragmentation and excessive sarcomere shortening prevention were important to the improvement in fillet tenderness associated with the ES and MT treatments. (Key words: fragmentation, electrical stimulation, muscle tensioning, tenderness, broiler meat)
578
BIRKHOLD AND SAMS
and MT on tenderness, sarcomere length, and post-mortem pH decline of earlyharvested broiler breast fillets and to examine the effects of these treatments on muscle fragmentation using both a myofibrillar fragmentation index procedure and light microscopy. MATERIALS AND METHODS
3
Cervin Electrical Systems, Minneapolis, MN 55410. 4 Brower, Houghton, IA 52631. 5 Instron Corp., Canton, MA 02021.
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 5, 2015
In each of two 72-bird trials, male broilers were raised to 7 wk of age in litter-covered floor pens and fed a corn and soybean-based diet (20% CP; 3,234 kcal of ME/kg of feed). Eight to 12 h prior to processing, birds were cooped (six birds per coop) without access to feed and water. Twelve broilers were hung on the shackle line per replication, stunned using a Cervin3 Model S stunner on low (220 V, 1 A) for 5 s, and allowed to bleed through a neck cut for 90 s. Then three carcasses from each replicate were assigned to either no treatment, 15 s of ES, prechill MT, or the combination treatment of 15 s of ES and prechill MT. For the ES-treated carcasses (six per replicate), a 12-gauge multistrand copper wire attached to the shackles was clipped to the posterior end of the keel of each carcass with a copper clip. The shackle rail system functioned as the ground. Electrical stimulation (440 V, 1 A) was applied by placing the kill knife of the Cervin3 Model S stunner against the dry skin across the extreme anterior portion of both right and left Pectoralis muscles. Current was pulsed on for 2 s and off for 1 s for five pulses. After ES, all carcasses were subscalded together at 60 C for 45 s, defeathered in a Brower4 Model SP 30SS rotary drum picker, and manually eviscerated. Muscle tensioning was applied to all six birds in each replication that was to receive MT or the combination treatment immediately after evisceration and just prior to entering the prechill tank (approximately 15 min post-mortem). Muscle tensioning was
applied by binding the wings behind the back at the elbow with plastic cable ties. All 12 carcasses were chilled together using a two-stage chilling regimen of 15 min prechill in tap water at 20 C, followed by 30 min chill in 1 C ice slush. Pectoralis muscles were removed immediately following chilling (1 h postmortem) by severing the humeral-scapular joint and pulling firmly downward on the wing to strip the muscle from the carcass (Hamm, 1981). Wings and skin were removed from fillets immediately after harvesting. The left fillet from all carcasses was aged in crushed ice for 24 h, cooked to an internal temperature of 85 C, and sheared using an Allo-Kramer shear cell on an Instron Universal Testing Machine5 as described by Sams (1990). A 2.5-cm sample for sarcomere length determination was cut from the anterior portion of the right fillet from each carcass, placed into a plastic bag, and immediately frozen in liquid nitrogen. After freezing, these samples were stored at -76 C until sarcomere length determinations were made using the laser diffraction method of Cross et al. (1981). A second sample (10 to 15 g) for fragmentation index was cut from the right fillet just posterior to the first, placed in a plastic bag, aged on ice 24 h, and frozen in liquid nitrogen. After freezing, these samples were held at -76 C until fragmentation index was determined using the oven-dried, gravimetric method described by Sams et al. (1991). A third sample (10 to 15 g) was cut from the right fillet just posterior to the second, placed in a plastic bag, and immediately frozen in liquid nitrogen. After freezing, samples were held at -76 C until pH was determined using the iodoacetate procedure described by Sams and Janky (1986). Five additional replications of 12 broilers each were processed and the entire experiment was repeated (Trial 2). Shear value, sarcomere length, pH, and fragmentation index data were subjected to ANOVA and Duncan's multiple range test (SAS Institute, 1985). The residual mean square error was used as the error term and because no significant interaction between trial and treatment was detected, data from the two trials were pooled.
