3rd International Poultry Meat Congress

efficiency associated with an increased rate of post mortem pH decline [2, 3]. Broiler breast
muscle in which the rate of pH decline is sub-optimal are characterised by a dark, firm and
dry (DFD) muscle that is associated with atypical colour, flavour, texture and decreased shelf
life [4, 5]. A physiological myopathy in broiler muscle was identified in the 1990s [6, 7] that
was consistent with these changes. Mitchell and colleagues have characterised some of the
physiological mechanisms underlying these changes [8, 9]. MacRae et al. [10] argued that as
muscle fibre diameter was substantially larger in broiler than in layer muscle, and in broilers the
fibre diameter in breast muscle was larger than in leg muscle, that the myopathy might be related
to the lack of efficient diffusion of nutrients from the vascular system leading to subsequent
muscle cell damage. Other evidence supports the conclusion that short term or long term prior
stress increases the incidence respectively of both PSE-like and DFD broiler muscle [11].
More recently, there have been reports that a proportion of commercial broiler carcasses exhibit
white striping, characterised by white parallel striations in the direction of the muscle fibres
[12], or “wooden breast muscle” [13], conditions that lead, in some cases, to the condemnation
of severely affected carcasses. In some cases, white striping has resulted in the rejection of a
significant proportion of carcass after veterinary inspection at the slaughter plant: in one large
study of 28,000 breast fillets randomly chosen from 56 broiler flocks, over 12% of samples were
affected and showed similar characteristics to PSE meat [12].Pathological studies have shown
that muscle abnormalities in samples affected by white striping are similar to those previously
observed in birds at ambient temperatures that are exacerbated under conditions of heat stress [7]
and lead to PSE-like muscle. These include degeneration, necrosis and regeneration of muscle
fibres, angular myofibres, the presence of inflammatory cells, eosinophilic strands of collagen
and thick cords of adipose tissue separating myofibres and muscle fascicles and [14, 15]. The
breast muscle of broiler chickens is lighter and less red and yellow compared with traditional
and layer lines (Table 2) However, scores for haemorrhages were higher and creatine kinase
activity (CK), a marker of muscle cell damage, was elevated in the broiler chickens. Consistent
with the elevated CK activity as an indicator of muscle cell damage, the concentrations of Na,
K, Mg and Ca in plasma and muscle ash were higher in broilers compared with traditional and
layer lines of chickens [16]. Furthermore both initial and final breast muscle pH were lower in
broilers, and the decline in pH was greater, compared with the other two groups. Nevertheless the
meat was more tender, requiring less shear force to cut the cooked meat, and taste panel scores
were higher for texture, flavour and overall acceptability of meat from broilers compared with
layers and traditional breeds [1, 16]. The differences in breast muscle function between broilers
and layers (the latter presumably reflecting the wild type of breast muscle) remain if evaluated
at the same age or body weight and are exacerbated by heat stress [17]. Taken together these
results indicate that the normal physiological functions of broiler muscle cells are compromised
leading to substantial decrements in the quality of broiler meat. This raises the question as to
whether these differences are inherent in genetic selection for greater body weight and breast
muscle yield or if they are merely circumstantial. This question can be answered at least partly
by determining genetic parameters (heritabilites and genetic correlations) and by the results of
selection experiments.
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