Effect of 3 postmortem electrical stimulation treatments on the quality of early deboned broiler breast meat1

Effect of 3 postmortem electrical stimulation treatments on the quality of early deboned broiler breast meat1

Effect of 3 postmortem electrical stimulation treatments on the quality of early deboned broiler breast meat1 H. zhuang,2 E. M. Savage, and K. Lawrenc...

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Effect of 3 postmortem electrical stimulation treatments on the quality of early deboned broiler breast meat1 H. zhuang,2 E. M. Savage, and K. Lawrence Quality and Safety Assessment Research Unit, USDA, Agricultural Research Service, Richard B. Russell Research Center, PO Box 5677, Athens, GA 30604-5677 ABSTRACT The present experiment was carried out to evaluate the effects of electrical stimulation (ES) immediately prescalding (PS), ES immediately postdefeathering (PD), or PS combined with PD (PSPD) on the quality of early deboned (2 h) broiler breast muscles, pectoralis major (fillets), and pectoralis minor (tenders). No stimulation, early-deboned (2 h), and 24-h deboned (24 h) fillets were used for the comparison. The 42-d-old broiler carcasses were electrically stimulated with pulsed current at 200 V for 30 s over a 90-s time interval (total of 1 min over 180 s for PSPD), and breast meat was deboned 2 h postmortem. Quality indicators evaluated were CIE L*, a*, and b* color and pH of the raw fillets and cook yields and Warner-Bratzler (WB) shear force of the fillets and tenders. There were no differences in raw fillet color, pH, and cook yields of both the fillets and tenders between the 3 ES

treatments. Effects of different ES treatments on meat WB shear force values varied with breast muscles. For the fillets, the average WB shear force values of both the PS and PSPD samples, which were not different from each other, were significantly lower than those of the PD samples. For the tenders, there were no differences in the average shear force values between the 3 ES treatments. Regardless of ES treatment and breast muscle, early deboned broiler breast meat from ES carcasses required significantly less force to shear than the 2-h control. These results indicate that ES can tenderize early deboned poultry breast muscles; however, the effectiveness of ES tenderization varies with ES treatments for the fillets. The PS treatment is more effective in reducing fillet shear values than PD, and there is no further reduction in shear values with PSPD compared with the PS treatment.

Key words: broiler, electrical stimulation, breast meat, quality, shear force 2010 Poultry Science 89:1737–1743 doi:10.3382/ps.2009-00460

INTRODUCTION Postmortem electrical stimulation (ES) of animal carcasses has been used by the red meat industry to minimize toughening associated with cold shortening for the past 4 decades (Barbut, 2002) and has been adapted for commercial use with poultry (Sams, 2002). The newest commercially available ES equipment stimulates poultry carcasses after defeathering (Young et al., 2005). Webb et al. (1989) suggested applying ES twice during processing (during bleed-out or prescalding followed by additional ES after scalding) to further reduce the postmortem aging time required for tenderness of boneless skinless poultry breast fillets (pectoralis major). This

©2010 Poultry Science Association Inc. Received September 15, 2009. Accepted May 15, 2010. 1 Mention of a product or specific equipment does not constitute a guarantee or warranty by the USDA and does not imply its approval to the exclusion of other products that may also be suitable. 2 Corresponding author: [email protected]

double-ES concept has recently been obtaining application interest from poultry processors (W. Lee, Pilgrim’s Pride Corporation, Athens, GA; personal communications). However, most published research involving ES effects on quality, including color, pH, shear force (or tenderness), and cook yield, of early deboned poultry breast fillets used single prescalding ES (Li et al., 1993). Young et al. (2004, 2005) evaluated the effects of single preevisceration ES (immediately after defeathering) on marination performance of nonaged fillets and on the texture and yields of early deboned fillets (2 and 3.5 h postmortem). However, the effects were compared with only a non-ES control. There is a lack of research published in peer-reviewed journals that directly compares the effects of ES stages of processing before evisceration and times (once or single vs. twice or double). The present experiment was conducted to evaluate the quality effects of 3 different ES treatments: prescalding ES after bleed-out (PS), ES immediately postdefeathering (PD), and PS combined with PD (PSPD) on broiler breast muscles. Our previous research (zhuang et al., 2008, 2009) showed that deboning time can significantly

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affect the shear force and texture quality of pectoralis minor (tenders), and we found no published reports on the effect of ES on muscle shear force of poultry breast tenders. Therefore, effect of ES on the texture of early deboned tenders was also investigated in the present study. Meat quality measurements included color and pH of the raw fillets and cook yields and shear force of the fillets and tenders deboned 2 h postmortem. Nonstimulated 2-h deboned fillets were used as a control, and 24-h deboned breast meat was used as a reference for well-aged tender fillets.

