Hot-boning enhances cook yield of boneless skinless chicken thighs1 H. Zhuang,2 B. C. Bowker, R. J. Buhr, and G. Sanchez Brambila USDA, Agricultural Research Service, Russell Research Center, 950 College Station Road, Athens, GA 30604-5677 24 h PM. Thighs were separated from the legs (drumsticks) at either the same time the whole legs were removed from the carcasses or at 24 h PM. Intact thighs (bone in) were cooked fresh at 24 h PM. Color of fresh thigh muscles, cook yield, and Warner-Bratzler shear force of cooked samples were measured. Cook yield of the thighs cut from the backbone before chilling was significantly higher than those cut from the carcasses at 2 and 24 h PM, which did not differ from each other. The PM time at which intact thighs were separated from the leg (drumstick) did not influence cook yield. These results demonstrate that postmortem deboning time can significantly affect cook yield of boneless skinless chicken thigh products. Deboning chicken thighs after chilling reduces the cook yield. Differences in the cook yield of thighs may also result from the removal of whole chicken legs from the carcass backbone.
Key words: chicken thigh, postmortem deboning time, cook yield, iliotibialis, color 2014 Poultry Science 93:1553–1560 http://dx.doi.org/10.3382/ps.2013-03662
INTRODUCTION Cook yield or cook loss is one of the most commonly measured quality attributes for meat products. Cook yield directly affects product yield and profitability of further processed products, indicates meat waterholding capacity (WHC), and is related to the texture quality of finished products (Lawrie, 1998; Toscas et al., 1999; Van Oeckel et al., 1999; Cheng and Sun, 2008). Water-holding capacity, often measured with cook yield, of meat can be influenced by numerous variables from farm to table, including genetic strains, muscle types, preprocessing growing conditions and stresses, operations and treatments during processing (stunning, electrical stimulation, and chilling), postprocessing handling (aging, cutting, and storage), precooking treatments (marination), cooking techniques
©2014 Poultry Science Association Inc. Received September 30, 2013. Accepted February 11, 2014. 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]
(microwave, frying, or stewing), and cooking conditions (time and temperature; Cheng and Sun, 2008). In chicken breast meat, cook yield or cook loss is influenced by postmortem (PM) deboning time. Thomson et al. (1986) showed that regardless of bird stunning method and experimental replication, fillets deboned and cooked at 20 min PM consistently lost less weight than samples deboned and cooked at 24 h PM. Lyon and Wilson (1986) also found that cook yield of chicken breast meat was affected by rigor condition. Prerigor samples cooked immediately after deboning exhibited higher yields than postrigor samples (24 h PM) regardless of cooking method. Lyon et al. (2003) reported that cook yield of 24-h deboned hen breast meat was lower than that deboned 2 h postchill. Zhuang and Savage (2011) found that the cook yield of hot-boned breast meat was higher than either 2 or 24 h cold-deboned chicken fillets. northcutt et al. (2001) demonstrated that cook yield of broiler breast meat deboned at 2, 4, and 6 h postchill was higher than samples deboned 0 h postchill. Boneless skinless chicken thighs are a relatively new poultry product in the retail market and their market share is rapidly growing in the United States (Aho, 2009). There is no scientific information available con-
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ABSTRACT Three experiments were conducted to evaluate the effects of postmortem deboning time on cook yield of boneless skinless chicken thighs. In experiment 1, chicken thigh meat was deboned at 0.75 (hotbone), 2, and 24 h postmortem (PM) and trimmed to obtain mainly iliotibialis muscle. Samples were cooked directly from a frozen state. Cook yield of the muscle was significantly influenced by PM deboning time. Hotboned thighs exhibited a 7% greater cook yield than the samples deboned at 24 h. In experiment 2, boneless skinless chicken thighs were deboned at 0.3, 2, and 24 h PM and cooked directly from a fresh, never-frozen state at 24 h PM. Cook yield of the hot-boned thighs was significantly higher than those of the 2 and 24 h deboned samples, which did not differ from each other. In experiment 3, whole legs (thigh + drumstick) were cut from the carcass backbone at 0.3 (hot-cut), 2, and
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MATERIALS AND METHODS Experiment 1 During each of 3 replications, commercially processed and eviscerated carcasses from broilers (approximately 42 d old) were obtained from a local processing plant before carcass chilling. Carcasses were placed in coolers and transported to the laboratory within 20 min. Carcasses were immersed in a prechill water tank at 14°C for 0.25 h, followed by submersion in water immersion chill tanks at 0 to 4°C for 60 min. For each replica-
tion, 10 prechill carcasses were hot-boned at 45 min PM (average carcass temperature, 35.2°C), 10 postchill carcasses were cold-deboned at 2 h PM (average carcass temperature, 3.6°C), and 10 chilled carcasses were placed in Ziploc freezer bags and held at 1 to 2°C for 22 h before being deboned at 24 h PM (average carcass temperature, 1.9°C). Broiler thighs were deboned manually. Due to muscle variation and irregular alignment of muscle fibers, muscles from the medial side of each thigh were removed and discarded. Muscles from the lateral side, consisting primarily of the iliotibialis, iliofibularis, and femoritibialis (Howell, 1938), were trimmed to a similar length (Figure 1). Trimmed thigh samples were placed in cooking bags (Seal-a-Meal bag, The Holmes Group, El Paso, TX) and immediately frozen at −20°C until further analysis (14–21 d). Bagged thigh meat was cooked directly from the frozen state to an endpoint temperature of 77 to 78°C (12–15 min at 85°C in a Henny Penny combi oven, Henny Penny Co., Eaton, OH). Cooked thighs were allowed to cool at room temperature for 3 to 5 min before they were removed from bags, drained, and weighed. Cook yield was calculated as a percentage of the prefreezing weight (Zhuang et al., 2007).
Experiment 2 Broilers (n = 18) were slaughtered at the Richard B. Russell Agricultural Research Center poultry processing facility according to an approved protocol of the Institutional Animal Care and Use Committee. Birds were stunned at 15 V and 50 Hz of pulsed direct current in a prestunner (model SF-7000, Simmons Engineering Co., Dallas, GA) for 8 s, and killed by exsanguination using an automatic knife (model SK5, Simmons Engineering Co.). After bleeding (2 min), carcasses were scalded in a triple-pass 3-stage scalder (model SGS-3Ca, Stork-Gamco Inc., Gainesville, GA) for 1.5 min and were automatically picked in a 4-bank picker for 30 s (model D-8, Stork-Gamco Inc.). The heads, feet, viscera, and lungs were removed manually and carcasses were rinsed with water to remove blood, feathers, or other loose debris before chilling. Carcasses were immersion-chilled until body temperature reached 4°C and then stored in Ziploc bags on ice in a cold room until deboning. Whole legs (a total of 36 yielded from 18 carcasses) were removed from carcasses and thighs were skinned and deboned prechill at 20 min PM (hotboned; n = 12), postchill at 2 h PM (n = 12), and at 24 h PM (n = 12). To minimize variation in the results, paired thigh halves from the same carcass were used for the 3 different deboning time treatments. For each deboning time, 12 thighs were removed from different individual carcasses and 6 of them were paired with one of the other treatments. After deboning, thighs were placed in individual Ziploc bags and stored on ice in a cold room (4°C) until cooking. At 24 h PM, the boneless skinless thigh samples were weighed and placed in cooking bags. The samples were then cooked
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cerning the effects of deboning time on the cook yield of boneless skinless chicken thigh meat. Although much research has been conducted on chicken breast meat, thigh and breast products are known to contain inherently different types of muscles. Chicken breast muscles consist of almost entirely white, fast-twitch, glycolytic (type IIB) muscle fibers (Edman et al., 1988; Barbut, 2002), whereas thigh muscles have a high abundance of heterogenous red, slow-twitch, oxidative (type I) muscle fibers (Howell, 1938; Barbut, 2002). Compared with white fibers, red fibers are smaller in diameter, contain more myoglobin, less glycogen, and have a higher lipid content (Cassens and Cooper, 1971). While important physiologically, vast differences in muscle fiber type also influence PM meat quality development. Experiments have shown that PM conditions have different effects on WHC of poultry breast and thigh meat. For example, Lesiak et al. (1996) found that turkey thigh muscles were more susceptible to cold shortening than breast muscles. Lower storage temperature (0°C) produced better WHC in breast meat regardless of storage time, whereas storage temperatures at 12°C for a longer time (more than 24 h) resulted in better WHC than either lower (0°C) or higher (30°C) storage temperature in turkey thighs. These indicate that effects of PM deboning time on cook yield of boneless chicken thighs may differ from that on cook yield of boneless chicken breast fillets. While it is hypothesized that deboning time influences the cook yield of chicken thigh meat, it is further speculated that the fabrication process for producing thigh products may also have an influence on cook yield. For the production of boneless thigh products, before removing the thigh meat from the femur bone, intact thighs need to be removed from the carcass backbone and separated from the leg (drumstick). It is unknown if deboning time effects on cook yield or other quality measurements in boneless thigh products result from these fabrication steps. The objective of this 3-part study was to investigate the effects of PM deboning time on the cook yield of boneless skinless chicken thighs cooked from either the frozen state (experiment 1) or fresh at 24 h PM (experiment 2), and to understand the role that cutting up plays in cook yield differences of deboned thigh meat due to PM aging (experiment 3).
