- Email: [email protected]
Muscle Growth and Meat Quality
©2007 Poultry Science Association, Inc.
Muscle Growth and Meat Quality M. J. Duclos,1 C. Berri, and E. Le Bihan-Duval Station de Recherches Avicoles, INRA, 37380 Nouzilly, France
Primary Audience: Broiler Breeders and Selectioners, Poultry Processors
Although poultry products are diverse, the general trend is for portioned and further-processed products to increase their market share. In this context, technological quality of poultry meat is an important aspect. It is largely determined by the acidification process of the meat postmortem. Defects in meat acidification have been described in poultry, and the purpose of this report was to evaluate whether they are linked to growth rate or stress susceptibility and whether they are under genetic control. Key words: meat quality, growth, genetics, stress, metabolism 2007 J. Appl. Poult. Res. 16:107–112
DESCRIPTION OF PROBLEM General Trends of the Poultry Meat Industry Due to genetic improvement and progress in nutrition and poultry management, meat-type chickens may exhibit very high growth rates and feed efficiency. At the same time, poultry production has been greatly diversified with the use of various genetic types reared in either intensive or extensive conditions. Although standard fast-growing birds are mainly used for portions or further-processed products, in some countries, alternative productions using intermediate or slow-growing genotypes are also developing. At the moment, these birds are mainly consumed as whole carcasses. Growth rates can be improved to reach an acceptable level of productivity and ensure a satisfying carcass and meat quality according to the specific targets of each production system. The quality of poultry products can be partitioned into several attributes, mainly the sensory (color, tenderness, flavor, juiciness) and the physical (muscle yield, water-holding capacity, 1
Corresponding author: [email protected]
cooking loss) attributes of chicken carcasses and meat, which vary with growth rate and body composition. The first concern is to produce a carcass of good quality, specifically, obtain a maximal meat yield with a limited fatness. Given the general trend for consuming more chicken parts and further-processed products, rather than whole carcass, an important aspect today is the technological quality of the meat. This is largely determined by postmortem metabolism through its effect on the color and the water-holding capacity. Biological Basis of Poultry Meat Quality Postmortem metabolism of the muscle tissue influences the characteristics of the meat [1]. In particular, the rate and the amplitude of acidification have a strong effect on both organoleptic and technological parameters of meat quality. After bleeding, cessation of O2 supply modifies muscular metabolism during the initiation of rigor mortis. The muscle relies on the anaerobic glycolytic pathway to use the i.m. glycogen stores for ATP regeneration, which leads to the
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SUMMARY
108
RELATION BETWEEN GROWTH AND MEAT QUALITY Several studies have shown that selection for greater BW or muscle development has induced histological and biochemical modifications of the muscle tissue. Some results have been ob-
tained by comparing 2 genotypes divergently selected for high or low growth rates [21] or a genotype selected for higher breast muscle development and a control genotype [22]. In both models, increased breast muscle development was associated with a marked increase in muscle fiber size [21, 22]. Modifications in muscle fiber number would be difficult to quantify and therefore cannot be ruled out, but they must be modest. In the divergent selection for high or low growth rates, a 20% increase in muscle fiber number was measured in an indicator muscle, the anterior latissimus dorsi [21], and it was hypothesized that a similar increase could occur in the pectoralis muscle. In the experimental selection for increased breast meat yield, the difference in fiber size seemed to arise during the first 2 wk posthatch, when the growth of the breast muscles was maximal [22]. Alterations in muscle biochemistry have also been observed. The meat from the genotypes with greater muscle mass showed a decreased content of heminic pigments and a decreased level of glycogen stores [23]. As a result, those genotypes showed a paler breast meat (less redness and greater lightness) with higher pHu values. A more recent study was designed to relate breast muscle development, including muscle fiber size, to the postmortem metabolism and breast meat quality [24]. Phenotypic and genetic relationships between fiber and meat traits were estimated for a total of 600 commercial broilers. For all birds, muscle fiber cross-sectional area, glycolytic potential, lactate content, postmortem pH fall, and classical meat traits (color, drip and thawing-cooking loss, and Warner-Bratzler shear force) were measured. As the fiber size increased, both the glycogen reserve at death (glycolytic potential) and the postmortem glycolytic activity of muscle decreased. As a consequence, breast muscles with the largest fibers exhibited the highest pH15 and pHu. Therefore, breast muscles with the largest fibers exhibited a darker lightness value, lower drip and thawingcooking losses, and greater tenderness after cooking. Commercial selection relies on several characteristics, including leanness, which prompted us to examine whether this parameter could by itself influence muscle metabolism. We observed that lean chickens showed a compara-
