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Author: CHRISTINE ALVARADO - Food Science and Technology, Virginia Tech (Courtesy of Alltech Inc.)
Publication date: 02/26/2007
With genetic selection for rapid growth and a shift from whole bird processing to further processed products, turkey processors have observed an increase in the number of meat quality problems.
Specifically, processors have observed an increase in turkey products such as formed breast loaves and rolls with poor water holding capacity, pale color, poor texture, and poor cohesiveness. Meat with these pale, soft, and exudative characteristics is referred to as PSE meat.
Products made with pale, soft, and exudative meat are unacceptable not only to producers but also to consumers who object to the pale color and increased drip loss found in packages (Ferket and Foegeding, 1994). Pale, soft, and exudative meat increases the amount of purge in cook-in bags, which results in reduced cook yield, poor meat binding, and a soft product texture with poor sliceability.
These products create economic losses for processors in reduced yield, wasted materials, extra labor required to repackage these products, and customer dissatisfaction.
The development of PSE meat is directly related to biochemical changes occurring in the muscle during rigor mortis development. Birds with PSE meat are normally characterized by a rapid postmortem metabolism resulting in a rapid pH decline early postmortem. This low pH combined with high carcass temperatures early postmortem may cause protein denaturation resulting in PSElike characteristics. Many studies in pork have found that antemortem stress factors and postmortem processing conditions can alter postmortem biochemical changes in the muscle and can be associated with the development of PSE meat.
Meat quality characteristics
The inability of the muscle cells to rid themselves of metabolic by-products such as lactic acid causes several metabolic and structural changes within the muscle, the most important of which is a decrease in pH (Judge et al., 1989). This decrease in pH is the most significant postmortem change and can affect important meat quality attributes such as color, water holding capacity, and texture (Ferket and Foegeding, 1994; Pearson, 1994). These three attributes are used to evaluate meat quality by both processors and consumers. Two conditions caused by the rate of pH decline are dark, firm, and dry (DFD) and pale, soft and exudative (PSE) meat.
Dark, firm and dry meat develops when muscle glycogen is depleted prior to slaughter resulting in high muscle pH from reduced postmortem glycolysis (Hedrick et al., 1989; Gregory, 1994; Lawrie, 1998).
Dark firm and dry meat is characterized as having a high water holding capacity, even though the meat appears dry (Pearson and Young, 1989).
Pale soft and exudative meat is characterized as having a pale color, soft texture when cooked, and a low water holding capacity. This condition develops due to accelerated postmortem metabolism, which leads to a rapid decline in postmortem muscle pH (Pearson and Young, 1989; Hedrick et al., 1989; Lawrie 1998). Rapid postmortem glycolysis has been observed in both pig and turkey meat in which pH of PSE muscles was below 5.8 within 45 min postmortem in pigs and 15 min postmortem in turkeys, compared to normal muscle pH at this postmortem time of greater than 6.0 (Enfalt et al., 1993; Rathgeber et al., 1999).
This rapid decline in pH occurring while muscle temperatures are still elevated denatures proteins and causes a decrease in solubility and enzymatic activity (Briskey and Wismer-Pedersen, 1961; Penny, 1969). Muscles with pH values below 6.0 at 45 min postmortem had a higher ATPase activity indicating a more rapid glycolytic rate and were found to produce poor meat quality (Wismer- Pedersen, 1959). Penny (1967) reported that porcine muscle kept at 37 °C for 6 hrs postmortem resulted in a rapid pH decline to below 5.8 and a loss of half of Mg-ATPase activity compared to muscles incubated at 34 °C.
COLOR
Color is an important quality attribute as consumers are often willing to pay more for poultry products based on color. Several factors influence meat color including pH, myoglobin concentration, nitrites, and oxidation state of the heme iron (Hedrick et al., 1989; Fletcher, 1999). Even though myoglobin has been shown to be important in determining color, there is no difference in myoglobin concentration in PSE, DFD, and normal pork meat (Pearson and Young, 1989).
Swatland (1993) found that color was most associated with pH decline and light scattering properties. The degree of lightness of the meat depends upon the amount of light scattered and absorbed; pale meat has an increased amount of scattered light and a decreased absorption. In PSE meat, light does not penetrate far into the meat prior to being scattered; therefore, the meat appears pale. In DFD meat, light scatters a small extent and the meat appears darker (Offer et al., 1989).
