Stunning and stress
Proper handling of animals and birds prior to slaughter has a tremendous impact on the quality of meat obtained and nowadays there is much research and investment into abbatoir design and construction. Even the input of specialists such as animal psychiatrists is vital to establish a well functioning and efficient slaughtering process. The fact of the matter is that, if the functional quality of the meat is destroyed during the slaughtering and cooling process, neither the use of an additive nor processing with modern equipment can remedy this damage afterwards.
Stunning with the help of carbon dioxide (C02) is the preferred method nowadays for knocking out pigs, because the pigs lose consciousness gently and, in comparison with electrical stunning, much less internal muscle bleeding occurs and fewer bones are broken. The pigs are placed in a cage and lowered into a chamber where the concentration of carbon dioxide gradually increases and remains at a high concentration for a certain period of time.
Stunning pigs using a gas such as argon is an even more gentle way to knock out pigs and an improved quality of meat could be obtained using this method. However, stunning pigs with argon is in its infancy and further research as well as testing is required. It is also usually more expensive to use argon than carbon dioxide. To avoid acute ’stress’ before slaughtering, gentle showers, such as a fine mist, are sprayed over the pigs to keep them cool and waiting areas are of light blue or light green colour, which also calms pigs down. In addition, when pigs have to walk to the stunning area on their own feet, they prefer to walk slightly uphill, and they walk faster if they are walking towards a well-lit area; so abbatoirs often are designed with this in mind. Research continues to find better and more efficient ways to improve both meat quality as well as slaughtering efficiency.
Selective breeding and especially over breeding in pigs nowadays has produced significantly larger amounts of muscle meat in breeds such as Pietrain and German Landrace, and others, compared with 20 years ago. The aim of selective breeding is to maximize growth of the high value cuts of a pig such as the loin and the leg. Muscle tissue needs to be supported by much oxygen and the skeleton as well as the size of the heart from pigs has stayed the same over the same period of time while the amount of muscle tissue increased dramatically. As a result, overbred animals are very sensitive to stress or stressful situations.
Pale soft exudative meat, red soft exudative meat and dry firm dark meat
Pale soft exudative (PSE) meat is the term used to describe a defective type of meat, seen predominantly in pork, but also in poultry. Unfortunately it is high-value cuts, such as loin and leg meat, that are predominantly affected and it is the degree of the PSE condition of meat which is crucial, as the borderline between PSE and non-PSE meat is not clearly defined.
The combination of two factors, namely a low pH value shortly after slaughtering and a high temperature within meat (above 37 °C) at the same time, are the cause of the PSE condition. In acute stressful situations prior to slaughter, pigs produce lactic acid from glycogen anaerobically whilst they are still alive and they breathe heavily to form the large amounts of ATP required. The formation of lactic acid whilst the animal is still alive, in conjunction with fast post-mortem glycolysis, causes a rapid drop in pH. This results in an abnormally low pH value in muscle tissue shortly after slaughter, contributing to the formation of PSE meat. Electrical stunning of pigs most commonly speeds up post-mortem glycolysis and therefore also contributes to the formation of PSE meat. Stressful conditions prior to slaughter also cause a rise in the pigs’ temperature and the temperature of meat remains high after slaughter. PSE-susceptible pigs can exhibit body temperatures of around 42 °C if heavily stressed shortly before slaughter, whereas 37 °C is the normal body temperature. A halothane test is frequently applied in order to determine PSE susceptibility in pigs. Halothane-positive pigs, commonly Pietrain and Landrace, demonstrate great tendency to produce PSE meat when the animal is not handled properly prior to slaughter.
PSE meat is usually of pale colour, wet in appearance and very soft in texture and the PSE condition is caused by partially denatured proteins. Denatured proteins cannot hold, or bind, muscular water as well as fully native proteins. More specifically, the length of the myosin filament is reduced by around 8-10% during this process of denaturation and the WHC of meat (the capacity of meat itself to retain, or hold, its own tissue water) is greatly reduced as a result. The reduced WHC explains the fact that PSE meat appears to be ‘wetter’ than normal pork meat. In actual fact, the level of water within PSE pork is in most cases the same as in normal pork, but the WHC is reduced owing to the smaller quantity of native protein in the meat. Firmness of PSE pork is also reduced in comparison with normal pork owing to partial denaturation of proteins; denatured proteins exhibit a change in their three-dimensional structure, which leads to a less firm structure overall.
The light, or lighter, colour of PSE pork is explained by the small myofibrillar volume in the muscle tissue. Muscle tissue with a small myofibrillar volume has a high light-scattering ability; so light is reflected differently in PSE meat from normal pork. Light is unable to penetrate into the meat and so becomes scattered right on the surface; the myoglobin cannot absorb the light thus making the meat appear pale. It is an interesting fact that the colour of PSE pork is generally lighter even though the content of myoglobin in PSE pork is in most cases the same as in normal pork. The small myofibrillar volume causes the open-meat structure of PSE meat as denatured proteins shrink, thus resulting in larger gaps between the individual fibres. To some degree, myoglobin is also denatured in PSE meat, and denatured myoglobin does not contribute to the formation of curing colour in cured meat products any longer.