Downloaded from http://japr.oxfordjournals.org/ at University of Alberta on April 25, 2015
accumulation of lactic acid and protons. Therefore, the acidification process depends upon the amount of glycogen stores (estimated by the glycolytic potential) and the rate of the glycolysis [1]. In the chicken, normal pH values at 15 min postmortem (pH15) are around 6.2 to 6.5 [2, 3], whereas normal ultimate pH (pHu) values are around 5.8 [4, 5]. If the pH15 value is low (below 6.0) when the muscle is still warm, the proteins are subject to denaturation [1], which leads to a decreased water-holding capacity and a decoloration of the meat. Such defects have been amply described in pigs, but also in turkeys [6, 7, 8, 9, 10] and chickens [5, 11]. These meats are often described as pale, soft, and exudative by analogy with those described in pigs. They show exudative water loss [8, 12, 13, 14] and a decreased technological yield [5, 14, 15]. Although raw pale, soft, and exudative meats show a rather soft texture, they tend to be less tender after cooking because of excessive exudation [12, 13, 14]. The results concerning the color of those meats are more controversial. Comparing carcasses with fast and normal rates of acidification, some authors have observed paler meat for fast glycolyser [8, 15], whereas others have observed no difference [14, 16]. Acid meats are characterized by a low ultimate pH (pHu < 5.7), which induces structural alterations in the muscle with an impairment of the technological processing ability. Artificially acidifying turkey meat [17] induces a destructuration of the myofibrillar network, which also induces a marked decrease in water-holding capacity. At the other extreme, meats with a high ultimate pH also show defects in their color, texture, and water-holding capacity. High pHu meat is dark, firm, and dry. These types of meats can occur in poultry [18, 19]; they show enhanced water-holding capacities, but an increased sensitivity to microbial development described in other species has not yet been observed in dark poultry meats [20].
JAPR: Symposium
DUCLOS ET AL.: POULTRY MEAT AND EGG QUALITY SYMPOSIUM
EFFECT OF STRESS-GENOTYPE INTERACTIONS ON MEAT QUALITY Studies conducted in different species indicate that stressors during transportation and at the slaughterhouse have a strong influence on meat quality. The fact that meat quality defects are sometimes described under commercial conditions but do not arise under less stressful experimental conditions also suggests this. We have, therefore, constructed experiments to have an insight on the effect of stressors on meat quality parameters of different chicken genotypes. A first study compared commercial fastgrowing and slow-growing broilers submitted to transport or acute heat stressors [26]. The data showed a muscle-specific effect of stressors, breast muscle being less sensitive than thigh muscle. It was, however, observed that the slowgrowing birds showed a higher level of struggle during shackling and an accelerated rate of postmortem glycolysis detrimental to the quality of their breast meat. In a second study, 2 fast-growing genotypes were considered, one showing a higher breast yield (heavy line) in comparison with a slow-growing genotype [3, 27]. The birds were handled with minimal stress, submitted to
a hanging stressor of 2 min, or submitted to 3.5 h of heat stressor followed by hanging. The measure of circulating corticosterone confirmed the additive effects of both stressful conditions. Observation of the birds during shackling with recording of vocalizations, wing-flapping duration, and attempts to right showed that the slowgrowing line was the most active, the heavy line was the least active, and the fast-growing line was intermediate. The meat from the slow-growing genotype was redder. The hanging stressor also induced a redder meat; it is therefore likely that the differences of activity explained partly the differences in meat color among the genotypes. Both genotypes and stressors altered postmortem acidification of the meat. The values of pH15 were higher for the heaviest birds in relation to their lower level of activity on the shackle line, although they decreased with the hanging duration. Heat stressor induced no further decrease. Within each genotype, we observed a strong negative correlation between pH15 and wing-flapping duration. The pHu values were lowest for the slow-growing genotype, highest for the standard fast-growing genotype, and intermediate for the heavy genotype, and they decreased following the heat plus hanging stressors. Within each genotype, the pHu was negatively correlated with the glycolytic potential (−0.42 to −0.66). Finally, the technological yield was mostly altered by stressors in the slowgrowing genotype, whereas it was almost unaltered in the other 2 genotypes.
