Pale, soft, exudative meat, or PSE meat, describes a carcass quality condition known to occur in pork, beef, and poultry. It is characterized by an abnormal color, consistency, and water holding capacity, making the meat dry and unattractive to consumers. The condition is believed to be caused by abnormal muscle metabolism following slaughter, due to an altered rate of glycolysis and a low pH within the muscle fibers. Genetic predispositions and stress levels prior to slaughter are known to affect the incidence of PSE meat.
Normally, calcium ions are used by the body to activate muscles, composed of myofibril cells. Ca2+ is transported out of the sarcoplasmic reticulum surrounding muscle fiber cells, and leads to metabolism and contractions of the myofibers. Under PSE conditions, twice the amount of Ca2+ is released post-mortem, which causes excessive muscle glycolysis, and the buildup of lactic acid. This lactate accumulates in the postmortem muscle, and leads to a very low pH.
As the pH drops, proteins in the myofibers are denatured, leading to abnormal cell structure and function. The result is a pale tissue color, and a soft, almost mushy texture. The muscle cells shrink, and less water is held within the cell membrane. Subsequently, the myofibers will continue to lose water content as the meat is cooled and stored, leading to excessive drip loss.
Pigs susceptible to porcine stress syndrome, or PSS, have an increased likelihood of developing PSE meat. These animals become easily stressed pre-slaughter, which leads to exaggerated glycolysis, an increase in body temperature, and higher production of lactic acid. In particular, the Halothane gene, Hal, induces PSS in swine. It is a single point mutation in this gene that causes abnormal calcium channels within the muscle. Hal+ pigs are five times more likely to develop PSE meat than Hal- hogs.
The incidence of PSE in poultry meat is believed to have increased over the past several decades because of the incredible advancements in growth rates. Intense breeding selection for breast size and feed efficiency is likely responsible for the increase in meat quality issues. Conditions behind the PSE poultry meat are believed to be the same as observed in pork; higher rates of glycolysis postmortem lead to a sudden pH drop, which in turn causes protein denaturation and a loss of function.
Acute stress immediately prior to slaughter may result in the abnormal Ca2+ diffusion seen in PSE postmortem muscle. This in turn will induce the increase in glycolysis and cause the decline in pH. Stressful conditions may include handling, transportation, loading and unloading from a truck, mixing with unfamiliar animals and individuals, entering an unfamiliar facility, and stunning.
It has also been suggested that excessive heat during summer months results in higher incidences of meat quality problems.
For these reasons, animal welfare often correlates to the incidence of PSE meat or other carcass quality issues. The length of transport, time period between loading and unloading, and the rest time in lairage are known to affect meat quality. Physical activity and psychological stress associated with transportation, as well as incidence of fighting between individuals in lairage, can confound these factors.
Hogs susceptible to porcine stress syndrome (PSS) commonly develop PSE meat postmortem.
Within the swine industry, two separate genes have found to influence the incidence of PSE meat conditions. The first gene discovered was named the Halothane gene, Hal, because researchers noticed that pigs with this specific genotype developed PSE meat after being anesthetized with the halothane drug. However, the Hal gene is only responsible for about 25-35% of the PSE meat processed at abattoirs. Another gene, RN- is likely responsible for the majority of PSE pork problems. The RN- gene increases the glycogen content of the muscle, resulting in lower pH. The poultry industry is still currently trying to identify and eliminate genes that may be responsible for predisposing birds to PSE meat.
Advancements in swine and poultry breeding can result in decreases in PSE meat. Selecting hogs with a genotype lacking the Hal and RN- genes should help eliminate the problem. Breeding advancements have led to fewer marketed pigs having genotypes containing these two genes, so that PSE rates are decreasing. Using microbiology techniques, it is also possible to "knockout" these genes, to reduce the likelihood of carcass quality problems. In addition, breeding animals not susceptible to porcine stress syndrome is highly beneficial. Poultry breeding companies can lower meat qualities issues, including PSE, by selecting for health and cardiovascular wellness in addition to yield and efficiency.
Since stress is highly correlated to carcass quality, reducing causes of stress during the slaughter process can drastically improve meat quality. In addition, calmer animals will have a lower temperature, which will help eliminate protein denaturation. Eliminating wing flapping, vocalizations, and excessive movement as animals move from the farm into the abbatoir can help reduce PSE. Lower meat pH's are correlated with the length and roughness of transportation. Stress levels are also known to decline as human handling decreases. Therefore, automatic loaders and unloaders may be useful for the poultry industry. Following transportation and unloading, animals should be given an extensive rest period before stunning (referred to as lairage). Stunning methods can play a large role in meat quality. Current research suggests that a two phase gas chamber may be best for minimizing stress 
Abbatoir chilling conditions
By quickly chilling pork and poultry meat, in order to bring the muscle temperature down to an acceptable level, will reduce myofibril glycolysis and stop muscle metabolism. Slower chilling results in a lower pH, lighter colored meat, and greater yield losses after cooking.
Meat quality issues cost the pork industry $200 million annually in the US. Of this total, the incidence of PSE meat accounts for a decline in profits by approximately $0.90 per pig.
In 2005, the incidence of PSE meat on the market in the United States was found to be 3.4%, significantly less than during a similar survey three years earlier. However, the range of PSE incidence ranges from 0.1% to 10% throughout the industry.
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