POSTMORTEM DEBONING AND THIGH COOK YIELD
in a water bath at 85°C to the endpoint temperature of 80°C. Similar to sample handling procedures used for sensory panel evaluation (Zhuang et al., 2007), cooked samples were cooled in the sealed bags for 3 to 5 min at room temperature, removed from bags, drained, and reweighed for cook yield calculations.
Experiment 3 Broiler birds were slaughtered and processed following the same procedure as described for experiment 2. Two replications of the entire experiment were conducted with 12 birds yielding 24 whole legs (thigh + drumstick) per replicate. Within each replicate, 8 of the 24 whole legs were cut (removed) from the different carcasses at 20 min PM, 8 at 2 h PM, and 8 at 24 h PM. For whole leg samples that were cut from carcasses at 20 min PM, half of the intact (bone-in) thighs were separated from the legs (drumsticks) at 20 min PM and the other half of the samples were separated from the legs at 24 h PM. Similarly, for whole leg samples that were cut from the carcasses at 2 h PM, half of the intact thighs were separated from the legs at 2 h PM and half were separated at 24 h PM. Intact thighs (bone-in)
were separated from the legs by cutting at the joint between the thigh and leg (Figure 2), skin was removed, and the samples were trimmed. All samples (whole legs and intact thighs) taken before 24 h PM were weighed at the time of sampling (20 min or 2 h PM), placed in a Ziploc bag, and stored on ice in a cold room until 24 h PM when they were reweighed to calculate drip loss. At 24 h PM, the thighs that were still attached to legs were cut, trimmed, and weighed again (for cook loss) before being sealed in individual cook bags. After cooking intact thigh samples to 80°C in an 85°C water bath, the samples were cooled at room temperature for 3 to 5 min before they were drained and weighed. The cooked thigh muscles were then removed from the bone and cut into a 19 mm wide strip for Warner-Bratzler (WB) shear force analysis (Zhuang and Savage, 2009). Color (skin side) and pH (with a piercing probe at the cranial end) of raw skinless broiler thighs were measured both after the samples were removed from carcasses, and at 24 h PM according to the method of Zhuang and Savage (2010). Because thighs consist of multiple muscles, they were divided into muscle A (iliotibialis) and muscle B (sartorius; Figure 2) for color, pH, and WB shear force measurements. Color was measured at 2 locations on the exposed surface of each thigh muscle (Aa, Ab, Ba, and Bb) and the mean of the 2 color measurements for each muscle was used for the statistics. For each muscle, pH was measured in the proximal end. For WB shear force values (N), each muscle was sheared one time and the mean of the muscles was used for the statistical analysis.
Statistics All data were analyzed using the GLM procedures of SAS (SAS version 9.1, SAS Institute Inc., Cary, NC). The main factors were PM time (deboning time for experiments 1 and 2), leg separation (for experiment 3 only), muscle (for color and pH in experiment 3 only), and replication (for experiments 1 and 3). Means were separated with the Tukey option at a significance level of 0.05.
RESULTS Experiments 1 and 2 Table 1 shows the effect of PM deboning time on cook yield of thigh muscles (mainly iliotibialis) cooked from a frozen state. There were significant differences between the 3 different deboning times. Hot-boned thigh samples had the highest cook yield (81.1%), followed by the 2-h samples (78.5%). Thigh samples deboned at 24 h PM had the lowest cook yield (73.5%). Table 1 also shows the effect of PM deboning time on cook yield of boneless skinless chicken thighs cooked fresh at 24 h PM. There were significant differences among the 3 different deboning times. The cook yield of hot-
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Figure 1. Deboned and trimmed thigh muscle (iliotibialis) used to evaluate cook yield in experiment 1. Color version available in the online PDF.
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boned skinless chicken thighs (79.9%) was significantly higher than those of the 2-h (72.9%) and 24-h samples (73.4%), which did not differ from each other.
Experiment 3 Table 2 shows color and pH values of 2 different muscles from the intact chicken thighs (bone-in) at the time the whole legs were cut from the carcass and at 24 h PM. There were significant differences for pH, L* (lightness), a* (redness), and b* (yellowness) values between the 2 muscles. Regardless of cut-up time, L*
Table 1. Effect of deboning time on cook yield of boneless skinless chicken thighs Cook yield1 (%) Deboning time Hot-boned Chill-boned (2 h PM) Chill-boned (24 h PM)
Experiment 12
Experiment 23
81.1 ± 3.6a 78.5 ± 4.6b 73.5 ± 4.5c
79.9 ± 2.4a 72.9 ± 2.4b 73.4 ± 4.3b
a–cMean values with no common superscript in the same column are significantly different (P < 0.05). 1Cook yield = 100 × (cooked thigh weight/raw thigh weight immediately after deboning and trimming). 2In experiment 1, the hot-boned thigh muscle iliotibialis was removed from bones 45 min postmortem (PM). The samples were cooked directly from a frozen state (mean ± SD, n = 30). 3In experiment 2, the hot-boned whole boneless skinless chicken thighs were removed from bones 20 min PM. The samples were cooked fresh at 24 h PM (mean ± SD, n = 12).
DISCUSSION Results from the present study show that PM deboning time can significantly affect cook yield of deboned
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Figure 2. A diagram showing cuts, color sampling locations, and muscles used for shear force measurements in a whole chicken leg in experiment 3. Color version available in the online PDF.
and a* values of the muscle B were significantly higher (more than 3 units for L*) and b* values lower (more than 2 units) than those of the muscle A. These results indicate that surface color of thigh muscle region A was consistently darker (low L* value), more yellow (high b* value), and less red (low a* value) than that of location B. For pH, the average values of the muscle B were significantly higher (more than 0.1 units) than those of the muscle A for early PM samples (hot-cut and 2 h removed); however, there were no differences between the 2 muscles for pH measured at 24 h PM. The color and pH of the thigh muscles also depended upon the time at which the whole legs were cut from the carcass and aging time. For muscle A, the L* value of the hot-cut thighs was significantly lower than those of the 2- and 24-h-cut thighs at both the time of whole leg removal from the carcass and after the samples were aged to 24 h PM (hot-cut + 24 h and chill-cut at 2 h + 22 h). The L* value of the 2-h-cut thighs was significantly lower than those of the 24-h-cut samples and the 2 h + 22 h thighs, which did not differ from each other. However, the L* value of the hot-cut thighs (hot-cut + 24 h) was significantly lower than both the 2 h + 22 h and 24-h-removed thighs at 24 h PM. For muscle B, regardless of aging time, the L* values of the hotcut thighs were significantly lower than the 2 and 24 h samples. There was no difference between the hot-cut and hot-cut + 24 h samples and there were no differences between the 2, 24, and 2 + 22 h samples. Average a* values were consistently higher at 24 h PM, and the hot-cut thighs measured at 24 h PM had the highest a* value regardless of muscle. For pH in muscle A, only significant difference was observed between the 2 h and hot-cut + 24 h samples. However, for muscle B, pH of the early-cut thighs (including both hot-cut and 2-h-cut) was significantly higher than those measured at 24 h PM. There was no difference between the hot-cut and 2-h-cut thighs, and there were no differences between the 3 samples that were measured at 24 h PM. Table 3 shows the effect of PM cut-up time on drip loss, cook yield, and shear force of the cut whole chicken thighs. Overall, samples lost less than 1% of moisture after stored at a refrigerated temperature for more than 22 h, and there was no difference between the hot-cut and 2-h-cut samples. Cook yield of the hotcut thighs was significantly higher than 2- and 24-h cold-cut samples, which did not differ from each other. Shear force values of cooked intact thighs averaged less than 40 n and there were no significant differences between the 3 p.m. times. There were no differences between the whole legs (thigh + drumstick) and intact thighs (bone-in) for drip loss, cook yield, or shear force (Table 4).