GENETIC CONTROL OF POULTRY MEAT QUALITY As discussed above, the comparison of genotypes suggests that poultry meat quality traits are under genetic control. Heritability values and genetic correlations have first been measured in an experimental line of chickens selected for improved breast meat yield and decreased fat deposition [28]. All recorded parameters (pH15, pHu, luminance, redness, yellowness, and water loss) showed high heritability values ranging from 0.35 to 0.57. In this model, the highest genetic correlations were between pHu and luminance (−0.91), pHu and drip loss (−0.83), and luminance and drip loss (+0.81), in agreement with data obtained in other species like pigs. Notably, a rather high negative correlation be-
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tively lower level of glycogen stores than fat chickens, with decreased amplitude of acidification of the meat postmortem and decreased exudation [25]. This suggests that selection for increased muscle yields and against fat deposition could exert cumulative effects on muscle metabolism, decreasing glycogen storage and thereby reducing the amplitude of acidification postmortem. Finally, the water-holding capacity of the meat, and therefore its processing yield, was ameliorated. Together, these studies did not identify any antagonism between growth rate or muscle development and breast meat quality parameters such as water retention and processing ability. However, the effect of muscle structural changes on other product characteristics such as the sliceability remains to be investigated. More generally, mechanisms underlying the decrease of glycogen stores in intensively selected birds need to be elucidated and put in relation to the bird’s general metabolism and stress sensitivity.
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avoiding too high or too low of values. This parameter, however, requires slaughtering birds and therefore selecting on siblings, which is costly and of reduced efficiency. For this reason, we believe that there is a need for identifying molecular markers allowing for marker-assisted selection. Identifying molecular markers of meat quality traits requires resource populations with pedigree families. Divergently selected lines of broilers with high or low growth rates or with high or low adiposity at the same BW are adequate models. By crossing those lines 2 by 2, the alleles governing meat quality traits can be tracked in an F2 population. Such populations have been recently obtained, and preliminary analysis on the identification of quantitative trait loci governing growth and body composition have been reported [31, 32]. Simultaneously, microarrays are being used to compare global gene expression profiles [33] within each of the 2 sets of divergent genotypes (fast-growing compared with slow-growing and fat compared with lean chickens with the same BW). The combination of the results from both strategies is expected to identify double functional and positional candidates as a step toward the identification of the genes explaining the differences among genotypes. This strategy is being applied simultaneously to the 2 genetic models, which should permit identification of genes that are specific to 1 model or common to both for similar criteria (i.e., ideally, this should permit identification and discriminate between general mechanisms and other more specific to each “genetic background”). In the future, a major task will be to identify the mutation(s) underlying a favorable quantitative trait loci allele so that it can be used by the breeders in marker-assisted selection schemes.
CONCLUSIONS AND APPLICATIONS 1. We accumulated data showing that variations in growth rate among genotypes have an effect on meat quality parameters. This relation could result from alterations in muscle structure or metabolism. 2. We observed no evidence of any antagonism between growth rate or muscle development and breast meat quality parameters such as water retention and processing ability. 3. A part of the differences among genotypes is likely to result from the differences in their susceptibility to preslaughter stressors. The slowest-growing genotypes appear to be the most reactive, and the fastest-growing genotypes appear to be the least reactive.
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tween pHu and abdominal fat was also recorded (−0.54). By contrast, the correlations with pH15 were rather low, due to the relatively high values for this parameter observed under experimental conditions. The correlations between growth and meat quality traits were also rather low, suggesting that the genetic mechanisms underlying those traits could be different and further indicating that selection for growth rate or breast meat yield would not negatively alter meat quality. A comparable study was performed on turkeys [29], and similar data were obtained. In this last study, however, the heritability values were lower, possibly because the birds had been slaughtered in a commercial plant. More recently, a study was conducted on a commercial strain of heavy broilers [30], confirming the correlations previously observed in the experimental strain [28] and significant heritability values for meat parameters (from 0.25 to 0.35). Three additional traits, glycolytic potential, cooking loss, and toughness of cooked meat, were recorded that also showed high heritability values (from 0.34 to 0.43). Notably, the glycolytic potential was negatively correlated to the pHu, with a value close to −1. Luminance, exudate, cooking loss, and toughness were negatively correlated with pHu (−0.65 to −0.89), whereas only luminance and exudate were negatively correlated with pH15 (about −0.5). Again, no correlation was observed between pHu and pH15, suggesting a distinct genetic control of these 2 parameters. The genetic studies suggested that pHu was a highly heritable character that was highly correlated with several meat quality traits. It could therefore be considered as a good selection parameter to improve meat color, water-holding capacity, and texture. One strategy could be selecting for an optimal window of pHu values,
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4. Within a fast-growing genotype, we observed only low genetic correlations between growth and meat quality traits. As quality traits are highly heritable, this suggests a possibility to select for meat quality without altering growth rate. 5. The measure of meat quality parameters, however, requires slaughtering birds and therefore selecting on siblings, which is costly and of reduced efficiency. For this reason, we believe that there is a need for identifying molecular markers allowing for marker-assisted selection.
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2. Kijowski, J., and A. Niewiarowicz. 1978. Emulsifying properties of proteins and meat from broiler breast muscles as affected by their initial pH values. J Food Technol 13:451–459.
