**3. Existing and emerging muscle myopathies**

#### **3.1 Breast muscle myopathy**

Deep pectoral muscle myopathy, also known as green muscle disease and Oregon disease, was first identified in turkeys [76] and later in broiler breeders [77] and 7-week-old broiler chickens [78]. This disease affected the wing elevating muscle (*M. supracoracoideus* or pectoralis minor) and was characterized by the death of the muscle (tenders) but did not cause the death of the bird. Dead muscle decay, while the bird was still alive, resulted in the appearance of a yellowish-green color due to the breakdown of hemoglobin and myoglobin to bile salts; muscle myopathy could affect just one (unilateral) or both (bilateral) pectoralis minor muscles. Since affected tenders were located deep in the breast, this defect resulted in consumer complaints when the carcasses were sold as a whole.

The pectoralis minor muscle is confined in a tight space between the sternum and the pectoralis major muscle (large breast fillet). It is also encased in a rigid fibrous sheath that restricts any increase in muscle volume in response to any physiological changes caused by muscle exercise such as wing-flapping [79] which requires increased blood flow to supply the oxygen and nutrients needed by the muscles. The incidence of green muscle disease has also been reported to be higher in high yielding crosses, especially males.

On the other hand, the incidence of focal pectoral myopathy has increased, and it has been associated with increased growth rate and muscle size [12, 80]. Further investigation is required to determine the causes of this muscular defect since focal myopathy has an even more detrimental effect on the poultry industry. It has affected the pectoralis major muscle leading to consumer complaints and industry economic loss.

#### **3.2 Pale, soft, and exudative-like condition in poultry muscles**

The incidence of pale, soft, and exudative (PSE) meat has been well-documented in swine, where meat has a very light gray color, soft texture, and cannot hold water

#### *An Overview of Poultry Meat Quality and Myopathies DOI: http://dx.doi.org/10.5772/intechopen.104474*

[81, 82]. This condition has been associated with heavy muscling [83]. In poultry, similar PSE characteristics have been reported in turkey meat [84, 85], chickens [86, 87], and ostriches [88]. However, it is more difficult to distinguish and identify these characteristics in poultry meat compared to pork. This condition has been referred to as PSE since characteristics were similar to PSE in pork, which is misleading since both conditions were not exactly the same. Poultry researchers have preferred to refer to the condition in poultry as "PSE-like" or "Pale poultry muscle syndrome" [86, 89]. The PSE and PSE-like conditions are detrimental to the industry profitability since it affects important meat quality attributes involved in the production of value-added products and further processed meat. Affected muscles have been reported to lose their rheological properties and become unable to hold water. For example, mortadella prepared with PSE-like chicken meat has reduced water-holding capacity, altered texture, diminished emulsion stability, and required additives to restore the functional properties of normal meat [90]. In addition, poultry processors have been concerned with the appearance of PSE-like meat in fresh tray packs. The pale color affected color uniformity within the package and, thus, consumer acceptance. The occurrence of PSE-like in poultry meat has been believed to be the result of accelerated postmortem glycolysis (rapid pH decline), while the carcass was still warm [91]. In poultry, normal pH values at 15 min postmortem (pH15) are around 6.2–6.5 [92, 93], whereas normal ultimate pH (pHu) values are approximately 5.8 [60, 88, 94]. If the pH15 value is low (below 6.0) when the muscle is still warm, the proteins are subject to denaturation, which leads to a decreased water-holding capacity and a lighter color of the meat.

The reasons for PSE-like condition have remained unclear, but up to 30% of broiler breast meat and up to 40% of turkey breast meat have shown this defect in commercial processing plants [95–97]. Furthermore, it has been reported that the occurrence of PSE-like meat in birds may be affected by alteration to the intracellular calcium homeostasis caused by a mutation in the ryanodine receptor gene, which is different from the ryanodine receptor gene in swine, and also depends upon the several aspects of preslaughter and postslaughter management practices [98, 99]. It is thought that the application of "snow chilling" with carbon dioxide intensified meat quality abnormalities [100]. In addition, other factors have been thought to contribute to this problem, such as heat stress during the finisher period or the preslaughter period [86], and stress and struggling before slaughter [101].

Differentiating PSE-like meat from normal meat has been based on the instrumental or visual assessment of color lightness (L\*). However, the cutoff value for classifying meat as PSE-like has differed among researchers. Petracci et al. [102] considered an L\* value of 56 as the cutoff, while Barbut [28, 103, 104] suggested classifying turkey breast meat as PSE-like when L\* values were greater than 52 at 24 h postmortem. Fraqueza et al. [105] classified breast meat as PSE-like when the L\* was greater than 50 and pHu was less than 5.8, while Woelfel et al. [106] used L\* values greater than 54 in broilers as their standards.

Using L\* *per se* as an indicator of PSE-like condition has not been considered accurate and could be misleading because several factors influence poultry meat color. Feed ingredients used in poultry have been reported to change breast meat color (e.g., wheat-based versus corn-based diets). In addition, it has been shown that genetic selection for increased growth and breast meat yield resulted in a marked increase in muscle fiber size [107, 108] with a shift toward a greater proportion of white fibers (glycolytic) and reduced dark fibers (oxidative), which produced meat that appears

pale but still has a high pHu. Muscle thickness [48, 49] and color measurement position on the fillet [109] also affects color measurement. Therefore, color, pHu, and waterholding capacity should be considered when classifying poultry as PSE-like meat.