6.12 ± 0.13b,x
0.20a,x 0.20a,x 0.12b,x 0.17b,x ± ± ± ± 6.39 6.34 6.14 6.06 ± ± ± ± 3.81 1.79 3.05 4.40
6.14 ± 0.22ab,x 2.34 ± 1.92b,y
6.12 6.21 6.14 6.09
± ± ± ±
0.18ab,y 0.21a,y 0.12ab,x 0.25b,x 1.91a,y 1.22b,y 1.73ab,y 2.36a,y
values with no common superscript in the same column are significantly different (P < 0.05). values with no common superscript in the same row for the same parameter are significantly different (P < 0.05). 1Postmortem (PM) time at which whole legs were removed from carcasses. 2L* = lightness; a* = redness; b* = yellowness. 3The color and pH were measured after the hot-cut thighs were aged for 24 h at a refrigerated temperature. 4The color and pH were measured after the 2-h-cut thighs were aged for 22 h at a refrigerated temperature.
6.67 ± 2.51a,x −0.09 ± 0.80ab,y 60.5 ± 2.59a,y
± ± ± ± 53.1 58.1 61.7 55.8
Hot-cut (20 min PM) Chill-cut (2 h PM) Chill-cut (24 h PM) Hot-cut (20 min PM + 24 h aging)3 Chill-cut (2 h PM + 22 h aging)4
a–dMean
± ± ± ± 60.8 63.8 65.4 61.1
64.8 ± 2.41a,x
−0.53 −0.71 −0.38 0.52
± ± ± ±
1.01b,y 0.54b,y 0.98b,y 0.99a,y
Muscle A Muscle B
3.21b,x 2.81a,x 3.69a,x 3.58b,x 2.68d,y 2.63b,y 2.88a,y 1.90c,y
Muscle A Cut-up time/aging time
x,yMean
± ± ± ± 0.96 0.85 1.55 3.08
1.85 ± 0.80b,x
7.46 4.49 7.01 7.35
± ± ± ±
2.04a,x 1.73b,x 2.55a,x 1.78a,x 0.95cd,x 0.85d,x 1.48bc,x 0.94a,x
Muscle B
Muscle A
b* a* L*
Table 2. Effect of cut-up time1 on color and pH of intact (bone-in) chicken thigh muscles (mean ± SD, n = 16; experiment 3)2
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chicken thighs. For both the thigh samples consisting of mainly iliotibialis muscle (experiment 1) and the thigh samples consisting of multiple muscles (experiment 2), the cook yields of hot-boned thighs were significantly higher than the chilled samples. For the boneless skinless products, the difference can be as large as 6%. It is commonly noted that WHC of hot-boned meat is superior to cold-deboned meat (Pisula and Tyburcy, 1996) regardless of muscle type. Neel et al. (1987) and Woltersdorf and Troeger (1988) found that hot-boned pork had higher WHC than the cold-boned samples. Similarly, Lyon et al. (1983) reported that deboned leg meat from un-eviscerated, un-chilled chicken carcasses released less moisture when heated than conventionally processed and aged meat. With chicken breast fillets, Zhuang and Savage (2011) found that hot-boned samples had significantly higher cook yields than coldboned meat. It is postulated that the higher WHC associated with hot-boned meat is due to higher muscle pH before cooking (Lyon and Wilson, 1986; Pisula and Tyburcy, 1996). There is a significant negative correlation between meat ultimate pH and cook loss in both poultry meat (McCurdy et al., 1996) and red meat (Bendall and Swatland, 1988; Andrews et al., 2000). Low pH in meat results in denaturation of meat proteins which can lead to reduced WHC and cook yield. During PM aging, pH of hot-boned or early PM meat is consistently higher than cold-boned or well-aged meat (McCurdy et al., 1996; Lyon and Wilson, 1986). Thus, the higher cook yields in hot-boned meat are thought to be due to a pH effect. For samples that are cooked immediately after deboning (Thomson et al., 1986; Lyon and Wilson, 1986), this would be the case. However, in our study, samples were aged until 24 h PM to allow pH to reach its minimum threshold. As shown in experiment 3 with bone-in thigh samples, pH differences between the samples were likely minimized before cooking. Thus, variations in cook yield between hot-boned and cold-boned samples were likely not due to pH differences and suggest that other factors may affect the differences in cook yield due to PM deboning time. In pork, Lan et al. (1995) showed that the muscle contractile state could affect cook loss, and Choi et al. (1987) found that hot-boned meat that had higher WHC also contained more soluble myosin heavy chains compared with cold-boned meat. The differences in the cook yield results between experiment 1 (cook yield of the 2-h samples was higher than the 24-h samples) and experiment 2 (no difference between the 2- and 24-h samples for cook yield) could be due to the different raw muscles used for the experiments, the sample preparation methods, or both. In experiment 1, only the iliotibialis muscles were used and the samples were cooked directly from the frozen state. However, in experiment 2, whole boneless skinless chicken thighs were used and samples were cooked from a fresh, never-frozen state. It is well known that prerigor and postrigor red meat respond differently to
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Muscle B
Muscle A
pH
Muscle B
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Zhuang et al. Table 3. Effect of cut-up time1 on drip loss, cook yield, and Warner-Bratzler shear force of intact (bone-in) chicken thighs (mean ± SD, n = 16; experiment 3) Cut-up time Hot-cut (20 min PM) Chill-cut (2 h PM) Chill-cut (24 h PM)
Drip loss2 (%)
Cook yield3 (%)
Shear force (N)
0.59 ± 0.63 0.75 ± 0.45 —–
79.5 ± 5.1a 77.2 ± 3.4b 76.8 ± 3.5b
36.0 ± 12.4 31.9 ± 8.7 29.0 ± 7.6
a,bMean
values with no common superscript in the same column are significantly different (P < 0.05). (PM) time at which whole legs were removed from carcasses. 2Drip loss = 100 × (W initial – W24 h)/Winitial, where W means weight. 3Cook yield = 100 × (cooked thigh weight/raw thigh weigh precooking). 1Postmortem
tion of iliotibialis and sartorius from the backbone at different PM times, the effect might also result from the rate of pH changes in cut whole chicken legs during PM aging, a cold-shortening effect for the cold-removed samples, or both. Stewart et al. (1984) found that pH of breast meat hot-boned from broilers somehow declined more slowly than breast meat left intact on the carcass. Rapid pH drop during early PM rigor mortis has been hypothesized to cause pale, soft, exudative poultry breast fillets and poor meat WHC, including cook yield (Barbut, 1993, 2009). These results indicate that PM biochemical changes in bone-in chicken thigh muscle follow a similar pattern to boneless skinless chicken thighs during meat aging, and that the whole chicken thighs can be used to predict quality changes in boneless skinless chicken thighs PM. Our results also demonstrate that color of thigh muscles significantly changes during the first 24 h PM aging. The thigh muscle L* values significantly increased regardless of muscle types. This result is in agreement with the data published by Petracci and Fletcher (2002), in which changes in L* values of thigh muscles increased rapidly in the first 3 h PM and plateaued after 6 h PM. An interesting observation from the present study was that lightness values of thigh muscles (regardless of muscle type) were significantly lower and redness values were the highest in the hot-cut sample [treatment hot-cut (20 min PM + 24 h aging)] measured after aging for 24 h compared with those of the other 2 treatments [treatment chill-cut (24 h PM) and treatment chill-cut (2 h PM + 22 h aging)] at the same PM time. This result indicated that the hot-cut thigh muscles would have darker color compared with the cold-deboned thighs after PM aging. For chicken breast meat, raw meat color lightness is associated with poor meat quality (Barbut, 1993, 2009). Although not statis-
Table 4. Effect of separation of legs from thighs when legs were cut from carcasses at either 20 min or 2 h postmortem (PM) on drip loss, cook yield, and Warner-Bratzler shear force of intact (bone-in) chicken thighs (mean ± SD, n = 16; experiment 3)1 Cut
Drip loss (%)
Cook yield (%)
Shear force (N)
Thigh only Whole leg
0.72 ± 0.53 0.63 ± 0.57
78.1 ± 4.6 78.6 ± 4.4
35.1 ± 8.3 32.8 ± 12.9
1The cooked thighs that were separated at 20 min (hot-cut) and 2 h PM (chill-cut) were combined for statistical analysis.