579
BROILER BREAST FILLETS 1
TABLE 1. Shear value, sarcomere length, pH, and fragmentation index means ± SD of Pectoralis harvested 1 h post-mortem from broiler carcasses that received 440 V electrical stimulation (ES) and muscle tensioning (MT) Treatment Control ES (15 s) MT (during chill) ES + MT
Shear value
Sarcomere length
(kg/g) 12.55 ± 5.61a 8.52 ± 2.26< 10.71 ± 4.34b 7.66 ± 1.81c
1.64 1.85 1.76 1.96
w ± .26<* ± .10" ± .27c ± .14'
Fragmentation index
pH 6.05 5.81 5.99 5.79
± .30a ± .18" ± .24a ± .12b
102.95 105.13 83.16 108.49
± 44.00a ± 46.39' ± 38.88b ± 42.83"
a_d
Means within a column with no common superscripts differ significantly (P £ .05). n = 36 per mean.
x
RESULTS AND DISCUSSION Both the ES and MT treatments significantly reduced shear values when compared with controls (Table 1), but the combined ES and MT treatment yielded fillets that would be considered tender by consumers. This conclusion was based on the observation that their average shear value was below the 8 kg/g tough-tender threshold of Simpson and Goodwin (1974), within the 8.8 to 6.0 kg/g "moderately tender" to "slightly tender" range of
Lyon and Lyon (1990), and below the 8.1 kg/g threshold for "good" to "very good" overall texture acceptability (Lyon and Lyon, 1990). This trend was similar to that reported by Birkhold et al. (1992) when ES was applied at a higher voltage (820 V), suggesting that this lower voltage would be effective for tenderizing earlyharvested broiler breast fillets. The ES treatments significantly reduced pH values at 1 h post-mortem when compared with unstimulated carcasses (Table 1). This suggested an acceleration in the rate of rigor mortis development, which has been associated with an accelerated depletion of adenosine triphosphate (ATP) via the conversion of adenosinecontaining nucleotides to inosinecontaining nucleotides (Thompson et al., 1987; Lyon et al, 1989). Sarcomere length is a measure of the state of muscle contraction and has been associated with tenderness (Locker, 1960). Average sarcomere lengths were shortest in the control birds (Table 1). Control carcasses had significantly shorter sarcomeres than MT carcasses, which had significantly shorter sarcomeres than ES carcasses (Table 1). The carcasses receiving the combined ES and MT treatment had sarcomere lengths that were significantly longer than the average for either treatment alone. This pattern also followed the reduction in shear values, indicating that the prevention of excessive sarcomere shortening is an important factor in improving the tenderness of early-harvested fillets from treated carcasses. Average fragmentation index values were not significantly different for con-
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 5, 2015
An additional replication was processed as previously described. The left fillets were removed at 1 h post-mortem and aged in crushed ice for 24 h. From each left fillet, one sample for light microscopy was dissected from the anterior portion of the aged Pectoralis in approximately 10 mm x 10 mm x 30 mm strips, with the long axis running parallel to the fibers. The Pectoralis strips were then fixed in 10% formalin, imbedded in paraffin, and cut with a microtome into longitudinal sections of 5 to 7 /xm that were stained with eosin and hematoxylin as described by Dougherty (1981) and photographed at 200x magnification. Upon visual examination of the entire area of each section, selected fields were photographed to represent the subjective determination by three meat scientists of distinct and consistent differences in fragmentation due to treatments. Because of the subjective nature of these micrographs, only general, qualitative conclusions can be obtained from their presentation.
