Causes of PSE and DFD Meats and Treatments
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What is PSE & DFD meat? The terms pale, soft and exudative PSE and dark, firm and dry DFD describe two undesired quality characteristics that can be exhibited in the meat from most species of meat producing animals. However the PSE condition is predominantly found in pigs whilst the DFD condition occurs across all species. Within this essay the incidence rates of both PSE /DFD, the causative factors, consequences and commercial factors as well as prevention shall be considered. Pictures of the two conditions can be seen in fig 1.
These undesirable conditions are very important when considering both functional and specified meat quality. They are significant factors that contribute to the economic, social and environmental wellbeing of the entire sector and encompass wide ranging issues affecting meat production, profits, yields, animal welfare considerations, processing, customer perception, consumption, food safety, spoilage, waste and levels of carbon dioxide production.
Both conditions affect the colour of meat, as it can be considered abnormally pale or dark relative to what is considered normal. Colour is one of the major discriminatory factors in a buyer's selection, excessively pale or dark meat is unlikely to be purchased. The texture of the meat with PSE and DFD is also affected detrimentally leading to unacceptable levels of softness or firmness. Undesired changes in water holding capacity of the meat occur, leading to problems in processing and the commercial yields of products, as well as detrimental effects on the sensory attributes of meat such as juiciness or succulence. There is thought to be a link between DFD and a lack of flavour in a cooked product. "The development of both PSE and DFD meat can be associated with earlier development of rigormortis"  .
Products may subjectively exhibit one or more of the descriptive characteristics e.g. too pale or too dark, too soft or too firm, too exudative or too dry relative to the characteristics of normal meat. Normal acidification of muscles after an animal is slaughtered occurs through glycolysis. Glycogen and creatine phosphate are depleted. Glycogen conversion to lactic acid is described here.
C6H10O5 +H2Oïƒ 2C3H6O3
There is no oxygen for aerobic metabolic processes to occur. The circulatory system can no longer remove lactic acid and it accumulates within the muscle. The abnormal conditions of PSE and DFD are linked to abnormally high or low acidity, thus the pH value of the meat can be used to more objectively describe the conditions at different times after slaughter (table 1). However these values can be different in different muscles within a carcass and different packaging methods may affect pH values. Accurate pH recordings are also particularly difficult with various methods having various problems such as lack of homogeneity of sample, fat smearing, dilution of sample etc.
"Using objective indices of PSE and DFD tends to produce higher estimates of the prevalence of the conditions in a population than using subjective assessment."  Subjective methods of gauging DFD / PSE using people may be less sensitive. People may only be able to judge the extremes of the conditions.
There appears to be an increase in the levels of incidence of PSE and DFD. Within the United Kingdom one quarter of pigs may show evidence of PSE and one tenth DFD. "The cost of PSE to the American pork industry was estimated at 30 million dollars in 1992, that of DFD pork 0.2 million dollars..."  estimated at 16% and 10% of total United States production respectively. This gives an indication of the economic significance of the problems.
The levels of glycogen available in the muscle prior to slaughter will determine the ultimate pH. These levels can be changed through stress, theses stresses prior to slaughter are the main factors involved in causing the PSE and DFD conditions. Although there are genetic, muscle composition and processing factors that can be instrumental, these shall be discussed later and these ultimately link to abnormalities in post mortem acidification of the muscles anyway, the effects of long term and short term stress shall be considered now. "It is well known that the handling of meat animals prior to slaughter is not only important from a welfare point of view but also affects the quality of the resulting meat"  . It has also been known for many years that hunted animals keep less well than those kept in relative calm conditions.
With PSE the cause appears to be acute (or short term) stress to the animal prior to slaughter. This acute stress leads to rapid acidification as glycogen breaks down quickly after slaughter to give lactic acid, resulting in a low ultimate pH. If carcass temperatures are high e.g. within deep muscle regions of the carcass, or rates of cooling are not rapid enough, conditions can develop that allow denaturing of the proteins within these muscles. The muscles are characterised by having lower water holding capacity due to the myofibrillar components of the muscles shrinking. This expels fluids into the space between the muscle fibres. When cut the meat will exude or lose this fluid, this is called drip. Excessive drip represents a loss in total yield but is also unsightly in shelf ready packaging and less likely to be purchased by a potential consumer.