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freezing and thawing (Locker and Hagyard, 1963), and it is likely that chicken dark muscle behaves similar to red meat. In beef, cook losses of fresh samples tend to decrease with increased rigor temperatures, whereas they tend to increase for frozen samples (Farouk and Swan, 1998). The whole boneless skinless thighs used in experiment 2 contained multiple muscles in addition to the thigh iliotibialis muscle used in experiment 1 and probably contained more fat tissue. Lyon et al. (1984) attributed the difference in cook yield of ground fowl meat between breast and legs to the higher fat content in leg meat. Unlike deboning chicken breast meat, deboning thigh meat typically requires several steps. Deboning chicken thighs from carcasses involves removing whole chicken legs (thigh + drumstick) from carcass backbones and separating whole thighs from drumsticks before thigh muscle is deboned from the femur bone. For boneless skinless chicken thighs, the differences in cook yield due to different deboning times could result from the removal, separation step, or both. Experiment 3 was designed to evaluate the effect of cutting steps on cook yield of chicken thighs. Results demonstrated that cutting whole legs from carcasses before chilling can result in significant differences in cook yield (Table 3), and that separating the thighs from drumsticks at the time when whole legs were cut from carcasses (it was either 20 min or 2 h PM in the current study) has no effect (Table 4). This suggests that the early removal of the thigh from the backbone improves cook yield of hot-boned thighs and that the attachment of thigh muscles to the backbone somehow could at least partially result in loss of muscle WHC during aging. In addition to potential differences in contractile state (Lan et al., 1995; Cason et al., 2002) and solubility of myosin heavy chains (Choi et al., 1987) due to separa-
POSTMORTEM DEBONING AND THIGH COOK YIELD
ACKNOWLEDGMENTS The authors express their sincere thanks to Beth Barton, Andrew Ross, Taylor Kronn, Dianna V. Bourassa, and Gerald Heitschmidt of USDA-Agricultural Research Service (Athens, GA) for their technical assistance during the study.
REFERENCES Aho, P. 2009. Broiler economics. Arbor Acres, XVII (6). Andrews, B. S., J. A. Unruh, M. C. Hunt, and C. L. Kastner. 2000. Effects of freezing pork chops on Warner Bratzler shear force and cookery traits. Accessed Sep. 2013. http://www.asi.ksu.edu/ doc4982.ashx. Barbut, S. 1993. Color measurements for evaluating the pale soft and exudative (PSE) occurrence in turkey meat. Food Res. Int. 26:39–43. Barbut, S. 2002. Poultry Products Processing—An Industry Guide. CRC Press LLC, Boca Raton, FL. Barbut, S. 2009. Pale, soft, and exudative poultry meat—Reviewing ways to manage at the processing plant. Poult. Sci. 88:1506– 1512. Bendall, J. R., and H. J. Swatland. 1988. A review of the relationships of pH with physical aspects of pork quality. Meat Sci. 24:85–126. Cason, J. A., C. E. Lyon, and J. A. Dickens. 2002. Tenderization of hot-boned broiler breast meat by clamping during chilling. Poult. Sci. 81:121–125. Cassens, R. G., and C. C. Cooper. 1971. Red and white muscle. Adv. Food Res. 19:1–74. Cavitt, L. C., J.-F. C. Meullenet, R. Xiong, and C. M. Owens. 2005. The relationship of razor blade shear, Allo-Kramer shear, Warner-Bratzler shear and sensory tests to changes in tenderness of broiler breast fillets. J. Muscle Foods 16:223–242. Cheng, Q., and D.-W. Sun. 2008. Factors affecting the water holding capacity of red meat products: A review of recent research advances. Crit. Rev. Food Sci. Nutr. 48:137–159. Choi, Y. I., C. L. Kastner, and D. H. Kropf. 1987. Effects of hot boning and various levels of salt and phosphate on protein solubility, functionality, and storage characteristics of preblended pork used in frankfurters. J. Food Prot. 50:1025–1036. Edman, A.-C., J. Lexell, M. Sjostrom, and J. M. J. Squire. 1988. Structural diversity in muscle fibres of chicken breast. Cell Tissue Res. 251:281–289. Farouk, M. M., and J. E. Swan. 1998. Effect of rigor temperature and frozen storage on functional properties of hot-boned manufacturing beef. Meat Sci. 49:233–247. Howell, B. A. 1938. Muscles of the avian hip and thigh. Auk 55:71– 81. Lan, Y. H., J. Novakofski, R. H. Mccusker, M. S. Brewer, T. R. Carr, and F. K. Mckeith. 1995. Thermal gelation of stretched and coldshortened bovine sternomandibularis muscle and myofibrils. J. Food Sci. 60:661–663. Lawrie, R. 1998. Meat Science, 6th ed. Woodhead Publishing Ltd., Cambridge, UK. Lesiak, M. T., D. G. Olson, C. A. Lesiak, and D. U. Ahn. 1996. Effects of postmortem temperature and time on the water-holding capacity of hot-boned turkey breast and thigh muscle. Meat Sci. 43:51–60. Locker, R. H., and C. J. Hagyard. 1963. A cold shortening effect in beef muscles. J. Sci. Food Agric. 14:787–793. Lyon, B. G., C. E. Lyon, and D. Hamm. 1984. The effect of salt addition and postmortem holding time on cooked yield and texture of hot-stripped ground fowl meat. Poult. Sci. 63:1154–1159. Lyon, C. E., D. Hamm, J. E. Thomson, J. P. Hudspeth, J. L. Ayres, and J. E. Marion. 1983. Effects of hot or cold deboning on functional-properties of broiler dark meat and quality of sausage. Poult. Sci. 62:965–970. Lyon, C. E., B. G. Lyon, and E. M. Savage. 2003. Effect of postchill deboning time on the texture profile of broiler breeder hen breast meat. J. Appl. Poult. Res. 12:348–355. Lyon, C. E., and R. I. Wilson. 1986. Effects of sex, rigor condition, and heating method on yield and objective texture of broiler breast meat. Poult. Sci. 65:907–914. McCurdy, R. D., S. Barbut, and M. Quinton. 1996. Seasonal effect on pale soft exudative (PSE) occurrence in young turkey breast meat. Food Res. Int. 29:363–366. Neel, S. W., J. O. Reagan, and J. W. Mabry. 1987. Effects of rapid chilling and accelerated processing on the physical and sensory characteristics of fresh pork loins. J. Anim. Sci. 64:765–773.