### **3.3 White striping, woody breast, and spaghetti meat**

White striping, woody breast, and spaghetti meat can be collectively referred to as the myopathies of modern broiler. These nomenclatures were simply based on the appearance of the defective muscles. White striping is a condition described in broiler chickens and characterized by white striations parallel to the direction of muscle fibers on both breast fillets and thighs of broilers. White striping is considered to be an emerging issue by the poultry meat industry that could be associated with enhanced growth rate and heavier body weight in birds [110–112], especially in the age of 6–8 weeks [110], and higher fat content in broiler breast fillets [111]. The incidence of white striping was evaluated under commercial conditions, and the overall incidence in broiler breast meat was 12.0%, of which 3.1% had severe striping [113]. It is possible that the intense selection for rapid growth rate in birds could have accidentally been accompanied by the selection for inadequate capillary/fascial growth or muscle fiber defects leading to myopathic changes referred to as growth-induced myopathy [13], under which these three different myopathies can be classified.

The precise etiology of white striping has not been defined yet [114]; however, several speculations have been reported. In turkeys, Wilson et al. [80] reported that rapid growth rate may have led to the limited ability of muscle support systems leading to a condition called focal myopathy, which affected the major pectoral muscle.

Ischemia could also result from a rapid growth rate and lead to muscular damage in turkeys [115]. It is also possible that reduced oxygen supply to breast muscle resulted from lower capillary density in fast-growing chickens [116]. A higher growth rate could also lead to defective cation regulation in muscles leading to an increased sodium, potassium, magnesium, and calcium in muscle tissue [117]. An increased level of calcium in muscle tissue could initiate several tissue changes, including the activation of intracellular proteases or lipases resulting in myopathic changes [13, 118–120]. Kuttappan et al. [114] reported that breast fillets showing severe white striping had reduced protein content and myopathic lesions, while Petracci et al. [113] observed poor cohesion beneath the striation area.

Poultry producers started noticing and complaining about woody breast in the late 1990s [12, 121]. The woody breast muscle is usually characterized by increased firmness in all or parts of the pectoralis major muscle that can start in the live birds and can be detected by palpating the breast muscle. Sihvo et al. [121] reported that woody breast might result from fibrosis, which leads to an accumulation of interstitial connective tissue. This myopathy affects consumer acceptability and meat quality; even when trying to mitigate by diverting to further processed poultry products, woody breast meat is still required to be mixed with normal meat to maintain the quality of the further processed product [122, 123].

Spaghetti meat, or previously known as mushy breast, is the most recent emerging myopathy of breast meat in poultry. As the name implies, the breast muscle loses its structure and firmness. One distinct feature the spaghetti meat has that would differentiate it from white striping and woody breast is the loss of endomysial and perimysial connective tissue that compromises the fiber bundles cohesion, coupled with a loose connective tissue deposition [124] leading to the separation of the fascicles into "spaghetti" strings.

*An Overview of Poultry Meat Quality and Myopathies DOI: http://dx.doi.org/10.5772/intechopen.104474*

Sanden et al. [23] investigated the collagen of muscles with either woody breast or spaghetti meat abnormalities. They showed that collagen in woody breast muscle was a mix of thin and thick fibers, whereas spaghetti meat had thinner, fewer, and shorter. However, both myopathies generally resulted in a higher content in connective tissue (mainly in perimysium) compared to normal muscle.

Several researchers have investigated these myopathies to understand their etiology and effect on meat products quality [114, 121, 124, 125]. It is believed that cellular stress and hypoxia (ischemia) caused by muscle hypertrophy are the main triggering factors behind white striping and woody breast, in addition to being strapped within a relatively rigid connective tissue that limits the hypertrophy capabilities. However, what is interesting is that spaghetti meat, where the opposite issue is faced concerning connective tissue, started appearing. It is possible that geneticist, while trying to reduce the rigidness of the connective tissue, led to the emergence of the most recent abnormality of spaghetti meat, which is worth investigating in the future with poultry strain companies.

### **4. Nutrition and muscle myopathies**

Researchers have investigated multiple factors that may have either contributed or helped in eliminating the emerging myopathies starting at different incubation conditions [126] all the way to management during growing [127, 128] and nutritional manipulations [129–133].

Several white muscle defects and myopathy have been reported. According to the literature, these problems spiked in the 1970s and 2000s concurrent with increased feed prices. It was suggested that producers were driven to use less expensive feed and use alternative feed ingredients (e.g., DDGS) to control costs. One significant consequence of feeding less expensive feed was that the essential amino acids (e.g., lysine and methionine) became a primary concern when formulating these diets, while the nonessential amino acids (e.g., arginine, glycine, and proline) were neglected despite their essential role in connective tissue formation, which may have contributed to the emerging of muscle defects as genetics for enhanced growth and muscle accretion were improved even further.

The spectacular advancements in genetics witnessed by the broiler industry have resulted in broilers with a higher growth rate, while the role of nutrition has become even more critical in supporting the increased growth demands of what may have become a relatively fragile animal. Profit-driven decisions about formulating feed in a least-cost manner while neglecting the essentiality of nonessential amino acids in nutrition would eventually be evidenced by increased condemnation at the processing plant and increased consumer complaints.

*Broiler Industry*