MATERIALS AND METHODS Chicken Samples For each of 3 replicates, 28 commercially grown, mixed sex, approximately 42-d-old broilers free of obvious flaws were obtained at the holding shed of a commercial processing facility in the morning. Mixed sex birds were used to follow Young et al. (2004, 2005) experimental methods and to assimilate industrial practice. Birds were transported 12 km to the research facility in conventional plastic coops each containing 7 broilers. Before the processing, 1 to 3 birds were used to test the operation of the pilot line and electrical stimulator. Birds were then randomly selected, suspended from overhead shackles in groups of 4, stunned at 12 V of pulsed direct current in a prestunner (model SF7000, Simmons Engineering Co., Dallas, GA) for 8 to 10 s, and killed by exsanguination using an automatic knife (model SK-5, Simmons Engineering Co.). After bleeding, carcasses were scalded in a 56°C triple-pass 3-stage scalder (model SGS-3Ca, Stork-Gamco Inc., Gainesville, GA) for a total of 105 s and picked for 15 s in a 4-bank picker (model D-8, Stork-Gamco Inc.). All times reported here were measured-confirmed using a digital timer. Poultry identification tags (Heartland Animal Health, Fair Play, MO) were inserted into 1 wing of each carcass for carcass tracking. The heads and shanks were removed after ES treatments. All birds were manually eviscerated and rinsed with water to remove blood, feathers, or other loose debris before chilling.

ES Three of 4 birds in each group were subjected to ES (the fourth was used as a 2-h control) either immediately after bleeding (before scalding, PS) or immediately after defeathering (before evisceration, PD), or both. The fourth 2-h control bird in each set was randomly assigned a shackle and was left hanging on the line when the others were removed for ES. Carcasses to be stimulated were removed from the line in groups of 3 and hung by the feet on grounded shackles of a site-built European-style stimulator (in 2008) with the breast in the area of the sternum contacting a charged

stainless steel plate (similar to the prototype stimulator as described by Young et al., 2004, 2005). For the PS birds, the legs, feet, and breast areas were sprayed with water before stimulation. For the PD carcasses, ES was performed directly after defeathering (without further wetting with water) and before heads and shanks were removed. For the PSPD treatment, PS-stimulated birds were hung back on the shackles and were scalded and defeathered along with the control, after which they were removed again for application of the PD ES. There were 2 batches for each ES treatment on each day. The order of processing for the 3 ES treatments was carried out in a balanced design across the study, which resulted in 2 occurrences of each treatment being processed first, second, and third (18 birds total for each treatment). Birds were stimulated with a controlled, pulsed potential of 200 V (60 Hz of alternating current). Pulse duration was 0.5 s on and 1 s off for 90 s (Lyon et al., 1989; Sams, 2002; Young et al., 2004). Actual current was measured with each set of birds and varied under the conditions of this study with ranges of 240 to 320 mA for PS carcasses and 260 to 290 mA for PD carcasses. The additional birds (of the 84) were used for a 24-h debone reference treatment and were processed separately in single batches (3 or 4 at a time) without stopping. The 24-h treatment was added to show the potential for meat tenderness for this experimental set of birds. Before the study, the animal use proposal for the experiment was approved by the Institutional Animal Care and Use Committee at the Richard B. Russell Research Center, Athens, Georgia.