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tically significant, the texture analysis of cooked whole thigh meat showed that hot-cut thighs tended to have the highest WB shear force value (36 N), followed by the 2-h samples. Thighs cut at 24 h PM had the lowest shear force value (29 N; Table 3). A similar relationship between deboning time and sensory texture characteristics of cooked boneless skinless chicken thigh meat was reported by Zhuang and Savage (2011). They found that for sensory texture attributes hardness, cohesiveness of mass, and chewiness, hot-boned thighs had the highest average intensity score, whereas 24-h-deboned samples had the lowest score. Results from the present study indicate that the removal of the whole chicken leg from the backbone has the same effect on texture quality of chicken thigh meat as deboning, and further demonstrate that the removal of the whole leg may play a key role in quality changes in deboned chicken thighs due to aging PM. Because there were no significant differences in WB shear force between the cooked hot-cut thighs and aged cold-cut samples and high WHC, including high cook yield, has been associated with better sensory texture quality (Van Oeckel et al., 1999), these results indicate that hot-boning chicken thighs might not have the same detrimental effects on eating texture quality observed in chicken breast fillets, in which hot-deboning consistently results in tougher meat texture than cold-deboning at 24 h PM (Lyon and Wilson, 1986; Cavitt et al., 2005; Xiong et al., 2006). In summary, PM aging can significantly affect cook yield of deboned chicken thighs and the difference in cook yield between hot-boned and cold-deboned thighs can be as much as 6%. Aging before deboning resulted in significantly reduced cook yield in chicken thigh iliofibialis muscles, boneless skinless whole chicken thighs, and whole intact chicken thighs (bone in). The PM time at which whole legs were cut from the carcass backbone influenced cook yield of whole intact thighs. However, separation of whole intact thighs from drumsticks did not affect the cook yield. In addition, thighs cut from carcass backbones before chilling were darker and redder at 24 h PM than thighs that were coldcut from carcasses and measured at the same PM time (24 h). There were no differences between the hot-cut and cold-removed whole thighs for muscle shear force. These results suggest that hot processing, either hotboning or hot-cutting of whole chicken legs from carcass backbone before chilling, can significantly increase cook yield or product yield of boneless skinless chicken thighs during further processing (processing beyond cut-up part) without detrimental effects on meat color and texture quality.
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Zhuang et al. Woltersdorf, W., and K. Troeger. 1988. Improving the quality of PSE pork by means of very rapid chilling. Fleischwirtschaft. 68:803–808. Xiong, R., L. C. Cavitt, J. F. Meullenet, and C. M. Owens. 2006. Comparison of Allo-Kramer, Warner-Bratzler and razor blade shears for predicting sensory tenderness of broiler breast meat. J. Texture Stud. 37:179–199. Zhuang, H., and E. M. Savage. 2009. Variation and Pearson correlations of Warner-Bratzler shear force measurements within broiler breast fillets. Poult. Sci. 88:214–220. Zhuang, H., and E. M. Savage. 2010. Comparisons of sensory descriptive flavor and texture profiles of cooked broiler breast fillets categorized by raw meat color lightness values. Poult. Sci. 89:1049–1055. Zhuang, H., and E. M. Savage. 2011. Comparison of sensory descriptive flavor profiles between cooked hot-boned and cold-deboned broiler breast fillets. Int. J. Poult. Sci. 10:426–432. Zhuang, H., E. M. Savage, S. E. Kays, and D. S. Himmelsbach. 2007. A survey of the quality of six retail brands of boneless, skinless chicken breast fillets obtained from retail supermarkets in the Athens, Georgia area. J. Food Qual. 30:1068–1082.
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Northcutt, J. K., R. J. Buhr, L. L. Young, C. E. Lyon, and G. O. Ware. 2001. Influence of age and postchill carcass aging duration on chicken breast fillet quality. Poult. Sci. 80:808–812. Petracci, M., and D. L. Fletcher. 2002. Broiler skin and meat color changes during storage. Poult. Sci. 81:1589–1597. Pisula, A., and A. Tyburcy. 1996. Hot processing of meat. Meat Sci. 43S1:125–134. Stewart, M. K., D. I. Fletcher, D. Hamm, and J. E. Thomson. 1984. The influence of hot boning broiler breast muscle on ph decline and toughening. Poult. Sci. 63:1935–1939. Thomson, J. E., C. E. Lyon, D. Hamm, J. A. Dickens, D. L. Fletcher, and A. D. Shackelford. 1986. Effects of electrical stunning and hot deboning on broiler breast meat quality. Poult. Sci. 65:1715–1719. Toscas, P. J., F. D. Shaw, and S. L. Beilken. 1999. Partial least squares (PLS) regression for the analysis of intrument measurements and sensory meat quality data. Meat Sci. 52:173–178. Van Oeckel, M. J., N. Warnants, and C. V. Boucque. 1999. Comparison of different methods for measuring water holding capacity and juiciness of pork versus on-line screening methods. Meat Sci. 51:313–320.
Key words: chicken thigh, postmortem deboning time, cook yield, iliotibialis, color 2014 Poultry Science 93:1553–1560 http://dx.doi.org/10.3382/ps.2013-03662
INTRODUCTION Cook yield or cook loss is one of the most commonly measured quality attributes for meat products. Cook yield directly affects product yield and profitability of further processed products, indicates meat waterholding capacity (WHC), and is related to the texture quality of finished products (Lawrie, 1998; Toscas et al., 1999; Van Oeckel et al., 1999; Cheng and Sun, 2008). Water-holding capacity, often measured with cook yield, of meat can be influenced by numerous variables from farm to table, including genetic strains, muscle types, preprocessing growing conditions and stresses, operations and treatments during processing (stunning, electrical stimulation, and chilling), postprocessing handling (aging, cutting, and storage), precooking treatments (marination), cooking techniques
©2014 Poultry Science Association Inc. Received September 30, 2013. Accepted February 11, 2014. 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]
(microwave, frying, or stewing), and cooking conditions (time and temperature; Cheng and Sun, 2008). In chicken breast meat, cook yield or cook loss is influenced by postmortem (PM) deboning time. Thomson et al. (1986) showed that regardless of bird stunning method and experimental replication, fillets deboned and cooked at 20 min PM consistently lost less weight than samples deboned and cooked at 24 h PM. Lyon and Wilson (1986) also found that cook yield of chicken breast meat was affected by rigor condition. Prerigor samples cooked immediately after deboning exhibited higher yields than postrigor samples (24 h PM) regardless of cooking method. Lyon et al. (2003) reported that cook yield of 24-h deboned hen breast meat was lower than that deboned 2 h postchill. Zhuang and Savage (2011) found that the cook yield of hot-boned breast meat was higher than either 2 or 24 h cold-deboned chicken fillets. northcutt et al. (2001) demonstrated that cook yield of broiler breast meat deboned at 2, 4, and 6 h postchill was higher than samples deboned 0 h postchill. Boneless skinless chicken thighs are a relatively new poultry product in the retail market and their market share is rapidly growing in the United States (Aho, 2009). There is no scientific information available con-
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ABSTRACT Three experiments were conducted to evaluate the effects of postmortem deboning time on cook yield of boneless skinless chicken thighs. In experiment 1, chicken thigh meat was deboned at 0.75 (hotbone), 2, and 24 h postmortem (PM) and trimmed to obtain mainly iliotibialis muscle. Samples were cooked directly from a frozen state. Cook yield of the muscle was significantly influenced by PM deboning time. Hotboned thighs exhibited a 7% greater cook yield than the samples deboned at 24 h. In experiment 2, boneless skinless chicken thighs were deboned at 0.3, 2, and 24 h PM and cooked directly from a fresh, never-frozen state at 24 h PM. Cook yield of the hot-boned thighs was significantly higher than those of the 2 and 24 h deboned samples, which did not differ from each other. In experiment 3, whole legs (thigh + drumstick) were cut from the carcass backbone at 0.3 (hot-cut), 2, and
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MATERIALS AND METHODS Experiment 1 During each of 3 replications, commercially processed and eviscerated carcasses from broilers (approximately 42 d old) were obtained from a local processing plant before carcass chilling. Carcasses were placed in coolers and transported to the laboratory within 20 min. Carcasses were immersed in a prechill water tank at 14°C for 0.25 h, followed by submersion in water immersion chill tanks at 0 to 4°C for 60 min. For each replica-
tion, 10 prechill carcasses were hot-boned at 45 min PM (average carcass temperature, 35.2°C), 10 postchill carcasses were cold-deboned at 2 h PM (average carcass temperature, 3.6°C), and 10 chilled carcasses were placed in Ziploc freezer bags and held at 1 to 2°C for 22 h before being deboned at 24 h PM (average carcass temperature, 1.9°C). Broiler thighs were deboned manually. Due to muscle variation and irregular alignment of muscle fibers, muscles from the medial side of each thigh were removed and discarded. Muscles from the lateral side, consisting primarily of the iliotibialis, iliofibularis, and femoritibialis (Howell, 1938), were trimmed to a similar length (Figure 1). Trimmed thigh samples were placed in cooking bags (Seal-a-Meal bag, The Holmes Group, El Paso, TX) and immediately frozen at −20°C until further analysis (14–21 d). Bagged thigh meat was cooked directly from the frozen state to an endpoint temperature of 77 to 78°C (12–15 min at 85°C in a Henny Penny combi oven, Henny Penny Co., Eaton, OH). Cooked thighs were allowed to cool at room temperature for 3 to 5 min before they were removed from bags, drained, and weighed. Cook yield was calculated as a percentage of the prefreezing weight (Zhuang et al., 2007).