19. Mallia, J. G., S. Barbut, J. P. Vaillancourt, S. W. Martin, and S. A. McEwen. 2000. A dark, firm dry-like condition in turkeys condemned for cyanosis. Poult. Sci. 79:281–285.
3. Berri, C., M. Debut, C. Sante´-Lhoutellier, B. Arnould, B. Boutten, N. Sellier, E. Bae´za, N. Jehl, Y. Je´go, M. J. Duclos, and E. Le Bihan-Duval. 2005. Variations in chicken breast meat quality: A strong implication of struggle and muscle glycogen level at death. Br. Poult. Sci. 46:572–579.
20. Allen, C. D., S. M. Russell, and D. L. Fletcher. 1997. The relationship of broiler breast meat color and pH to shelf-life and odor development. Poult. Sci. 76:1042–1046.
4. Fletcher, D. L. 1999. Broiler breast meat color variation, pH, and texture. Poult. Sci. 78:1323–1327. 5. Van Laack, R. L., C. H. Liu, M. O. Smith, and H. D. Loveday. 2000. Characteristics of pale, soft, exudative breast meat. Poult. Sci. 79:1057–1061. 6. Barbut, S. 1996. Estimates and detection of the PSE problem in young turkey breast meat. Can. J. Anim. Sci. 76:455–457. 7. Barbut, S. 1997. Occurrence of pale soft exudative meat in mature turkey hens. Br. Poult. Sci. 38:74–77. 8. Pietrzak, M., M. L. Greaser, and A. A. Sosnicki. 1997. Effect of rapid rigor mortis processes on protein functionality in pectoralis major muscle of domestic turkeys. J. Anim. Sci. 75:2106–2116. 9. Sosnicki, A. A., M. L. Greaser, M. Pietrzack, E. Pospiesch, and V. Sante. 1998. PSE-like syndrome in breast muscle of domestic turkeys: A review. J. Muscle Food 9:13–23. 10. Owens, C. M., E. M. Hirschler, S. R. McKee, R. MartinezDawson, and A. R. Sams. 2000. The characterization and incidence of pale, soft, exudative turkey meat in a commercial plant. Poult. Sci. 79:553–558. 11. Barbut, S. 1997. Problem of pale soft exudative meat in broiler chickens. Br. Poult. Sci. 38:355–358. 12. McKee, S. R., and A. R. Sams. 1997. The effect of seasonal heat stress on rigor development and the incidence of pale, exudative turkey meat. Poult. Sci. 76:1616–1620. 13. Hahn, G., M. Malenica, W. D. Mu¨ller, E. Taubert, and T. Petrak. 2001. Influence of postmortal glycolysis on meat quality and technological properties of turkey breast. Pages 325–328 in Proc. XV Eur. Symp. Qual. Poult. Meat, Kusadasi, Turkey. R. W. A. Mulder and S. Bilgili, ed. WPSA, Turkish Branch, Kusadasi, Turkey. 14. Molette, C., H. Remignon, and R. Babile. 2003. Effect of rate of pH fall on turkey breast meat quality. Br. Poult. Sci. 44:787–788. 15. Fernandez, X., V. Sante, E. Baeza, E. Lebihan-Duval, C. Berri, H. Remignon, R. Babile, G. Le Pottier, and T. Astruc. 2002. Effects of the rate of muscle post mortem pH fall on the technological quality of turkey meat. Br. Poult. Sci. 43:245–252. 16. Rathgeber, B. M., J. A. Boles, and P. J. Shand. 1999. Rapid postmortem pH decline and delayed chilling reduce quality of turkey breast meat. Poult. Sci. 78:477–484. 17. Barbut, S. 1997. Microstructure of white and dark turkey meat batters as affected by pH. Br. Poult. Sci. 38:175–182.