The paler appearance of PSE meat is likely due to the different refractive index of the myofibrils and the sarcoplasm. The reduction in the size of the myofilament lattice increases the light reflected from the meat, less light is absorbed by the meat, more is reflected and crucially blood pigments like myoglobin absorb less green light making the meat look more yellow as opposed to red. Also the low ultimate pH promotes oxidation of myoglobin and oxyglobin to metmyoglobin which makes the meat appear browner rather than red or purple. Again this discoloration makes the product less likely to be purchased at point of sale
DFD is caused by chronic (or long term) stress to the animal. Glycogen levels are depleted prior to death, meaning that less glycogen is converted to lactic acid after slaughter and the ultimate pH of the meat remains high. Being closer to pH neutral there are significant issues with potential spoilage organisms and food borne pathogens as well as the quality issues discussed here. With a high ultimate pH there is less denaturing of the proteins leading to increased binding of the fluids and less exudates or drip giving a firm dry appearance.
The lattice of myofilaments which shrank in the case of PSE does not in the case of DFD. This means that the affects of the refractive index differences of the myofibrils and sarcoplasm are reduced. More light is absorbed with less reflected leading to a darker colour. The tightly packed structure with less extracellular space between the fibres means that less oxidation of the myoglobin can occur, surface oxidation only may occur as oxygen cannot permeate the structure, this leads to a thin translucent / red outside layer with the reduced purple myoglobin pigment predominantly showing through from underneath this thin layer.
The stress factors to consider are many fold but are worth at this stage categorising as acute and chronic in terms fear, pain and physical stress and their potential affect on the PSE DFD conditions. These might include noise, temperature variations, fasting or starvation, overcrowding or being put with animals of different social groups. Practical causes of short term stress might include reaction to goading, striking, restraining systems or conveyors, long term stressors associated with DFD might include long transport journeys and being exposed to other social groups of animals for periods of time. Young Bulls and veal calves have been linked to higher incident rates of DFD perhaps due to storage conditions and their fractious nature when exposed to other social groups, respectively. Different species are known to show different sensitivities to different stress factors, for example sheep are known to be less sensitive to noise than pigs.
With pigs in particular there appears to be a genetic link to them being susceptible to stress, this is sometimes known as porcine stress syndrome. It exists as a double recessive gene that when apparent as an abnormal homozygote can be exposed and witnessed as a reaction to Halothane. Typically pigs with this double recessive gene react by becoming rigid and tense as opposed to the usual symptoms of anaesthesia through halothane "In pigs with this mutation, Ca2+ is released from the sarcoplasmic reticulum at a rate that is equivalent to twice that of normal"  Glycogen conversion to lactic acid happens much more readily and there are higher incidence of PSE amongst this genetic grouping. Measures to breed this characteristic out of pigs have been tried, it is thought that selective breeding for confirmation and fat level may have brought about this genetic mutation. The Halothane test does not work on the heterozygote parent and DNA tests are required to identify parents with the mutation to try and breed this sensitivity out.
Another gene has been identified in certain strains of the Hampshire breed of pigs, known as the RN- gene. It is thought that this mutation increases the glycogen content of the white muscles that contain a higher number of glycolytic fibres, again resulting in post mortem rapid acidification leading to a lower pHu upon death leading to the PSE condition.
There are other breed effects which can be demonstrated by a comparative study undertaken (Table 2) where traditional breeds were compared against modern commercially important breeds and an extreme of muscularity, this also seems to demonstrate a correlation between lighter more exudative muscles in those bred for confirmation or muscularity. In the search for modern breeds of the desired confirmation, stress susceptibility may have also been bred in.