In addition, myofibrils are birefringent, meaning they have two refractive indices (Hedrick et al., 1989). When viewed with polarized light, the isotropic band is singly refractive and the anisotropic band is doubly refractive. The path differences are strongly related to changes in pH, with a lower pH causing greater path differences and an increased scattering of light (Swatland, 1993). Therefore, both the scattering at the myofibrillar surface and the scattering of light through the myofibril are pH dependent and can cause the pale color observed in PSE meat.
In addition to an increase in light scattering, the amount of extracellular water can affect color. In PSE meat, there is more extracellular water than intracellular water causing less light to be absorbed and more light to be reflected (Swatland, 1993; Lawrie, 1998). On the other hand, DFD meat contains more intracellular water and thus absorbs more light than is reflected. This increased absorption of light gives the meat a darker appearance.
WATER HOLDING CAPACITY
Striated muscle contains approximately 75% water, which exists as three forms in the muscle; bound, immobilized and free. Since water is a polar compound, it can bind with other water molecules as well as charged protein groups. Bound or constitutional water is less than 1% of muscle water and is located within the protein molecules. This water has such a strong protein-water interaction that it cannot be lost except during ashing of the meat. The second form of water is immobilized or interfacial water, which is approximately 10-15% of water in meat. Immobilized water is attracted to the bound water layer creating multilayers of water each more loosely bound as the distance from the bound water increases. Due to the strong waterwater and water-protein interactions, immobilized water is typically lost with cooking. Free water accounts for the remaining portion of water in meat tissues and is associated with the extracellular space. Free water is held loosely through capillary forces and can be lost easily through mechanical actions such as cutting, grinding, cooking, and storage of meat.
Water binding capacity of meat is altered in two ways, by the net charge effect and the steric effect. As a polar molecule, water can bind to other charged amino acid side groups. The amount and type of charge on a protein change with pH. As pH decreases, the number of reactive groups available for water binding decreases to a minimum, called the isoelectric point or pI (Hedrick et al., 1989).
Since actin and myosin are the most predominant proteins in the muscle and since they are most responsible for water holding capacity, their pI is the pH at which water holding capacity is least. The pI of muscle tissue is approximately 5.1 where the number of positive charges equals the number of negative charges resulting in a net zero charge (Bendall, 1964). Since the ultimate pH of PSE meat is lower and closer to the pI than normal meat, water holding capacity is decreased due to a reduction in charge.
The steric effect, or degree of contraction, has the greatest influence on the water holding capacity of meat. As the amount of space between the muscle protein structures decreases, less space is available for water around the proteins. Contractile state and pH of the muscle can influence the amount of interstitial space. If the muscle is in a contracted state, there is less space within the muscle to hold water intracellularly due to the shortening of the sarcomeres (Lawrie, 1998). Therefore, water is expelled into the extracellular space.
Muscle pH can also affect the amount of space available to bind water molecules. If repulsion between charged groups is increased, as occurs in higher pH muscle, the protein network is enlarged and provides an increased amount of water holding capacity. In PSE meat, when the pH is closer to the pI of the myofibrillar proteins, attraction between charged groups increases and the protein network shrinks. Therefore, part of the immobilized water becomes free water, which may be lost as drip (Hamm, 1986).
Myofibrillar proteins such as myosin, the predominant muscle protein, have a large amount of water holding capacity. Any changes to this protein can decrease the ability of the muscle to retain water. Denaturation of myosin has been found to decrease water-holding capacity (Penny, 1969; Starbursvik et al, 1984; Offer, 1991). Offer (1991) suggested that the myosin head shrinks from 19 nm in normal muscle to 17 nm in PSE muscle. This shrinkage of the protein lattice causes expulsion of intracellular water into the extracellular space and decreases water-holding capacity. The extent of myosin shrinkage is also dependent upon the rate of pH decline, ultimate pH, and temperature during chilling with the faster rate of decline and higher temperatures early postmortem leading to increased denaturation and shrinkage (Offer, 1991).
Moisture retention: new regulations
A recent change in the US poultry industry has occurred with the new regulation regarding moisture absorption and retention. The Food Safety Inspection Service (FSIS) has issued a final rule requiring that plants produce poultry products with either no retained water or only the minimum amount required to meet food safety requirements.
FSIS requires that poultry meat be chilled to 4ºC within 4 hrs of slaughter of broilers and within 8 hrs for turkeys. In order to accomplish this decrease in meat temperature, carcasses are placed in an immersion chiller or ice water slush for approximately 80 min. During immersion chilling, the carcasses move through a continuous chilling system, with increasingly colder and cleaner water.