21. Remignon, H., M. F. Gardahaut, G. Marche, and F. H. Ricard. 1995. Selection for rapid growth increases the number and the size of muscle fibres without changing their typing in chickens. J. Muscle Res. Cell Motil. 16:95–102. 22. Guernec, A., C. Berri, B. Chevalier, N. Wacrenier-Cere, E. Le Bihan-Duval, and M. J. Duclos. 2003. Muscle development, insulin-like growth factor-I and myostatin mRNA levels in chickens selected for increased breast muscle yield. Growth Horm. IGF Res. 13:8–18. 23. Berri, C., N. Wacrenier, N. Millet, and E. Le Bihan-Duval. 2001. Effect of selection for improved body composition on muscle and meat characteristics of broilers from experimental and commercial lines. Poult. Sci. 80:833–838. 24. Berri, C., M. Debut, E. Le Bihan-Duval, V. Sante´-Lhoutellier, N. Haj Hattab, N. Jehl, and M. J. Duclos. 2004. Technological quality of broiler breast meat in relation to muscle hypertrophy. Pages 93– 96 in 50th Int. Congr. Meat Sci. Technol., Helsinki, Finland. 25. Berri, C., E. Le Bihan-Duval, E. Bae´za, P. Chartrin, N. Millet, and T. Bordeau. 2005. Effect of selection for or against abdominal fatness on muscle and meat characteristics of broilers. Pages 266– 270 in Proc. XVIIth Eur. Symp. Qual. Poult. Meat; Doorwerth, the Netherlands. WPSA, Utrecht, the Netherlands. 26. Debut, M., C. Berri, E. Baeza, N. Sellier, C. Arnould, D. Guemene, N. Jehl, B. Boutten, Y. Jego, C. Beaumont, and E. Le Bihan-Duval. 2003. Variation of chicken technological meat quality in relation to genotype and preslaughter stress conditions. Poult. Sci. 82:1829–1838. 27. Debut, M., C. Berri, C. Arnould, D. Gue´mene´, V. Sante´Lhoutellier, N. Sellier, E. Bae´za, N. Jehl, Y. Je´go, C. Beaumont, and E. Le Bihan-Duval. 2005. Behavioural and physiological response of three chicken breeds to pre-slaughter shackling and acute heatstress. Br. Poult. Sci. 46:527–535. 28. Le Bihan-Duval, E., C. Berri, E. Baeza, N. Millet, and C. Beaumont. 2001. Estimation of the genetic parameters of meat characteristics and of their genetic correlations with growth and body composition in an experimental broiler line. Poult. Sci. 80:839–843. 29. Le Bihan-Duval, E., C. Berri, E. Baeza, V. Sante, T. Astruc, H. Remignon, G. Le Pottier, J. Bentley, C. Beaumont, and X. Fernandez. 2003. Genetic parameters of meat technological quality traits in a grand-parental commercial line of turkey. Genet. Sel. Evol. 35:623–635. 30. Debut, M., C. Berri, C. Arnould, D. Gue´mene´, V. Sante´, N. Sellier, E. Bae´za, N. Jehl, Y. Je´go, C. Beaumont, and E. Le BihanDuval. 2005. Parame`tres ge´ne´tiques des caracte´ristiques musculaires,
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1. Monin, G. 1988. Evolution post-mortem du tissu musculaire et conse´quences sur les qualite´s de la viande de porc. J. Rech. Porc. Fr. 20:201–214.
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112 des indicateurs de qualite´ de la viande et de re´ponse au stress dans une ligne´e lourde de poulet de chair. J. Rech. Avicole 6:524–528. 31. Aggrey, S. E., W. Carre´, X. Wang, F. Pitel, A. Vignal, E. Le Bihan-Duval, C. Beaumont, M. Duclos, T. E. Porter, J. Simon, and L. A. Cogburn. 2005. Mapping of quantitative trait loci for fatness and breast meat yield in a novel resource population of broiler chickens. Page 553 in Proc. Plant Anim. Genomes XIII Conf. http:// www.intl-pag.org/13/ 32. Lagarrigue, S., F. Pitel, W. Carre´, B. Abasht, P. Le Roy, A. Neau, Y. Amigues, M. Sourdioux, J. Simon, L. Cogburn, S. Aggrey, B. Leclercq, A. Vignal, and M. Douaire. 2006. Mapping quantitative trait loci affecting fatness and breast muscle weight in meat-type chicken lines divergently selected on abdominal fatness. Genet. Sel. Evol. 38:85–97.
33. Cogburn, L. A., X. Wang, W. Carre, L. Rejto, T. E. Porter, S. E. Aggrey, and J. Simon. 2003. Systems-wide chicken DNA microarrays, gene expression profiling, and discovery of functional genes. Poult. Sci. 82:939–951.
Acknowledgments The work was made possible thanks to financial support by the French ACTA and OFIVAL organizations. The contribution of numerous colleagues from INRA was essential, especially that of Martine Debut. The perspectives in functional genomics rely on a collaborative work with the University of Delaware (Neward), University of Maryland (College Park), and University of Georgia (Athens) under an IFAFS-USDA grant and a Genanimal/French Ministry of Research grant.