Chilling and Deboning Eviscerated carcasses were chilled by immersion in ice water. The carcasses from each group were submersed in 151 L of a mixture of ice and tap water in a pilot-scale, paddle-type chill tank. The paddles in the chiller were operated at 4 rpm for the duration of the 50-min chill. The chilling water temperature was monitored using a temperature data logger in a waterproof case (Lascar Electronics, Erie, PA; average temperature was 0 ± 1°C) and carcass (breast) temperatures were checked at the end of chilling using a Digi-Sense handheld digital thermometer (Cole-Parmer Instrument Co., Vernon Hills, IL) fitted with a Physitemp hypodermic needle microprobe (Physitemp, Clifton NJ; mean body temperature at the end of 50-min chilling was 3.3 ± 0.9°C). After immersion chilling, carcasses were hung by their hocks in shackles and allowed to drip for 5 min before being weighed and sealed in 3.79-L Ziploc freezer bags (SC Johnson & Son, Inc., Racine, WI). The average weight of the postchill carcasses was 1,519 ± 219 g. All 2-h carcasses were held in ice slush for another 60 min and the 24-h reference carcasses were held for 23 h in a cold room (1°C) before deboning. All breast meat, including both the fillets and tenders, was manually removed from the carcasses at 2 or 24 h postmortem.

ELECTRICAL STIMULATION OF BROILER

Color and pH Measurements Color and pH measurements were performed only on the chicken breast fillets (left side only) immediately after deboning. Surface color measurements (L*, a*, and b* values) were carried out with a Minolta spectrophotometer CM-2600d (Konica Minolta, Ramsey, NJ) with settings of illuminant C, 10° observer, specular component excluded, and an 8-mm aperture. Surface areas were selected that were free from obvious defects (bruises, discolorations, hemorrhages, or any other conditions that might have prevented uniform color readings). Three measurements were taken on the bone or medial side of the fillet. Each measurement was the result of 3 averaged readings by the spectrophotometer. The pH of the fillets was determined at the cranial end (wing end) with a Sentron model 2001 pH meter and a LanceFET piercing probe (Sentron, Gig Harbor, WA). Between measurements, the probe tip was cleaned with a soft toothbrush and rinsed with deionized water (Zhuang and Savage, 2009). Breast fillets and tenders were individually weighed and vacuum-packed (508 mmHg) in prelabeled, separate cooking bags (Seal-aMeal bag, The Holmes Group, El Paso, TX) after color and pH were measured.

Warner-Bratzler Shear Force and Cook Yield Evaluation Vacuum-bagged samples were cooked in a Henny Penny MCS-6 combi oven (Henny Penny Corp., Eaton, OH) at 85°C (185°F) using the tender steam setting. The end point internal temperature was 78°C and cooking times were 20 to 25 min for the fillets and 7 to 10 min for the tenders. The temperature of each fillet was checked at 20 min and each tender at 7 min using a Digi-Sense hand-held digital thermometer fitted with a Physitemp hypodermic needle microprobe. Samples that had reached the targeted temperature were removed and samples not at the target temperature were cooked additional time or until they reached 78°C. The 2-h deboned fillets were cooked the same day as the processing and packaging. The 2-h deboned tenders were stored overnight in a refrigerator and were cooked the next day. The same cooking procedures were used for 24-h deboned fillets and tenders. Each fillet or tender was weighed after removal from its cooking bag (purge was left in the bag and the meat was weighed). Cook yield (%) was calculated by 100 × (cooked meat weight/raw meat weight). For the Warner-Bratzler (WB) shear force measurements, room temperature samples were sheared using a TA-XTPlus Texture Analyzer (Stable Micro Systems, Surrey, UK) fitted with a 30-kg load cell and Texture Exponent 32 version 2.0.5.0 software. A TA-7 WB shear-type blade was used. Test settings included a button-type trigger, 55-mm travel distance, 4 mm/s test speed, and calibration return distance of 1 mm (required by software operation). Maximum force

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measured to cut the strips was expressed in newtons (Zhuang et al., 2008). Fillet strips, 1.9 cm wide, were sheared perpendicular to the longitudinal orientation of the muscle fibers in 2 locations and heights at the shear points were recorded (average 18.6 ± 1.78 mm). For the tenders, samples were sheared in 2 locations perpendicular to the tendon or white fibrous connective tissue line that runs the length of the tender. The first shear location was about 2 cm from the cranial end (avoiding the thickest part of the tendon) and the second shear was about 2 cm from the first. Heights (average 11.8 ± 1.5 mm) and widths (23.4 ± 2.1 mm) were recorded at each shear location. Only 1 fillet and 1 tender from each bird were used for WB measurements.