There is also an air bubble agitation system, which is used to decrease the thermal layering and cools the carcass at a faster rate. This current/counter current flow method of chilling increases water uptake, increases heat exchange, and provides a method of cleaning the bird.
The new rule addresses both consumer issues and inconsistencies in meat processing. Consumer groups have long argued that retained water in poultry meat and subsequent leakage from the meat into packages could lead to spillage and contamination in the home. This increased contamination could result in an increase in foodborne illnesses from Salmonella and Campylobacter. The inconsistencies in meat processing have focused on the different methods of chilling carcasses. Beef, pork and lamb carcasses are air-chilled, not immersion-chilled, so water is not retained in meat products. Actually, water (moisture) is reduced due to shrinking that occurs with air chilling. Therefore, the red meat industry has argued that poultry meat is adulterated because water has been added to increase its bulk weight and therefore consumers are paying for water along with meat. Their reasoning is that if water uptake is not regulated to be at a minimum, then the product is not only adulterated but misbranded. Therefore, FSIS has proposed the final rule to help make the livestock and the poultry industries more consistent.
The final rule requires that retained water be held to a minimum in poultry meat and that the final product be labeled with the percentage of net weight water retained. The product would require the labeling “may contain up to _______ percent retained water” or “contains _________ percent retained water”. Poultry plants will have to bear the burden of in-plant testing to determine the amount of water retained in each product and change the labels to reflect this percentage. Therefore, total cost to the poultry industry will be an estimated $1.5 million to establish the limits and another $18.4 million to revise the labeling. This does not include the cost of reducing water retention, which could cost the industry an estimated $100 million.
Previously, processors have been allowed a maximum absorbed water of 8% for whole birds and 12% for cut-up parts. However, these numbers do not represent the amount of retained water following drip loss. Therefore, the cost figures for reducing retained water are estimated based on an average 5% retained water in broilers and 4% retained water in turkeys.
Most poultry plants currently limit water uptake and retention in products so the impact of the new ruling will be less than in plants that maximize the water retention in products. Either way, there will be a significant cost to the industry to reduce water retention. From a consumer viewpoint, there will probably not be a change in purchasing of poultry products with the revised labeling. The poultry industry is so highly competitive that it would be very difficult to raise the price of a product due to this new requirement and expect consumers to pay the difference. Therefore, this final rule is not expected to cause a difference in purchasing of poultry meat or meat products.
Pale, soft, and exudative meat
Pale, soft, and exudative poultry meat is a growing problem for the turkey industry due to the increase in further processed products (Owens et al., 1998; Woelfel et al., 1998). This PSE condition is characterized as meat with a pale color, soft texture and watery appearance. It is estimated that up to 41% of turkey meat and 37% of broiler meat could have PSE-like characteristics (Owens et al., 1998; Woelfel et al., 1998). Meat with PSE-like characteristics is unappealing to consumers as well as processors. It has been estimated that 60% of consumers purchase meat based on appearance of the product (Lee and Choi, 1999). For processors, PSE meat is undesirable not only because of the pale color, but also due to decreased yield from excess drip, increased cooking losses, reduced juiciness and poor binding ability. The use of PSE meat in further processed products can cost a processor approximately $2-4 million per year in lost meat yield alone (Owens et al., 1998; Woelfel, et al., 1998). This figure does not include the money lost due to added labor or materials needed for repackaging.
THE RYANODINE RECEPTOR AND CALCIUM RELEASE
In order for muscle contraction to occur, a change in voltage must be sensed by the transverse tubule, the change in membrane voltage must be transmitted to biological changes, and calcium must be released from the sarcoplasmic reticulum (Louis et al., 1993). The dihydropyridine receptor is located within the transverse tubule and is responsible for sensing voltage changes during contraction (Lamb, 1992). The sarcoplasmic protein involved in calcium release is termed the ryanodine receptor (Lai et al., 1988).
This homotetramer (560,000 MW) radiates from the sarcoplasmic membrane and is adjacent to the dihydropyridine receptor (MacLennen and Phillips, 1992). There are two isoforms of this protein in the avian species, α and β (Percival et al., 1994). A defect in this ryanodine receptor has been lined to Porcine Stress Syndrome (PSS) and expression of PSE-like characteristics. In pork, a single point mutation of cysteine instead of arginine in position 615 has been shown to cause an abnormal amount of calcium release during stress (Fuji et al., 1991).