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Muscle Growth and Meat Quality M. J. Duclos,1 C. Berri, and E. Le Bihan-Duval Station de Recherches Avicoles, INRA, 37380 Nouzilly, France
Primary Audience: Broiler Breeders and Selectioners, Poultry Processors
Although poultry products are diverse, the general trend is for portioned and further-processed products to increase their market share. In this context, technological quality of poultry meat is an important aspect. It is largely determined by the acidification process of the meat postmortem. Defects in meat acidification have been described in poultry, and the purpose of this report was to evaluate whether they are linked to growth rate or stress susceptibility and whether they are under genetic control. Key words: meat quality, growth, genetics, stress, metabolism 2007 J. Appl. Poult. Res. 16:107–112
DESCRIPTION OF PROBLEM General Trends of the Poultry Meat Industry Due to genetic improvement and progress in nutrition and poultry management, meat-type chickens may exhibit very high growth rates and feed efficiency. At the same time, poultry production has been greatly diversified with the use of various genetic types reared in either intensive or extensive conditions. Although standard fast-growing birds are mainly used for portions or further-processed products, in some countries, alternative productions using intermediate or slow-growing genotypes are also developing. At the moment, these birds are mainly consumed as whole carcasses. Growth rates can be improved to reach an acceptable level of productivity and ensure a satisfying carcass and meat quality according to the specific targets of each production system. The quality of poultry products can be partitioned into several attributes, mainly the sensory (color, tenderness, flavor, juiciness) and the physical (muscle yield, water-holding capacity, 1
Corresponding author: [email protected]
cooking loss) attributes of chicken carcasses and meat, which vary with growth rate and body composition. The first concern is to produce a carcass of good quality, specifically, obtain a maximal meat yield with a limited fatness. Given the general trend for consuming more chicken parts and further-processed products, rather than whole carcass, an important aspect today is the technological quality of the meat. This is largely determined by postmortem metabolism through its effect on the color and the water-holding capacity. Biological Basis of Poultry Meat Quality Postmortem metabolism of the muscle tissue influences the characteristics of the meat [1]. In particular, the rate and the amplitude of acidification have a strong effect on both organoleptic and technological parameters of meat quality. After bleeding, cessation of O2 supply modifies muscular metabolism during the initiation of rigor mortis. The muscle relies on the anaerobic glycolytic pathway to use the i.m. glycogen stores for ATP regeneration, which leads to the
Downloaded from http://japr.oxfordjournals.org/ at University of Alberta on April 25, 2015
SUMMARY
108
RELATION BETWEEN GROWTH AND MEAT QUALITY Several studies have shown that selection for greater BW or muscle development has induced histological and biochemical modifications of the muscle tissue. Some results have been ob-
tained by comparing 2 genotypes divergently selected for high or low growth rates [21] or a genotype selected for higher breast muscle development and a control genotype [22]. In both models, increased breast muscle development was associated with a marked increase in muscle fiber size [21, 22]. Modifications in muscle fiber number would be difficult to quantify and therefore cannot be ruled out, but they must be modest. In the divergent selection for high or low growth rates, a 20% increase in muscle fiber number was measured in an indicator muscle, the anterior latissimus dorsi [21], and it was hypothesized that a similar increase could occur in the pectoralis muscle. In the experimental selection for increased breast meat yield, the difference in fiber size seemed to arise during the first 2 wk posthatch, when the growth of the breast muscles was maximal [22]. Alterations in muscle biochemistry have also been observed. The meat from the genotypes with greater muscle mass showed a decreased content of heminic pigments and a decreased level of glycogen stores [23]. As a result, those genotypes showed a paler breast meat (less redness and greater lightness) with higher pHu values. A more recent study was designed to relate breast muscle development, including muscle fiber size, to the postmortem metabolism and breast meat quality [24]. Phenotypic and genetic relationships between fiber and meat traits were estimated for a total of 600 commercial broilers. For all birds, muscle fiber cross-sectional area, glycolytic potential, lactate content, postmortem pH fall, and classical meat traits (color, drip and thawing-cooking loss, and Warner-Bratzler shear force) were measured. As the fiber size increased, both the glycogen reserve at death (glycolytic potential) and the postmortem glycolytic activity of muscle decreased. As a consequence, breast muscles with the largest fibers exhibited the highest pH15 and pHu. Therefore, breast muscles with the largest fibers exhibited a darker lightness value, lower drip and thawingcooking losses, and greater tenderness after cooking. Commercial selection relies on several characteristics, including leanness, which prompted us to examine whether this parameter could by itself influence muscle metabolism. We observed that lean chickens showed a compara-
Downloaded from http://japr.oxfordjournals.org/ at University of Alberta on April 25, 2015