Statistics Because 3 carcasses were electrically stimulated simultaneously, the measurements of each parameter including color, pH, cook yield, and WB shear force values were averaged for each treatment batch before the statistical analyses were conducted. All data were subjected to 2-way ANOVA using the GLM procedures of SAS (SAS Version 9.1, SAS Institute Inc., Cary, NC). Models contained treatment (included 4 treatments: 2-h control, 2-h PS, 2-h PD, and 2-h PSPD,), replication, and treatment × replication interaction (if applicable). Means were separated with Duncan’s multiple range test at a significance level of 0.05. The null hypothesis implied no difference in the average measurement means between the different treatments.

RESULTS AND DISCUSSION In our ES study, we made some changes in the basic parameters used by Young et al. (2004, 2005) for their PD treatment. We used 200 V instead of 220 V because both 200 and 220 V are high-voltage treatments, and in the literature, use of 200 V was reported more often for ES of broilers (Li et al., 1993). Under our experimental conditions, a much higher current (260 to 290 mA) was detected for the PD treatment compared with that (130 to 140 mA) reported in Young’s experiments. This difference could result from different electrical stimulators being used in the experiments. Li et al. (1993) and Sams (2002) in their review indicated that ES methods, including current type and frequency and electrode positions on carcasses, can influence the contact conductivity. In Young’s study, a prototype stimulator was used and a group of 4 birds was stimulated simultaneously; however, we used a site-built stimulator and a group of 3 were stimulated simultaneously. Another possible explanation for the difference could be due to the variations of live birds or the experimental conditions used for the studies. Woolley et al. (1986a,b) showed that whole-bird resistance of broilers varied significantly, ranging between 1,000 to 2,600 Ω. Dickens and Lyon (1995) reported a current of 130 mA when broilers were stimulated using 200 V; however, Lyon et al. (1989)

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found that the current was 213 mA under 200 V. In addition, the average amperage in the PD (273 mA/ bird) was similar with that in the PS treatments (279 mA/bird) in our experiment. This could result from the wetting method we used for the PS birds (PD samples were not wetted with water before ES). Sams (2002) concluded that excessively wet feathers can markedly reduce current flow through the carcass by allowing it to flow through the wet feathers. The effects of different ES methods on color (indicated by CIE L*, a*, and b* values) and pH of raw broiler breast fillets are shown in Table 1. There were no differences in average L*, a*, and b* values between the 3 ES treatments. There were also no significant differences in the L* and a* values between the stimulated samples and nonstimulated 2-h control. However, the average b* values of the 2-h controls were significantly higher than those of the stimulated samples. The CIE L*, a*, and b* values are commonly used for representing poultry fillet color. The L* value indicates meat lightness, the a* value indicates redness, and the b* value indicates yellowness. Effects of ES on raw poultry fillet L*, a*, and b* values can be found in a few published reports; however, results are inconsistent. Maki and Froning (1987) observed no differences in L* and b* values between stimulated and unstimulated turkey breast meat, although the a* values of the stimulated samples were significantly higher. Owens and Sams (1997), however, found no significant differences in L* and a* values between ES and control turkey breast meat regardless of deboning and aging time. Froning and Uijttenboogaart (1988) did not find any difference in L* values between the control and ES chicken fillet samples beyond the 60min deboning time. But the ES resulted in significantly increased a* values and did not consistently affect b* values compared with the control. Craig et al. (1999) reported that stimulation with 440 V resulted in significantly lower L* and b* values and higher a* values in the raw broiler breast muscle samples. There were no reports comparing the effects of different ES methods on L*, a*, and b* values of poultry fillets. Our present study demonstrates that the different ES methods had similar effects on the color of the early deboned broiler fillets. Regardless of stimulation method, ES with 200 V resulted in significant changes in raw fillet color, with the stimulated fillets being less yellow than the 2-h control. The reduced b* value might result from metabolic changes affecting yellow pigments, carotenoids, flavins,