Currently, research is being conducted to determine if this same mutation exists in avian species. During contraction of normal muscle, the sarcolemma is depolarized following an action potential. The action potential travels to the sarcoplasmic reticulum through the t-tubule system.
The depolarization of the sarcolemma causes the ryanodine receptor to open, releasing calcium into the cell allowing contraction to occur. Following contraction, calcium is re-sequestered into the sarcoplasmic reticulum by ATPase, the ryanodine receptor is closed and relaxation occurs. In muscle with mutated ryanodine receptors, the calcium channel stays open longer allowing more calcium to be released than can be re-sequestered by the calcium-activated ATPase (MacLennan and Phillips, 1992). Also, Louis et al. (1992) found that not only does PSS muscle have a more rapid release of calcium from the sarcoplasmic reticulum but also four times the amount of calcium was required to close the calcium release channel. They also found that calcium release in normal muscle was inhibited at pH 6.9 (physiological pH) compared to pH 6.6 in PSS muscle. The consequence of continued levels of elevated calcium in PSS muscle is continued muscle contraction, increased glycogenolysis, accelerated muscle metabolism, heat production, decreased pH and increased lactic acid production, all of which can result in PSE meat.
It must be noted, however, that both genetic and environmental factors can contribute to the development of PSE meat. It has been estimated that only 4.5 % of the PSE problem in pork is due to a mutated ryanodine receptor (Pommier and Houde, 1993). In pork, molecular genetic tests are being used to selectively remove this mutated gene from the population. However, this is not possible yet in avian species since there are two isoforms of this gene and the mutation has not been found. Therefore, much of the recent research has focused on not only antemortem conditions, but also processing conditions to reduce the incidence of PSE meat in poultry.
ANTEMORTEM FACTORS
There are several antemortem and postmortem causes of PSE meat. In turkeys, genetic selection for improved feed conversion and rapid breast muscle growth has led to faster growth of the muscle fibers as compared to the supportive connective tissue (Swatland, 1990; Sosnicki and Wilson, 1991).
This rapid growth and sedentary growing conditions can also lead to a decrease in the ratio of capillaries to muscle fibers resulting in a greater accumulation of lactic acid. Since there are fewer capillaries present, the muscle cannot rid itself of the lactic acid, causing muscle acidosis (Addis, 1986; Sosnicki and Wilson, 1991). This condition can lead to a faster rate of pH decline early postmortem prior to carcass chilling which can lead to protein denaturation. Sante et al. (1995) reported a 1.4 fold faster rate of pH decline in fast growing turkeys as compared to turkeys with a normal growth rate.
Heat and transportation stresses are other antemortem factors that can lead to the development of PSE meat. Stress susceptible pigs that were subjected to heat stress at 42-45 °C for 20 to 60 min prior to slaughter developed PSE-like characteristics (Sayre et al., 1963). Northcutt (1994) found similar results when chickens were exposed to temperatures of 40-41 °C for 1 hr.
McKee and Sams (1998) found that seasonal heat stress accelerated postmortem metabolism and causes PSE-like characteristics in turkeys. During the growth cycle of the bird, heavier birds respond to heat stress more severely than lighter weight birds and can develop PSE meat (Bohren et al., 1982).
McCurdy et al. (1996) evaluated the effect of season on the incidence of PSE in turkeys and reported the highest L* values in the summer season and the lowest in the winter. This suggests that the incidence of PSE turkey meat should increase during the hot summer months, which is consistent with industry reports.
Transportation has been extensively studied in both pigs and poultry as an antemortem stressor. Short durations of transportation stress can result in a rapid decline in postmortem pH due to accelerated glycolysis; and long durations of stress can result in a depletion of muscle glycogen stores that can result in DFD meat. It is well documented through physiological responses that animals become stressed during transportation to the slaughter facility. Kannan et al. (1998) reported significantly higher corticosterone levels in broilers transported for 3 hrs immediately before processing as compared to those transported but allowed to rest for 3 hrs prior to slaughter. This recovery agrees with Warriss et al. (1992), who concluded that at least 1 hr was needed for pigs to recover from transport stress. Gregory (1994) reviewed that the deaths of broilers during transportation increased at a non-linear accelerated rate as journey time increased. Mortality was also reported to be greater during hot weather. Sosnicki and Wilson (1991) found that birds exposed to transportation immediately prior to slaughter had a faster rate of pH and ATP decline and a higher lactic acid concentration at 90 min postmortem. However, Owens and Sams (2000) reported that transporting turkeys for 3 hrs prior to slaughter did not result in poor meat quality. The differences in meat quality observed in these studies could have been due to different transportation times, resting periods after transportation, temperature, and conditions immediately pre-slaughter.