Experiment 2 Broilers (n = 18) were slaughtered at the Richard B. Russell Agricultural Research Center poultry processing facility according to an approved protocol of the Institutional Animal Care and Use Committee. Birds were stunned at 15 V and 50 Hz of pulsed direct current in a prestunner (model SF-7000, Simmons Engineering Co., Dallas, GA) for 8 s, and killed by exsanguination using an automatic knife (model SK5, Simmons Engineering Co.). After bleeding (2 min), carcasses were scalded in a triple-pass 3-stage scalder (model SGS-3Ca, Stork-Gamco Inc., Gainesville, GA) for 1.5 min and were automatically picked in a 4-bank picker for 30 s (model D-8, Stork-Gamco Inc.). The heads, feet, viscera, and lungs were removed manually and carcasses were rinsed with water to remove blood, feathers, or other loose debris before chilling. Carcasses were immersion-chilled until body temperature reached 4°C and then stored in Ziploc bags on ice in a cold room until deboning. Whole legs (a total of 36 yielded from 18 carcasses) were removed from carcasses and thighs were skinned and deboned prechill at 20 min PM (hotboned; n = 12), postchill at 2 h PM (n = 12), and at 24 h PM (n = 12). To minimize variation in the results, paired thigh halves from the same carcass were used for the 3 different deboning time treatments. For each deboning time, 12 thighs were removed from different individual carcasses and 6 of them were paired with one of the other treatments. After deboning, thighs were placed in individual Ziploc bags and stored on ice in a cold room (4°C) until cooking. At 24 h PM, the boneless skinless thigh samples were weighed and placed in cooking bags. The samples were then cooked
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cerning the effects of deboning time on the cook yield of boneless skinless chicken thigh meat. Although much research has been conducted on chicken breast meat, thigh and breast products are known to contain inherently different types of muscles. Chicken breast muscles consist of almost entirely white, fast-twitch, glycolytic (type IIB) muscle fibers (Edman et al., 1988; Barbut, 2002), whereas thigh muscles have a high abundance of heterogenous red, slow-twitch, oxidative (type I) muscle fibers (Howell, 1938; Barbut, 2002). Compared with white fibers, red fibers are smaller in diameter, contain more myoglobin, less glycogen, and have a higher lipid content (Cassens and Cooper, 1971). While important physiologically, vast differences in muscle fiber type also influence PM meat quality development. Experiments have shown that PM conditions have different effects on WHC of poultry breast and thigh meat. For example, Lesiak et al. (1996) found that turkey thigh muscles were more susceptible to cold shortening than breast muscles. Lower storage temperature (0°C) produced better WHC in breast meat regardless of storage time, whereas storage temperatures at 12°C for a longer time (more than 24 h) resulted in better WHC than either lower (0°C) or higher (30°C) storage temperature in turkey thighs. These indicate that effects of PM deboning time on cook yield of boneless chicken thighs may differ from that on cook yield of boneless chicken breast fillets. While it is hypothesized that deboning time influences the cook yield of chicken thigh meat, it is further speculated that the fabrication process for producing thigh products may also have an influence on cook yield. For the production of boneless thigh products, before removing the thigh meat from the femur bone, intact thighs need to be removed from the carcass backbone and separated from the leg (drumstick). It is unknown if deboning time effects on cook yield or other quality measurements in boneless thigh products result from these fabrication steps. The objective of this 3-part study was to investigate the effects of PM deboning time on the cook yield of boneless skinless chicken thighs cooked from either the frozen state (experiment 1) or fresh at 24 h PM (experiment 2), and to understand the role that cutting up plays in cook yield differences of deboned thigh meat due to PM aging (experiment 3).
POSTMORTEM DEBONING AND THIGH COOK YIELD
in a water bath at 85°C to the endpoint temperature of 80°C. Similar to sample handling procedures used for sensory panel evaluation (Zhuang et al., 2007), cooked samples were cooled in the sealed bags for 3 to 5 min at room temperature, removed from bags, drained, and reweighed for cook yield calculations.
Experiment 3 Broiler birds were slaughtered and processed following the same procedure as described for experiment 2. Two replications of the entire experiment were conducted with 12 birds yielding 24 whole legs (thigh + drumstick) per replicate. Within each replicate, 8 of the 24 whole legs were cut (removed) from the different carcasses at 20 min PM, 8 at 2 h PM, and 8 at 24 h PM. For whole leg samples that were cut from carcasses at 20 min PM, half of the intact (bone-in) thighs were separated from the legs (drumsticks) at 20 min PM and the other half of the samples were separated from the legs at 24 h PM. Similarly, for whole leg samples that were cut from the carcasses at 2 h PM, half of the intact thighs were separated from the legs at 2 h PM and half were separated at 24 h PM. Intact thighs (bone-in)
were separated from the legs by cutting at the joint between the thigh and leg (Figure 2), skin was removed, and the samples were trimmed. All samples (whole legs and intact thighs) taken before 24 h PM were weighed at the time of sampling (20 min or 2 h PM), placed in a Ziploc bag, and stored on ice in a cold room until 24 h PM when they were reweighed to calculate drip loss. At 24 h PM, the thighs that were still attached to legs were cut, trimmed, and weighed again (for cook loss) before being sealed in individual cook bags. After cooking intact thigh samples to 80°C in an 85°C water bath, the samples were cooled at room temperature for 3 to 5 min before they were drained and weighed. The cooked thigh muscles were then removed from the bone and cut into a 19 mm wide strip for Warner-Bratzler (WB) shear force analysis (Zhuang and Savage, 2009). Color (skin side) and pH (with a piercing probe at the cranial end) of raw skinless broiler thighs were measured both after the samples were removed from carcasses, and at 24 h PM according to the method of Zhuang and Savage (2010). Because thighs consist of multiple muscles, they were divided into muscle A (iliotibialis) and muscle B (sartorius; Figure 2) for color, pH, and WB shear force measurements. Color was measured at 2 locations on the exposed surface of each thigh muscle (Aa, Ab, Ba, and Bb) and the mean of the 2 color measurements for each muscle was used for the statistics. For each muscle, pH was measured in the proximal end. For WB shear force values (N), each muscle was sheared one time and the mean of the muscles was used for the statistical analysis.
Statistics All data were analyzed using the GLM procedures of SAS (SAS version 9.1, SAS Institute Inc., Cary, NC). The main factors were PM time (deboning time for experiments 1 and 2), leg separation (for experiment 3 only), muscle (for color and pH in experiment 3 only), and replication (for experiments 1 and 3). Means were separated with the Tukey option at a significance level of 0.05.
RESULTS Experiments 1 and 2 Table 1 shows the effect of PM deboning time on cook yield of thigh muscles (mainly iliotibialis) cooked from a frozen state. There were significant differences between the 3 different deboning times. Hot-boned thigh samples had the highest cook yield (81.1%), followed by the 2-h samples (78.5%). Thigh samples deboned at 24 h PM had the lowest cook yield (73.5%). Table 1 also shows the effect of PM deboning time on cook yield of boneless skinless chicken thighs cooked fresh at 24 h PM. There were significant differences among the 3 different deboning times. The cook yield of hot-
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Figure 1. Deboned and trimmed thigh muscle (iliotibialis) used to evaluate cook yield in experiment 1. Color version available in the online PDF.
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boned skinless chicken thighs (79.9%) was significantly higher than those of the 2-h (72.9%) and 24-h samples (73.4%), which did not differ from each other.