Even within the defined subjective and objective norms there are variations of colour, texture, pH and water holding capacity within different muscles of the carcasses of all species. The muscles affected by a particular stressor may be specific to a region of the anatomy rather than throughout the entire carcass. A particularly undesirable effect can be seen in the two tone appearance of meat, where the PSE, DFD and normal conditions can be exhibited in the extreme within a single carcass. An animal that is susceptible to PSE may exhibit DFD characteristics in those muscles that have been subject to long term stressors and thus have glycogen depletion prior to death. However some of the other muscles e.g. m. longissimus dorsi, within the same pig that are less likely to have glycogen depletion may exhibit the PSE condition. Processing methods can also have an effect, carcass cooling regimes that don't achieve deep muscle temperatures that are cold enough or at the wrong cooling rates can contribute to the conditions required to denature the proteins and exhibit the PSE condition. Howard and Lawrie (1956) found that "...the rate of pH fall post mortem was inversely proportional to the tenderness of the meat on subsequent cooking"  Animal handling systems, shearing washing, crushing etc will have a direct effect on the levels of glycogen within the muscle systems and so ultimately the levels of PSE DFD occurring.
Being able to measure stress indicators and indices of PSE DFD is critical if there is to be an understanding of the prevention of stress and therefore a reduction in the incidence of PSE and DFD in meat. Measurements of the level blood lactic acid, levels of creatine kinase and the electrical characteristics of pork through electrical impedance can give good objective indicators. Levels of cortisol, creatine kinase, pH and colour characteristics measured through online light reflectance spectrophotometrey can give objective PSE DFD measurements. Signs of stress in the animals can give ante mortem indications. These might be obvious in the case of fallen or injured stock but may also include levels of vocalisation, mounting, biting etc as less obvious indicators.
Identifying an reducing these conditions is the key in improving functional and specified meat quality, most of the work undertaken to reduce the levels of PSE and DFD is involved in improving welfare conditions of animals in the short and longer term leading up to slaughter. It is a difficult process to reduce stressors, even with very careful handling the animals are subject to a degree of stress. Keeping handling to a minimum is the ideal situation, carefully controlling transport, design of transport to prevent loading and unloading stress, training and certification schemes to improve the skills and knowledge of animal handlers. With pigs it would be better to avoid breeding the susceptible genotypes although as seen there appears to be a direct correlation between improved confirmation and musculature and the stress susceptibility mutation. Keeping animals, especially young bulls in their own social groups would seem to be a logical preventative measure but can be very difficult to achieve.
Physical measures like cooling pigs with water sprays and covering the pens of young bulls may help stop such stress behaviour as fighting and mounting, adding supplements to feed and watering systems prior to slaughter in order to try and replenish glycogen levels may be one way in order to reduce the number of incidence of PSE , DFD as may using tranquilisers and muscle relaxants, however there are issues of potential residues in meat as well as negatively effecting the quality expectations of the consumer by treating an animal in this way.
In conclusion to reduce the levels of PSE DFD in meat of all species a combined approach of improving welfare conditions, reducing fear, physical stress and pain, training and education staff, as well as online monitoring and feedback to key stakeholders is required. Carcass cooling rates should be closely controlled. There is a need to develop new methods of objective measurement, both as soon as possible after slaughter and at point of sale, emerging technologies such as near infra red spectrophotometrey, nuclear magnetic resonance, developments in immunoassay techniques and genetic markers may help us identify and prevent the causes of PSE and DFD. Linking the relationship of animal welfare and profit is very important, Traditionally it has been thought that increasing welfare means increasing costs, for example in stocking densities of transport. In seeing that increasing welfare conditions could actually be a profitable activity, aiding reduced quality complaints, increased yields, better sales and less waste there is an opportunity to have large positive impact on the well being of the whole meat sector. "...there are few, if any, figures comparing the overall economics of these alternatives. However it seems the net effect of greater care could often be greater profitability"  .
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