POSTMORTEM TEMPERATURE
Postmortem temperature is an important processing factor involved in determining meat quality (Lee et al., 1979). The development of PSE is caused by protein denaturation resulting from a rapid rate of pH decline while carcass temperatures are still elevated (Penny, 1969). The elevated carcass temperatures may have a more damaging effect when meat pH is below 6.2 (Khan and Frey, 1971).
Offer (1991) and Fernandez et al. (1994) reported that carcasses with a normal pH decline may develop PSE meat if improperly chilled. Rapid chilling decreases the rate of postmortem glycolysis and pH decline, which can reduce protein denaturation. In pork, elevated postmortem temperature of 37 °C always resulted in PSE meat (Bendall and Wismer-Pedersen, 1962). When considering temperature induced PSE in pork, only a slight decrease in protein solubility was found in temperatures below 34 °C, whereas temperatures from 34 to 37 °C caused a significant loss of protein solubility (Penny, 1967). McKee and Sams (1998) reported that holding turkey carcasses at elevated temperatures of 40 °C will result in accelerated postmortem metabolism, a more rapid pH decline, and development of PSE meat.
PROTEIN DENATURATION AND PSE MEAT
Meat exhibiting PSE characteristics has a higher drip loss and paler color resulting from protein denaturation (Bendall and Wismer-Pedersen, 1962).
The insolubility of these denatured proteins is a result of aggregate formation. It is generally agreed that the pale color of PSE meat is due not only to the denaturation of the sarcoplasmic proteins, but also to the closer packing of the myofilaments at a lower pH. Both of these result in light scattering causing the muscle to appear more pale than normal.
The cause of decreased drip loss and poor binding ability in PSE meat is still under investigation. Sayre et al. (1963) reported that the solubility of sarcoplasmic proteins as compared to myofibrillar proteins may be better indicators of poor muscle quality. It has been further reported that precipitation of sarcoplasmic proteins may cause the decrease in drip loss associated with PSE meat (Bendall and Wismer-Pedersen, 1962). Pietrzak et al. (1997) noted that an SDS band at 90,000 D from an extract of sarcoplasmic proteins was larger in normal meat as compared to PSE samples. When comparing these to the myofibrillar samples, the 90,000 D protein in question was found to have increased.
Through Western Blotting techniques, the protein was found to be phosphorylase, a sarcoplasmic protein involved in the breakdown of glycogen to glucose for energy production. However, Pietrzak et al. (1997) found that phosphorylase undergoes low pH/ high temperature alterations and is precipitated only at the Z line and the central part of the A band. Therefore, according to this theory, phosphorylase precipitation is only partially responsible for reduced myosin extractability.
Another theory explaining reduced myosin extractability is that a more contracted state within the muscle causes thick and thin filaments to be closer (Irving et al., 1989; Offer, 1991). Thus, salt and phosphate may be less effective in penetrating the myofibrils to extract the salt soluble proteins in PSE pork. As muscle enters into rigor, myosin combines with actin to form an actomyosin complex that may protect myosin against the decrease in solubility usually found in PSE meat. Also, Offer (1991) reported that the myosin head retracted from 19 to 17 nm in PSE pork possibly decreasing the potential for extraction.
Need for research
Pale, soft, and exudative meat is a growing problem in the poultry industry due to the increase in further processed products. Products made with PSE meat have a decreased yield due to increased drip loss and poor binding ability. Since adding value through these further processed products is a means of increasing profit margins for companies, the problem of PSE meat is of great concern for processors.
Even though there is a genetic factor involved in PSE development in pork, the mutation has not yet been found in poultry and, if found, the genetic solution may involve several years to complete.
Therefore, processing factors leading to a solution or reduction in this condition may be a more expedient solution. There has been extensive research in pigs relating processing conditions to PSE meat, but very little in turkeys.
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Author: CHRISTINE ALVARADO
Food Science and Technology, Virginia Tech, Blacksburg, VA, USA
Author: CHRISTINE ALVARADO - Food Science and Technology, Virginia Tech (Courtesy of Alltech Inc.)
Publication date: 02/26/2007
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