Experiment 3 Table 2 shows color and pH values of 2 different muscles from the intact chicken thighs (bone-in) at the time the whole legs were cut from the carcass and at 24 h PM. There were significant differences for pH, L* (lightness), a* (redness), and b* (yellowness) values between the 2 muscles. Regardless of cut-up time, L*
Table 1. Effect of deboning time on cook yield of boneless skinless chicken thighs Cook yield1 (%) Deboning time Hot-boned Chill-boned (2 h PM) Chill-boned (24 h PM)
Experiment 12
Experiment 23
81.1 ± 3.6a 78.5 ± 4.6b 73.5 ± 4.5c
79.9 ± 2.4a 72.9 ± 2.4b 73.4 ± 4.3b
a–cMean values with no common superscript in the same column are significantly different (P < 0.05). 1Cook yield = 100 × (cooked thigh weight/raw thigh weight immediately after deboning and trimming). 2In experiment 1, the hot-boned thigh muscle iliotibialis was removed from bones 45 min postmortem (PM). The samples were cooked directly from a frozen state (mean ± SD, n = 30). 3In experiment 2, the hot-boned whole boneless skinless chicken thighs were removed from bones 20 min PM. The samples were cooked fresh at 24 h PM (mean ± SD, n = 12).
DISCUSSION Results from the present study show that PM deboning time can significantly affect cook yield of deboned
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Figure 2. A diagram showing cuts, color sampling locations, and muscles used for shear force measurements in a whole chicken leg in experiment 3. Color version available in the online PDF.
and a* values of the muscle B were significantly higher (more than 3 units for L*) and b* values lower (more than 2 units) than those of the muscle A. These results indicate that surface color of thigh muscle region A was consistently darker (low L* value), more yellow (high b* value), and less red (low a* value) than that of location B. For pH, the average values of the muscle B were significantly higher (more than 0.1 units) than those of the muscle A for early PM samples (hot-cut and 2 h removed); however, there were no differences between the 2 muscles for pH measured at 24 h PM. The color and pH of the thigh muscles also depended upon the time at which the whole legs were cut from the carcass and aging time. For muscle A, the L* value of the hot-cut thighs was significantly lower than those of the 2- and 24-h-cut thighs at both the time of whole leg removal from the carcass and after the samples were aged to 24 h PM (hot-cut + 24 h and chill-cut at 2 h + 22 h). The L* value of the 2-h-cut thighs was significantly lower than those of the 24-h-cut samples and the 2 h + 22 h thighs, which did not differ from each other. However, the L* value of the hot-cut thighs (hot-cut + 24 h) was significantly lower than both the 2 h + 22 h and 24-h-removed thighs at 24 h PM. For muscle B, regardless of aging time, the L* values of the hotcut thighs were significantly lower than the 2 and 24 h samples. There was no difference between the hot-cut and hot-cut + 24 h samples and there were no differences between the 2, 24, and 2 + 22 h samples. Average a* values were consistently higher at 24 h PM, and the hot-cut thighs measured at 24 h PM had the highest a* value regardless of muscle. For pH in muscle A, only significant difference was observed between the 2 h and hot-cut + 24 h samples. However, for muscle B, pH of the early-cut thighs (including both hot-cut and 2-h-cut) was significantly higher than those measured at 24 h PM. There was no difference between the hot-cut and 2-h-cut thighs, and there were no differences between the 3 samples that were measured at 24 h PM. Table 3 shows the effect of PM cut-up time on drip loss, cook yield, and shear force of the cut whole chicken thighs. Overall, samples lost less than 1% of moisture after stored at a refrigerated temperature for more than 22 h, and there was no difference between the hot-cut and 2-h-cut samples. Cook yield of the hotcut thighs was significantly higher than 2- and 24-h cold-cut samples, which did not differ from each other. Shear force values of cooked intact thighs averaged less than 40 n and there were no significant differences between the 3 p.m. times. There were no differences between the whole legs (thigh + drumstick) and intact thighs (bone-in) for drip loss, cook yield, or shear force (Table 4).
6.12 ± 0.13b,x
0.20a,x 0.20a,x 0.12b,x 0.17b,x ± ± ± ± 6.39 6.34 6.14 6.06 ± ± ± ± 3.81 1.79 3.05 4.40
6.14 ± 0.22ab,x 2.34 ± 1.92b,y
6.12 6.21 6.14 6.09
± ± ± ±
0.18ab,y 0.21a,y 0.12ab,x 0.25b,x 1.91a,y 1.22b,y 1.73ab,y 2.36a,y
values with no common superscript in the same column are significantly different (P < 0.05). values with no common superscript in the same row for the same parameter are significantly different (P < 0.05). 1Postmortem (PM) time at which whole legs were removed from carcasses. 2L* = lightness; a* = redness; b* = yellowness. 3The color and pH were measured after the hot-cut thighs were aged for 24 h at a refrigerated temperature. 4The color and pH were measured after the 2-h-cut thighs were aged for 22 h at a refrigerated temperature.
6.67 ± 2.51a,x −0.09 ± 0.80ab,y 60.5 ± 2.59a,y
± ± ± ± 53.1 58.1 61.7 55.8
Hot-cut (20 min PM) Chill-cut (2 h PM) Chill-cut (24 h PM) Hot-cut (20 min PM + 24 h aging)3 Chill-cut (2 h PM + 22 h aging)4
a–dMean
± ± ± ± 60.8 63.8 65.4 61.1
64.8 ± 2.41a,x
−0.53 −0.71 −0.38 0.52
± ± ± ±
1.01b,y 0.54b,y 0.98b,y 0.99a,y
Muscle A Muscle B
3.21b,x 2.81a,x 3.69a,x 3.58b,x 2.68d,y 2.63b,y 2.88a,y 1.90c,y
Muscle A Cut-up time/aging time
x,yMean
± ± ± ± 0.96 0.85 1.55 3.08
1.85 ± 0.80b,x
7.46 4.49 7.01 7.35
± ± ± ±
2.04a,x 1.73b,x 2.55a,x 1.78a,x 0.95cd,x 0.85d,x 1.48bc,x 0.94a,x
Muscle B
Muscle A
b* a* L*
Table 2. Effect of cut-up time1 on color and pH of intact (bone-in) chicken thigh muscles (mean ± SD, n = 16; experiment 3)2
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chicken thighs. For both the thigh samples consisting of mainly iliotibialis muscle (experiment 1) and the thigh samples consisting of multiple muscles (experiment 2), the cook yields of hot-boned thighs were significantly higher than the chilled samples. For the boneless skinless products, the difference can be as large as 6%. It is commonly noted that WHC of hot-boned meat is superior to cold-deboned meat (Pisula and Tyburcy, 1996) regardless of muscle type. Neel et al. (1987) and Woltersdorf and Troeger (1988) found that hot-boned pork had higher WHC than the cold-boned samples. Similarly, Lyon et al. (1983) reported that deboned leg meat from un-eviscerated, un-chilled chicken carcasses released less moisture when heated than conventionally processed and aged meat. With chicken breast fillets, Zhuang and Savage (2011) found that hot-boned samples had significantly higher cook yields than coldboned meat. It is postulated that the higher WHC associated with hot-boned meat is due to higher muscle pH before cooking (Lyon and Wilson, 1986; Pisula and Tyburcy, 1996). There is a significant negative correlation between meat ultimate pH and cook loss in both poultry meat (McCurdy et al., 1996) and red meat (Bendall and Swatland, 1988; Andrews et al., 2000). Low pH in meat results in denaturation of meat proteins which can lead to reduced WHC and cook yield. During PM aging, pH of hot-boned or early PM meat is consistently higher than cold-boned or well-aged meat (McCurdy et al., 1996; Lyon and Wilson, 1986). Thus, the higher cook yields in hot-boned meat are thought to be due to a pH effect. For samples that are cooked immediately after deboning (Thomson et al., 1986; Lyon and Wilson, 1986), this would be the case. However, in our study, samples were aged until 24 h PM to allow pH to reach its minimum threshold. As shown in experiment 3 with bone-in thigh samples, pH differences between the samples were likely minimized before cooking. Thus, variations in cook yield between hot-boned and cold-boned samples were likely not due to pH differences and suggest that other factors may affect the differences in cook yield due to PM deboning time. In pork, Lan et al. (1995) showed that the muscle contractile state could affect cook loss, and Choi et al. (1987) found that hot-boned meat that had higher WHC also contained more soluble myosin heavy chains compared with cold-boned meat. The differences in the cook yield results between experiment 1 (cook yield of the 2-h samples was higher than the 24-h samples) and experiment 2 (no difference between the 2- and 24-h samples for cook yield) could be due to the different raw muscles used for the experiments, the sample preparation methods, or both. In experiment 1, only the iliotibialis muscles were used and the samples were cooked directly from the frozen state. However, in experiment 2, whole boneless skinless chicken thighs were used and samples were cooked from a fresh, never-frozen state. It is well known that prerigor and postrigor red meat respond differently to
