Exploring the Factors Contributing to the High Ultimate pH of Broiler Pectoralis Major Muscles Affected by Wooden Breast Condition

Abstract

The elevated ultimate pH (pH _u ) found in wooden breast (WB) meat suggests an altered muscular energetic status in WB but also could be related to a prematurely terminated post-mortem pH decline. The aims of this study were to explore the factors contributing to the elevated pH _u and establish whether the occurrence of WB defect alters muscle post-mortem carbohydrate metabolism and determine if the contractile apparatus reflects such changes. A total of 24 carcasses from Ross 308 male chickens were obtained from a commercial producer and harvested using commercial processing procedures. Carcasses were categorized into unaffected (NORM) and WB groups (n = 12 each), and samples were collected from cranial bone-in pectoralis major (PM) muscles at 15 min and 24 h post-mortem for the determination of pH, glycolytic metabolites, adenonucleotides, buffering capacity, phosphofructokinase (PFK) activity, and in vitro pH decline. Twenty-four additional deboned PM samples (12 NORM and 12 WB) were collected from the same processing plant to assess muscle histology and sarcomere length at four different locations throughout the PM muscle. Data show that the reduced glycolytic potential of WB muscles only partially explains the higher (P < 0.001) pH _u of WB meat, as residual glycogen along with unaltered PFK activity suggests that neither glycogen nor a deficiency of PFK is responsible for arresting glycolysis prematurely. The dramatic reduction in ATP concentrations in the early post-mortem period suggests a defective ATP-generating pathway that might be responsible for the reduced pH decline in WB samples. Further, the addition of excess of ATPase extended post-mortem glycolysis of WB meat in an in vitro glycolytic system. WB-affected samples have longer (P < 0.001) sarcomeres compared to NORM, indicating the existence of compromised energy-generating pathways in myopathic muscles that may have had consequences on the muscle contraction and tension development, as in vivo, also during the post-mortem period. Considering the overall reduced glycolytic potential and the myodegenerative processes associated with WB condition, we speculate that the higher pH _u of WB meat might be the outcome of a drastically impaired energy-generating pathway combined with a deficiency and/or a dysfunction of muscle ATPases, having consequences also on muscle fiber contraction degree.

Keywords: glycolysis; pH; post-mortem metabolism; sarcomere length; wooden breast.

FIGURE 1

Schematic diagram of pectoralis major muscle sampling location for the evaluation of muscle histology and sarcomere length (AV, anteroventral; AD, anterodorsal; PV, posteroventral; PD, posterodorsal).

FIGURE 2

Average pH of unaffected (NORM) and wooden breast (WB) broiler pectoralis major muscles (n = 12/group) at 15 and 1,440 min post-mortem. Data represent means ± SEM. Asterisks indicate significant difference within the time point (***P < 0.001).

FIGURE 3

Average lactate (A, μmol/g), glycogen (B, μmol/g), glucose (C, μmol/g), and glucose-6-phosphate (D, μmol/g) of unaffected (NORM) and wooden breast (WB) broiler pectoralis major muscles (n = 12/group) at 15 and 1,440 min post-mortem. Data represent means ± SEM. Asterisks indicate a significant difference within the time point (***P < 0.001; **P < 0.01).

FIGURE 4

Glycolytic potential (μmol lactate/g meat) of unaffected (NORM) and wooden breast (WB) broiler pectoralis major muscles (n = 12/group). Data represent means ± SEM. Asterisks indicate a significant difference between experimental groups (***P < 0.001).

FIGURE 5

Average phosphofructokinase (PFK) activity [nmol nicotinamide adenine dinucleotide (NADH) * min^{– 1} * g^–1] of unaffected (NORM) and wooden breast (WB) pectoralis major muscles (n = 6/group). Data represent means ± SEM.

FIGURE 6

Average ATP (A, μmol/g), ADP (B, μmol/g), AMP (C, μmol/g), and IMP (D, μmol/g) of unaffected (NORM) and wooden breast (WB) broiler pectoralis major muscles (n = 12/group) at 15 and 1,440 min post-mortem. Data represent means ± SEM. Asterisks indicate a significant difference within the time point (***P < 0.001; *P < 0.05). BD, below limit of detection.

FIGURE 7

Buffering capacity (μmol H⁺⋅pH^–1⋅g^–1) (pH range 6.4–7.0) in unaffected (NORM) and wooden breast (WB) pectoralis major muscles (n = 12/group). Data represent means ± SEM. Asterisks indicate a significant difference within the same pH value (***P < 0.001).

FIGURE 8

Average pH of the in vitro model (n = 6/group). Data represent means ± SEM. (a–c) Means lacking a common letter significantly differ within a time point (P < 0.05).

FIGURE 9

Representative images of wooden breast (WB)-affected samples collected from anteroventral (AV), anterodorsal (AD), posteroventral (PV), and posterodorsal (PD) positions of the pectoralis major muscle. * inflammatory cells infiltrate; F, proliferation of connective tissue; L, fat tissue deposition; NF, necrotic fiber; FL, fiber lysis; PS, perimysial spacing; ES, endomysial spacing; G, giant hypertrophic fiber. Scale bar = 150 μm.

FIGURE 10

Average sarcomere length (μm) assessed in the anteroventral region of unaffected (NORM) and wooden breast (WB)-affected pectoralis major muscles (n = 6/group). Data represent means ± SEM. ***P < 0.001.

FIGURE 11

Average sarcomere length (μm) assessed in the anteroventral (AV), anterodorsal (AD), posteroventral (PV), and posterodorsal (PD) positions of pectoralis major muscles affected by WB myopathy (n = 6/position). Data represent means ± SEM. (a–c) Means lacking a common letter significantly differ (P < 0.05).