accumulation of lactic acid and protons. Therefore, the acidification process depends upon the amount of glycogen stores (estimated by the glycolytic potential) and the rate of the glycolysis [1]. In the chicken, normal pH values at 15 min postmortem (pH15) are around 6.2 to 6.5 [2, 3], whereas normal ultimate pH (pHu) values are around 5.8 [4, 5]. If the pH15 value is low (below 6.0) when the muscle is still warm, the proteins are subject to denaturation [1], which leads to a decreased water-holding capacity and a decoloration of the meat. Such defects have been amply described in pigs, but also in turkeys [6, 7, 8, 9, 10] and chickens [5, 11]. These meats are often described as pale, soft, and exudative by analogy with those described in pigs. They show exudative water loss [8, 12, 13, 14] and a decreased technological yield [5, 14, 15]. Although raw pale, soft, and exudative meats show a rather soft texture, they tend to be less tender after cooking because of excessive exudation [12, 13, 14]. The results concerning the color of those meats are more controversial. Comparing carcasses with fast and normal rates of acidification, some authors have observed paler meat for fast glycolyser [8, 15], whereas others have observed no difference [14, 16]. Acid meats are characterized by a low ultimate pH (pHu < 5.7), which induces structural alterations in the muscle with an impairment of the technological processing ability. Artificially acidifying turkey meat [17] induces a destructuration of the myofibrillar network, which also induces a marked decrease in water-holding capacity. At the other extreme, meats with a high ultimate pH also show defects in their color, texture, and water-holding capacity. High pHu meat is dark, firm, and dry. These types of meats can occur in poultry [18, 19]; they show enhanced water-holding capacities, but an increased sensitivity to microbial development described in other species has not yet been observed in dark poultry meats [20].
JAPR: Symposium
DUCLOS ET AL.: POULTRY MEAT AND EGG QUALITY SYMPOSIUM
EFFECT OF STRESS-GENOTYPE INTERACTIONS ON MEAT QUALITY Studies conducted in different species indicate that stressors during transportation and at the slaughterhouse have a strong influence on meat quality. The fact that meat quality defects are sometimes described under commercial conditions but do not arise under less stressful experimental conditions also suggests this. We have, therefore, constructed experiments to have an insight on the effect of stressors on meat quality parameters of different chicken genotypes. A first study compared commercial fastgrowing and slow-growing broilers submitted to transport or acute heat stressors [26]. The data showed a muscle-specific effect of stressors, breast muscle being less sensitive than thigh muscle. It was, however, observed that the slowgrowing birds showed a higher level of struggle during shackling and an accelerated rate of postmortem glycolysis detrimental to the quality of their breast meat. In a second study, 2 fast-growing genotypes were considered, one showing a higher breast yield (heavy line) in comparison with a slow-growing genotype [3, 27]. The birds were handled with minimal stress, submitted to
a hanging stressor of 2 min, or submitted to 3.5 h of heat stressor followed by hanging. The measure of circulating corticosterone confirmed the additive effects of both stressful conditions. Observation of the birds during shackling with recording of vocalizations, wing-flapping duration, and attempts to right showed that the slowgrowing line was the most active, the heavy line was the least active, and the fast-growing line was intermediate. The meat from the slow-growing genotype was redder. The hanging stressor also induced a redder meat; it is therefore likely that the differences of activity explained partly the differences in meat color among the genotypes. Both genotypes and stressors altered postmortem acidification of the meat. The values of pH15 were higher for the heaviest birds in relation to their lower level of activity on the shackle line, although they decreased with the hanging duration. Heat stressor induced no further decrease. Within each genotype, we observed a strong negative correlation between pH15 and wing-flapping duration. The pHu values were lowest for the slow-growing genotype, highest for the standard fast-growing genotype, and intermediate for the heavy genotype, and they decreased following the heat plus hanging stressors. Within each genotype, the pHu was negatively correlated with the glycolytic potential (−0.42 to −0.66). Finally, the technological yield was mostly altered by stressors in the slowgrowing genotype, whereas it was almost unaltered in the other 2 genotypes.
GENETIC CONTROL OF POULTRY MEAT QUALITY As discussed above, the comparison of genotypes suggests that poultry meat quality traits are under genetic control. Heritability values and genetic correlations have first been measured in an experimental line of chickens selected for improved breast meat yield and decreased fat deposition [28]. All recorded parameters (pH15, pHu, luminance, redness, yellowness, and water loss) showed high heritability values ranging from 0.35 to 0.57. In this model, the highest genetic correlations were between pHu and luminance (−0.91), pHu and drip loss (−0.83), and luminance and drip loss (+0.81), in agreement with data obtained in other species like pigs. Notably, a rather high negative correlation be-
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tively lower level of glycogen stores than fat chickens, with decreased amplitude of acidification of the meat postmortem and decreased exudation [25]. This suggests that selection for increased muscle yields and against fat deposition could exert cumulative effects on muscle metabolism, decreasing glycogen storage and thereby reducing the amplitude of acidification postmortem. Finally, the water-holding capacity of the meat, and therefore its processing yield, was ameliorated. Together, these studies did not identify any antagonism between growth rate or muscle development and breast meat quality parameters such as water retention and processing ability. However, the effect of muscle structural changes on other product characteristics such as the sliceability remains to be investigated. More generally, mechanisms underlying the decrease of glycogen stores in intensively selected birds need to be elucidated and put in relation to the bird’s general metabolism and stress sensitivity.