and bile pigments in the ES samples. From the physiological point of view, it has been hypothesized that meat tenderization by ES could result from ES-induced disruption of the sarcoplasmatic reticulum and lysosomes in the tissues and release of Ca++ and digestive enzymes (del Valle et al., 2008). It is very likely that the enzymes released from lysosomes were also responsible for low contents of yellow pigments in the ES meat observed in our experiment. There were no significant differences in pH values of raw, early deboned broiler fillets between the 3 ES methods tested in our study. However, average pH values of the ES treatments were significantly lower than that of the 2-h control. The ES-induced reduction in pH of early deboned poultry breast fillets has been consistently reported in literature (Maki and Froning, 1987; Thompson et al., 1987; Lyon et al., 1989; Wakefield et al., 1989; Birkhold et al., 1992; Owen and Sams, 1997; Alvarado and Sams, 2000; Young et al., 2004; Castañeda et al., 2005). Our results are in agreement with these reports, indicating that the ES treatments under our experimental conditions had physiological effects on early deboned broiler breast meat similar to those in the other studies. Our results also suggest that no matter how ES is applied (double ES vs. single ES and PS vs. PD), different ES treatments have the same accelerating effects on postmortem glycolysis of broiler breast muscle. Table 2 shows the effects of ES on cook yields of early deboned broiler breast meat. There were no differences between the 3 ES treatments and there were no differences between the ES samples and the 2-h control regardless of breast muscle. The effect of ES on cook loss or cook yield of poultry fillets was inconsistent in literature (Owens and Sams, 1997). In fact, several published papers have shown that significant interactions existed between ES and processing variables. Owens and Sams (1997) found that the effect of ES on cook loss of turkey breast fillets varied with experimental replicates and postharvest time. For 2-h harvested samples, there was a significant difference in cook loss between ES (14.9%) and the control (10.9%) in replicate 1; however, in replicates 2 and 3, there were no differences. For 8-h harvested fillets, there were no significant differences between the 2 treatments regardless of replicate. Alvarado and Sams (2000) showed that there were no differences in cook loss of chicken fillets deboned at 0.25 and 24 h; however, ES resulted in a significant

Table 1. Color and pH of broiler breast fillets (mean ± SD) Treatment1 Non-ES + 2 h Prescalding ES + 2 h Postdefeathering ES + 2 h Prescalding and postdefeathering ES + 2 h Non-ES + 24 h a,bMean 1ES

n 6 6 6 6 7

L* 53.6 54.6 55.2 54.1 54.6

a* 2.2a 3.1a 2.1a 2.0a

± ± ± ± ± 5.3

−0.4 −0.5 −0.5 −0.3 −0.1

b* 0.7a 0.8a 0.5a 0.4a

± ± ± ± ± 0.8

11.5 9.7 10.0 10.2 9.9

pH 0.8a 1.1b 0.3b 0.6b

± ± ± ± ± 1.8

values with no common superscripts in the same column are significantly different from each other (P < 0.05). = electrical stimulation; 2 h = 2 h deboned; 24 h = 24 h deboned.

6.5 6.2 6.1 6.1 6.1

± ± ± ± ±

0.2a 0.1b 0.1b 0.1b 0.4

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ELECTRICAL STIMULATION OF BROILER Table 2. Average cook yields and Warner-Bratzler shear force values of broiler breast fillets and tenders (mean ± SD) Warner-Bratzler shear force value (N)

Cook yield (%) Treatment1 Non-ES + 2 h Prescalding ES + 2 h Postdefeathering ES + 2 h Prescalding and postdefeathering ES + 2 h Non-ES + 24 h

n 6 6 6 6 7

Fillet 83.0 83.0 84.3 82.2 82.2

1.8a 1.7a 1.5a 2.1a

± ± ± ± ± 4.1

Tender 84.7 84.8 85.7 86.2 88.2

2.5a 3.5a 2.0a 2.2a

± ± ± ± ± 5.6

Fillet 153 64 96 67 36

27a 6c 25b 7c

± ± ± ± ± 13

Tender 75 39 44 42 26

± ± ± ± ±

25a 2b 4b 2b 4

a–cMean 1ES

values with no common superscripts in the same column are significantly different from each other (P < 0.05). = electrical stimulation; 2 h = 2 h deboned; 24 h = 24 h deboned.