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Muscle B
Muscle A
pH
Muscle B
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Zhuang et al. Table 3. Effect of cut-up time1 on drip loss, cook yield, and Warner-Bratzler shear force of intact (bone-in) chicken thighs (mean ± SD, n = 16; experiment 3) Cut-up time Hot-cut (20 min PM) Chill-cut (2 h PM) Chill-cut (24 h PM)
Drip loss2 (%)
Cook yield3 (%)
Shear force (N)
0.59 ± 0.63 0.75 ± 0.45 —–
79.5 ± 5.1a 77.2 ± 3.4b 76.8 ± 3.5b
36.0 ± 12.4 31.9 ± 8.7 29.0 ± 7.6
a,bMean
values with no common superscript in the same column are significantly different (P < 0.05). (PM) time at which whole legs were removed from carcasses. 2Drip loss = 100 × (W initial – W24 h)/Winitial, where W means weight. 3Cook yield = 100 × (cooked thigh weight/raw thigh weigh precooking). 1Postmortem
tion of iliotibialis and sartorius from the backbone at different PM times, the effect might also result from the rate of pH changes in cut whole chicken legs during PM aging, a cold-shortening effect for the cold-removed samples, or both. Stewart et al. (1984) found that pH of breast meat hot-boned from broilers somehow declined more slowly than breast meat left intact on the carcass. Rapid pH drop during early PM rigor mortis has been hypothesized to cause pale, soft, exudative poultry breast fillets and poor meat WHC, including cook yield (Barbut, 1993, 2009). These results indicate that PM biochemical changes in bone-in chicken thigh muscle follow a similar pattern to boneless skinless chicken thighs during meat aging, and that the whole chicken thighs can be used to predict quality changes in boneless skinless chicken thighs PM. Our results also demonstrate that color of thigh muscles significantly changes during the first 24 h PM aging. The thigh muscle L* values significantly increased regardless of muscle types. This result is in agreement with the data published by Petracci and Fletcher (2002), in which changes in L* values of thigh muscles increased rapidly in the first 3 h PM and plateaued after 6 h PM. An interesting observation from the present study was that lightness values of thigh muscles (regardless of muscle type) were significantly lower and redness values were the highest in the hot-cut sample [treatment hot-cut (20 min PM + 24 h aging)] measured after aging for 24 h compared with those of the other 2 treatments [treatment chill-cut (24 h PM) and treatment chill-cut (2 h PM + 22 h aging)] at the same PM time. This result indicated that the hot-cut thigh muscles would have darker color compared with the cold-deboned thighs after PM aging. For chicken breast meat, raw meat color lightness is associated with poor meat quality (Barbut, 1993, 2009). Although not statis-
Table 4. Effect of separation of legs from thighs when legs were cut from carcasses at either 20 min or 2 h postmortem (PM) on drip loss, cook yield, and Warner-Bratzler shear force of intact (bone-in) chicken thighs (mean ± SD, n = 16; experiment 3)1 Cut
Drip loss (%)
Cook yield (%)
Shear force (N)
Thigh only Whole leg
0.72 ± 0.53 0.63 ± 0.57
78.1 ± 4.6 78.6 ± 4.4
35.1 ± 8.3 32.8 ± 12.9
1The cooked thighs that were separated at 20 min (hot-cut) and 2 h PM (chill-cut) were combined for statistical analysis.
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freezing and thawing (Locker and Hagyard, 1963), and it is likely that chicken dark muscle behaves similar to red meat. In beef, cook losses of fresh samples tend to decrease with increased rigor temperatures, whereas they tend to increase for frozen samples (Farouk and Swan, 1998). The whole boneless skinless thighs used in experiment 2 contained multiple muscles in addition to the thigh iliotibialis muscle used in experiment 1 and probably contained more fat tissue. Lyon et al. (1984) attributed the difference in cook yield of ground fowl meat between breast and legs to the higher fat content in leg meat. Unlike deboning chicken breast meat, deboning thigh meat typically requires several steps. Deboning chicken thighs from carcasses involves removing whole chicken legs (thigh + drumstick) from carcass backbones and separating whole thighs from drumsticks before thigh muscle is deboned from the femur bone. For boneless skinless chicken thighs, the differences in cook yield due to different deboning times could result from the removal, separation step, or both. Experiment 3 was designed to evaluate the effect of cutting steps on cook yield of chicken thighs. Results demonstrated that cutting whole legs from carcasses before chilling can result in significant differences in cook yield (Table 3), and that separating the thighs from drumsticks at the time when whole legs were cut from carcasses (it was either 20 min or 2 h PM in the current study) has no effect (Table 4). This suggests that the early removal of the thigh from the backbone improves cook yield of hot-boned thighs and that the attachment of thigh muscles to the backbone somehow could at least partially result in loss of muscle WHC during aging. In addition to potential differences in contractile state (Lan et al., 1995; Cason et al., 2002) and solubility of myosin heavy chains (Choi et al., 1987) due to separa-
POSTMORTEM DEBONING AND THIGH COOK YIELD
ACKNOWLEDGMENTS The authors express their sincere thanks to Beth Barton, Andrew Ross, Taylor Kronn, Dianna V. Bourassa, and Gerald Heitschmidt of USDA-Agricultural Research Service (Athens, GA) for their technical assistance during the study.
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tically significant, the texture analysis of cooked whole thigh meat showed that hot-cut thighs tended to have the highest WB shear force value (36 N), followed by the 2-h samples. Thighs cut at 24 h PM had the lowest shear force value (29 N; Table 3). A similar relationship between deboning time and sensory texture characteristics of cooked boneless skinless chicken thigh meat was reported by Zhuang and Savage (2011). They found that for sensory texture attributes hardness, cohesiveness of mass, and chewiness, hot-boned thighs had the highest average intensity score, whereas 24-h-deboned samples had the lowest score. Results from the present study indicate that the removal of the whole chicken leg from the backbone has the same effect on texture quality of chicken thigh meat as deboning, and further demonstrate that the removal of the whole leg may play a key role in quality changes in deboned chicken thighs due to aging PM. Because there were no significant differences in WB shear force between the cooked hot-cut thighs and aged cold-cut samples and high WHC, including high cook yield, has been associated with better sensory texture quality (Van Oeckel et al., 1999), these results indicate that hot-boning chicken thighs might not have the same detrimental effects on eating texture quality observed in chicken breast fillets, in which hot-deboning consistently results in tougher meat texture than cold-deboning at 24 h PM (Lyon and Wilson, 1986; Cavitt et al., 2005; Xiong et al., 2006). In summary, PM aging can significantly affect cook yield of deboned chicken thighs and the difference in cook yield between hot-boned and cold-deboned thighs can be as much as 6%. Aging before deboning resulted in significantly reduced cook yield in chicken thigh iliofibialis muscles, boneless skinless whole chicken thighs, and whole intact chicken thighs (bone in). The PM time at which whole legs were cut from the carcass backbone influenced cook yield of whole intact thighs. However, separation of whole intact thighs from drumsticks did not affect the cook yield. In addition, thighs cut from carcass backbones before chilling were darker and redder at 24 h PM than thighs that were coldcut from carcasses and measured at the same PM time (24 h). There were no differences between the hot-cut and cold-removed whole thighs for muscle shear force. These results suggest that hot processing, either hotboning or hot-cutting of whole chicken legs from carcass backbone before chilling, can significantly increase cook yield or product yield of boneless skinless chicken thighs during further processing (processing beyond cut-up part) without detrimental effects on meat color and texture quality.
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