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avoiding too high or too low of values. This parameter, however, requires slaughtering birds and therefore selecting on siblings, which is costly and of reduced efficiency. For this reason, we believe that there is a need for identifying molecular markers allowing for marker-assisted selection. Identifying molecular markers of meat quality traits requires resource populations with pedigree families. Divergently selected lines of broilers with high or low growth rates or with high or low adiposity at the same BW are adequate models. By crossing those lines 2 by 2, the alleles governing meat quality traits can be tracked in an F2 population. Such populations have been recently obtained, and preliminary analysis on the identification of quantitative trait loci governing growth and body composition have been reported [31, 32]. Simultaneously, microarrays are being used to compare global gene expression profiles [33] within each of the 2 sets of divergent genotypes (fast-growing compared with slow-growing and fat compared with lean chickens with the same BW). The combination of the results from both strategies is expected to identify double functional and positional candidates as a step toward the identification of the genes explaining the differences among genotypes. This strategy is being applied simultaneously to the 2 genetic models, which should permit identification of genes that are specific to 1 model or common to both for similar criteria (i.e., ideally, this should permit identification and discriminate between general mechanisms and other more specific to each “genetic background”). In the future, a major task will be to identify the mutation(s) underlying a favorable quantitative trait loci allele so that it can be used by the breeders in marker-assisted selection schemes.
CONCLUSIONS AND APPLICATIONS 1. We accumulated data showing that variations in growth rate among genotypes have an effect on meat quality parameters. This relation could result from alterations in muscle structure or metabolism. 2. We observed no evidence of any antagonism between growth rate or muscle development and breast meat quality parameters such as water retention and processing ability. 3. A part of the differences among genotypes is likely to result from the differences in their susceptibility to preslaughter stressors. The slowest-growing genotypes appear to be the most reactive, and the fastest-growing genotypes appear to be the least reactive.
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tween pHu and abdominal fat was also recorded (−0.54). By contrast, the correlations with pH15 were rather low, due to the relatively high values for this parameter observed under experimental conditions. The correlations between growth and meat quality traits were also rather low, suggesting that the genetic mechanisms underlying those traits could be different and further indicating that selection for growth rate or breast meat yield would not negatively alter meat quality. A comparable study was performed on turkeys [29], and similar data were obtained. In this last study, however, the heritability values were lower, possibly because the birds had been slaughtered in a commercial plant. More recently, a study was conducted on a commercial strain of heavy broilers [30], confirming the correlations previously observed in the experimental strain [28] and significant heritability values for meat parameters (from 0.25 to 0.35). Three additional traits, glycolytic potential, cooking loss, and toughness of cooked meat, were recorded that also showed high heritability values (from 0.34 to 0.43). Notably, the glycolytic potential was negatively correlated to the pHu, with a value close to −1. Luminance, exudate, cooking loss, and toughness were negatively correlated with pHu (−0.65 to −0.89), whereas only luminance and exudate were negatively correlated with pH15 (about −0.5). Again, no correlation was observed between pHu and pH15, suggesting a distinct genetic control of these 2 parameters. The genetic studies suggested that pHu was a highly heritable character that was highly correlated with several meat quality traits. It could therefore be considered as a good selection parameter to improve meat color, water-holding capacity, and texture. One strategy could be selecting for an optimal window of pHu values,
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4. Within a fast-growing genotype, we observed only low genetic correlations between growth and meat quality traits. As quality traits are highly heritable, this suggests a possibility to select for meat quality without altering growth rate. 5. The measure of meat quality parameters, however, requires slaughtering birds and therefore selecting on siblings, which is costly and of reduced efficiency. For this reason, we believe that there is a need for identifying molecular markers allowing for marker-assisted selection.
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Acknowledgments The work was made possible thanks to financial support by the French ACTA and OFIVAL organizations. The contribution of numerous colleagues from INRA was essential, especially that of Martine Debut. The perspectives in functional genomics rely on a collaborative work with the University of Delaware (Neward), University of Maryland (College Park), and University of Georgia (Athens) under an IFAFS-USDA grant and a Genanimal/French Ministry of Research grant.
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