reduction in cook loss (or increase in cook yield) by more than 5% compared with the control when the fillets were deboned at 1.25 h postmortem. Young et al. investigated the effect of PD on the functionality of early deboned chicken breast fillets and reported that there was no difference in the cook loss of marinated fillets between nonstimulated and stimulated chicken carcasses (Young et al., 2004). However, in a later report, Young et al. (2005) showed that regardless of chilling method, nonmarinated fillets from ES carcasses exhibited significantly higher cook yield (about 2%) than those from nonstimulated carcasses. Our results demonstrated that ES at the different stages of processing and with different instances did not significantly affect cook yield (or cook loss) of chicken breast muscles. Additionally, the ES treatments did not affect cook yields when compared with the nonstimulated controls under our experimental conditions. The effects of different ES treatments on texture quality, expressed as WB shear force values, of broiler breast meat are also shown in Table 2. The average WB shear force of 24-h deboned samples is included in the table (not included in the statistical analysis). The 24-h average shear value (36 N) of the fillets fell well within the range of 24-h values (31.6 to 43.1 N) published in literature (Lyon and Lyon, 1996; Liu et al., 2004; Xiong et al., 2006; Zhuang and Savage, 2009), indicating that the 24-h tenderness reference used in our study had typical texture properties for tender breast meat. The data in Table 2 show that there were no differences in average WB shear force values of the 2-h tenders between the 3 ES treatments; however, the shear force values of the PS and PSPD fillets or those that had been stimulated before scalding were significantly lower than those stimulated only immediately after defeathering (PD). There was no difference between the PS and the PSPD. Compared with the 2-h controls, no matter which ES treatment was applied, the average shear force values of both the ES fillets and tenders were significantly lower (more than 37%). However, they were higher than the 24-h reference samples. There were large differences in average WB shear force values between the fillets and the tenders regardless of treatment. These results are consistent with numerous findings published in the past decades. For example, Zhuang et al. (2008) found that average WB shear force values of fillets were twice as

high as shear values of tenders regardless of aging time and carcass chill method. Dickens et al. (2002) reported that WB shear force values of 24-h deboned, nonstimulated fillets were approximately 1/3 of the values of 2-h nonstimulated fillets and that shear values of ES fillets fell between the values of the 2- and 24-h nonstimulated samples. Alvarado and Sams (2000) found that the average Allo-Kramer shear force value of 1.25-h deboned, nonstimulated fillets was more than twice as high as 24-h deboned, nonstimulated fillets. The average shear value of the stimulated and 1.25-h deboned samples was about half that of the 1.25-h control but 36% higher than the 24-h control. Warner-Bratzler shear force is the most used instrumental measurement to predict meat tenderness and its values are strongly correlated with sensory affective and descriptive texture evaluation results of chicken fillets (Lyon and Lyon, 1991, 1997; Xiong et al., 2006). According to the classification by Xiong et al. (2006), the 2-h control fillets in our study would be perceived as very tough (153 N), whereas the cooked 24-h reference sample would be very tender (36 N). The PS and PSPD fillets would be considered slightly tender (64 and 67 N, respectively), and the PD fillets would be neither tough nor tender (96 N). Our study demonstrates that ES with 200 V can significantly tenderize not only early deboned chicken fillets but can also tenderize early deboned tenders regardless of application stage (prescalding vs. postdefeathering) and instance (single ES vs. double ES) during processing. In addition, our results also show that tenderization of early deboned chicken fillets by ES depended on the application stage during processing, even though the effects of the different ES treatments on tenders were not significantly different. Postmortem ES before scalding resulted in significantly more tender chicken fillets than postmortem ES after defeathering; double postmortem ES before evisceration could not further improve tenderness of early deboned fillets compared with single postmortem ES before scalding. These results suggest that a single PS method should be used for tenderization of early deboned poultry breast meat or for reduction of the on-frame aging time before deboning. Tenderness is one of the most important quality characteristics of meat. For deboned poultry breast, tender meat can be obtained by delaying deboning time or