580
BIRKHOLD AND SAMS
Light microscopy revealed no fragmentation in control fibers (Figure 1). Electrical stimulation caused longitudinal breaks in the fibers, but they were not accompanied by wide gaps between the fiber ends (Figure 2). Muscle tensioning caused a wavy appearance in muscle from these carcasses (Figure 3A). Some of the wrinkling may be the result of "passive contraction" as described by Voyle (1969) in which the broken uncontracted fiber is
FIGURE 1. Photomicrograph of longitudinal sections of Pectoralis from control carcasses. Magnification 200x, bar indicates 25 [im.
crumpled by the contraction of the adjacent fibers to which it is attached. Wrinkling similar to that observed in fibers from MT carcasses (Figure 3A) was described by Rowe (1982) as "macroscopic wrinkling," which occurred when raw bovine muscles entered rigor restrained, later shortening when freed due to the elastic nature of the perimysium. Muscle tensioning also caused longitudinal breaks in the fibers accompanied by wide gaps between the fiber ends (Figure 3B). Electrical stimulation combined with MT resulted in the combination of wrinkled, wavy fibers and longitudinal fragmentation with and without gap formation (Figure 4). These differing types of myofibrillar fragmentation may not all be
FIGURE 2. Photomicrograph of longitudinal sections of Pectoralis from carcasses treated with electrical stimulation. Arrows indicate points of longitudinal fragmentation without large gap formation. Magnification 200x, bar indicates 25 /jm.
Downloaded from http://ps.oxfordjournals.org/ at University of Georgia on June 5, 2015
trols, ES, or combined ES and MT carcasses. Carcasses receiving only MT had significantly more fragmentation, as indicated by lower index values (Table 1). Excessively shortened sarcomeres in control fillets (Table 1) may have provided protection to muscle fibers from the endogenous proteases that are responsible for increased tenderness during aging (Pearson and Young, 1989). This would be consistent with what Davey et al. (1967) noted as the inability of cold-shortened beef to successfully undergo aging. Additionally, because samples from carcasses receiving only MT had the most fragmentation and longer sarcomeres than controls (Table 1), it suggested that the amount of fiber overlap is important for allowing post-mortem proteolysis. Fiber overlap does not, however, explain why there was no increased fragmentation measured for ES-treated carcasses. It is possible that the type of fragmentation supposedly induced by ES is not detectable with this fragmentation index procedure. In support of this theory, the ES and the combined ES and MT treatments had significantly longer sarcomeres but significantly less fragmentation than the MT carcasses. Thus, the state of muscle contraction was not the only factor important for successful post-mortem aging. The lower pH of the ES-treated muscles may have inhibited myofibrillar fragmentation, despite the longer sarcomeres of these muscles compared with the controls. Sams et al. (1991) reported that ES and high temperature conditioning prevented post-mortem myofibrillar fragmentation. The authors postulated that because these post-mortem treatments accelerated the normal decline in intracellular pH, the activity of the calciumactivated, neutral protease was reduced.
581
BROILER BREAST FILLETS
W • N
equally detected by the fragmentation index procedure used in this experiment. This conclusion was based on the observation that the ES-containing treatments tenderized early-harvested fillets, at least partially by causing myofibrillar fragmentation that was observable with a light microscope but not detected by the gravimetric fragmentation index procedure used in this experiment (Table 1). ACKNOWLEDGMENT
REFERENCES
FIGURE 3. Photomicrographs of longitudinal sections of Pectoralis from carcasses treated with muscle tensioning. A) The difference between wavy fibers (W) and normal fibers (N). B) Presence of longitudinal fragmentation accompanied by large gaps. Magnification 200x, bar indicates 25 jtm.
W
w
FIGURE 4. Photomicrograph of longitudinal sections of Pectoralis from carcasses treated with electrical stimulation and muscle tensioning. Arrows indicate longitudinal fragmentation without large gap formation; G = longitudinal fragmentation with large gap formation, and W = wavy fibers. Magnification 200x, bar indicates 25 /im.
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The authors are grateful for the assistance of B. M. Hargis in the preparation and evaluation of microscopy samples.
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