PSE pork can be checked 45 min (pH45) or 60 min (pHx) after slaughtering by checking the pH value in the muscle of the loin (Musculus longissimus dorsi). If pH45 is at 6.0, or pHx is at 5.8, PSE meat is obtained. When pHx is below 5.8, severe PSE pork is the result. Checking the pH value after 24 h, once the rigor mortis in pork is completed and the meat is well chilled, does not give any indication towards PSE, given the fact that in PSE pork as well as in normal pork the final pH values are more or less the same.
Chilling pork carcasses quickly, commonly in blast chillers straight after slaughtering, helps to minimize the severity of PSE. If meat with a high pH value is chilled quickly after slaughter, the meat proteins are not damaged as much as they would be if the meat were chilled slowly. During such fast chilling, a temperature in the meat of 32-35 °C is commonly reached within 90 min. Despite the possibility of obtaining a slight degree of cold shortening during ‘fast’ chilling, a slight degree of cold shortening is of less economical disadvantage than obtaining PSE meat.
Red soft exudative (RSE) meat is another term used to describe meat that has a quality defect. RSE pork has the same characteristics as PSE pork, except that it preserves the natural red colour of meat better than PSE pork, possibly because the carcass was chilled quickly after slaughter. Light is not scattered as severely as it is under ‘more severe’ PSE conditions and so the meat appears redder in colour despite the fact that a similar amount of protein is denatured.
Neither PSE nor RSE meat offers technological advantages within the manufacture of meat products. Because of the reduced level of native proteins, WHC as well as water binding-capacity (WBC) are reduced and the ‘lighter’ colour is not of any benefit either. It is also important to emphasize that once proteins are denatured, as they are in PSE pork, no additive can make up for this shortcoming afterwards during the production of meat products and the non-functional proteins cannot be turned back into their native state. The level of PSE pork varies from country to country and some countries have as little as 2-4% PSE pork whilst others obtain 25-A0% of PSE calculated from the total number of pigs slaughtered.
Dry from dark (DFD) meat is the term used for another type of defective meat; DFD meat is also known as ‘dark-cutting meat’. DFD characteristics in meat can be seen predominantly in beef as well as lamb; however, some pigs nowadays also exhibit DFD character.
Contrary to pigs, which produce lactic acid out of glycogen in an anaerobic way if exposed to ’stress’ prior to slaughter, animals such as cattle, deer or lamb utilize glycogen in an aerobic way if stressed before slaughter. They simply burn energy (glycogen) under stressful situations for the formation of ATP and no lactic acid is obtained while the animal is still alive. As a result, insignificant amounts of glycogen are left in the muscle at the point of slaughter and no, or very little, lactic acid can be produced post-slaughter during rigor mortis. This results in an insufficient decline in pH value within meat after slaughter and upon completion of rigor mortis, by beef after around 24-36 h, the pH value in meat is still around 6.0-6.2. This phenomenon is also sometimes called ‘incomplete rigor mortis’ as proper and sufficient acidification within meat never takes place post mortem and a small number of cross-links between actin and myosin are established. The small number of cross-links explains the high solubility of DFD meat; the protein molecules are far less tightly bound together than they are in PSE meat or meat that has undergone a ‘normal’ rigor mortis. DFD meat demonstrates a ‘closed’ fibre structure once rigor mortis is completed and only small gaps are present between the muscle fibres actin and myosin.
Contrary to PSE meat, where the pH value has to be checked 45 min or 1 h after slaughter, DFD meat can be detected upon completion of rigor mortis, in beef after around 24-36 h post-slaughter. If at this point in time the pH value is at (or above) 6.0, DFD meat is obtained. DFD meat appears dark in colour owing to the ‘closed’ fibre structure (small gaps between actin and myosin) and has a slight slimy/tacky appearance, which is not microbiological sliminess caused by high numbers of bacteria. When cutting steaks of DFD meat, butchers use the phrase ‘the meat does not come off the knife’ to describe their character.
From a technological view point, DFD meat has the advantage of high protein solubility, as acidification during rigor mortis never really took place and an actomyosin complex was only obtained to a small degree. The WHC of DFD meat is also excellent; the high pH value correlates with a high WHC as the pH value of DFD meat is a long way from the IEP (pH value of 5.2). On the other hand, because of insufficient acidification of muscle tissue during rigor mortis, the shelf life of DFD meat is dramatically shortened as nearly all types of bacterium find favourable conditions for growth at elevated pH levels. Hence, the high pH value is an obstacle for the development of curing colour in cured meat products produced out of beef.
In poultry, both PSE and DFD character can be found. Poultry generally enter rigor mortis very quickly and post-mortem glycolysis seems to take place more rapidly in white muscles, such as breast, compared with red muscles from the leg.