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by on-frame aging up to 4 to 6 h before the deboning. According to the postmortem aging hypothesis, after the bird dies, poultry breast muscles undergo contraction, which results from actin-myosin binding (induced by calcium diffusion from the sarcoplasmic reticulum after pH decreases to 6.0 postmortem in muscle) and results in decreased muscle fiber sarcomere length (or sarcomere shortening), and relaxation, which results from dissociation of the actin and myosin and results in increased sarcomere length, until complete depletion of adenosine triphosphate (ATP), which works as a plasticizer to dissociate actin and myosin. If the poultry breast meat is attached to bone frames during aging, the skeletal restraints can prevent the sarcomeres from contracting and subsequent meat from toughening. If the breast meat is removed from bones before the aging process is completed, the lack of skeletal restraints to limit the sarcomere contraction results in shorter sarcomeres and tougher meat. However, if the meat is deboned after ATP is depleted or the aging is complete, there is no further sarcomere contraction and the deboned meat will be tender (Lyon and Buhr, 1999; Sams, 2002). It has been demonstrated that for poultry breast fillets, ES functions effectively to reduce the aging time requirement before deboning. The suggested mechanism is either mainly by accelerating the muscle contraction-relaxation and therefore accelerating ATP depletion, which is closely associated with pH decline in dead animal muscle, to allow early deboning when the low-voltage ES is used, or predominantly by physically disrupting muscle fibers and therefore disrupting sarcomere contraction to allow muscles to be removed from bones early when the high voltage (larger than 120 V) is used (Sams, 1999, 2002). Thompson et al. (1987) found greater improvement in tenderness observed with the high-voltage ES compared with the low-voltage ES in chill-boned fillets and suggested that high-voltage ES depended more on myofibrillar fragmentation than just on metabolic acceleration indicated by pH reduction or ATP disappearance, as was the case with low voltage. Sams et al. (1989) observed that the improved tenderness in early harvested broiler breast fillets from highvoltage ES-treated carcasses was not strongly associated with the acceleration of ATP depletion. Our study also shows that there were no associations between the pH values, an indicator for ATP depletion, and the WB shear force values (R2 = 0.05, P = 0.72) for the 3 ES treatments, suggesting that the PS treatment might result in more physical disruption of muscle fibers or muscle damage in chicken fillets, therefore reduced WB shear force values, than the PD treatment. In addition, the significant differences in WB shear values between PD and PS or between PD and PSPD could also result from variations in shear forces of early deboned fillets treated with different ES methods. Dickens et al. (2002) found that in addition to the significant difference in mean shear force values between no-ES control [13.8 kilogram force (kgf)] and ES (6.8 kgf), the no-ES

fillets also showed much larger SD (8.0 kgf) compared with the ES samples (2.9 kgf). In our study, the SD for fillet shear values from PD-treated carcasses (25 N), which had significantly higher average shear force values than the PS and PSPS samples, was similar to that for the 2-h control (27 N) and about 4-fold higher than those for fillets from PS and PSPD carcasses (6 to 7 N). This result indicates that delayed ES (from prescalding to postscalding) during poultry processing somehow results in inconsistent responses of pectoralis major muscle to ES. The PS treatment results in more consistent texture quality compared with the PD treatment. Quality consistency is always very important for food processors to obtain consumers’ satisfaction of their product. Much smaller variations in fillet muscle shear force in the PS treatment carcasses further support our recommendation that the prescalding electrical stimulation should be used for tenderness improvement of early deboned broiler fillets. In summary, under the experimental conditions used in this study, different ES treatments do not result in differences in raw fillet color and pH. Regardless of ES treatment and breast muscle, ES significantly reduces shear force values of early deboned chicken breast meat compared with the nonstimulated 2-h control; however, shear force values vary between the ES treatments and muscles. For the tenders, the different ES treatments do not result in any differences in meat shear force values. For fillets, the double-ES method (PSPD) does not have any advantage in tenderization (lowering shear force values) compared with the single PS treatment. The PD treatment does not tenderize the fillets to the same extent as the PS and the PSPD treatments and the PD also has much larger variations in shear force values compared with the PS and PSPD treatments, which are similar with each other. There are no differences in cook yield between the 3 ES treatments. The use of a single prescalding electrical stimulation is recommended for processing improvement for early deboned chicken breast meat.

ACKNOWLEDGMENTS We thank Allan Savage, Peggy Feldner, Jerrie Barnett, Joseph Stanfield, and Elece Turnipseed with the Quality and Safety Assessment Research Unit, USDAAgricultural Research Service, Athens, Georgia, for their technical assistance during the study.

REFERENCES Alvarado, C. Z., and A. R. Sams. 2000. The influence of post-mortem electrical stimulation on rigor mortis development, calpastatin activity, and tenderness in broiler and duck pectoralis. Poult. Sci. 79:1364–1368. Barbut, S. 2002. Poultry Products Processing: An Industry Guide. CRC Press, Boca Raton, FL. 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. Poult. Sci. 71:2106–2112.

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