The influence of pre- and post-slaughter factors on pork quality

A complex interaction of animal factors, pre-slaughter management of the pig and post-slaughter carcass processing can have a significant influence on pork quality. Pork quality attributes are dependent upon the concentration of glycogen in the muscle at slaughter and the rate and extent of post-slaughter muscle glycolysis. Acute stress just prior to slaughter can lead to rapid rate of glycolysis and therefore rapid accumulation of lactate and a low muscle pH and pale, soft, exudative (PSE) pork. On the other hand low muscle glycogen at slaughter can lead to insufficient lactic acid formation, high muscle pH and dark, firm, dry (DFD) pork. Major pork quality defects such as PSE and DFD have resulted in large economic losses for the Australian pork industry, where PSE pork alone has been estimated to cost the industry $24 million per year. This thesis comprises a review of the literature related to the biochemistry involved, particularly muscle glycogenolysis, in the conversion of muscle to meat and the influence of various animal, pre- and post-slaughter factors on pork quality. The experimental work comprises of six experiments which examine the relationships between on-farm management, pre-slaughter management and post-slaughter processing and their influence on muscle physiology and glycogenolysis and ultimate pork quality. 1. Determination of muscle glycogen and lactic acid concentrations in muscle samples removed from the live pig using a shot-biopsy procedure. A total of 14 male crossbred finisher pigs were used in this experiment. The shot-biopsy procedure was used to remove muscle samples from the live pig. The Longissimus thoracis and Biceps femoris muscle had similar glycogen (13.7 and 12.5mg/g wet weight respectively) and lactic acid (1.3 and 1.9mg/g wet weight respectively) concentrations. By determining resting muscle glycogen and lactic acid concentrations in the live pig, the results can be used as an indicator of the extent of glycogenolysis that occurs in the muscle pre-slaughter and in comparing the influence of pre-slaughter factors such as handling and mixing of unfamiliar pigs on muscle glycogen concentrations at slaughter. 2. The effect of on farm and pre-slaughter handling on pork quality. Thirty-six male crossbred pigs were used in a 2x2 factorial design which consisted of 2 on-farm handling treatments (positive and negative handling) and 2 abattoir handling treatments (minimal and negative handling just prior to slaughter). Pigs negatively handled on-farm had lower muscle glycogen concentrations at slaughter, but there was no difference in muscle paleness or % drip loss compared to pigs positively handled on-farm. Pigs negatively handled at the abattoir had lower muscle glycogen concentrations at slaughter, but muscle paleness, % drip loss and the incidence of PSE were similar compared to pigs minimally handled at the abattoir. The results from this experiment indicate that pigs negatively handled on-farm have the potential to respond adversely to handling at the abattoir and hence may result in inferior pork quality. 3. The effect of mixing pigs during lairage and negative handling just prior to slaughter on pork quality. Forty-eight male crossbred pigs were used in a 2x2 factorial design which consisted of 2 lairage treatments (unmixed pigs during lairage and pigs mixed during lairage) and 2 abattoir handling treatments (minimal and negative handling just prior to slaughter). Mixing of pigs during lairage induced a high level of fighting among pigs and hence the pigs had lower muscle glycogen concentrations at slaughter, lower % drip, darker pork, a higher incidence of DFD and a lower incidence of PSE pork compared with unmixed pigs kept together during lairage. Negative handling just prior to slaughter did not affect muscle glycogen concentrations at slaughter or pork quality indicators in pigs that were unmixed or mixed during lairage. The unmixed pigs had a higher incidence of PSE irrespective of the handling treatment just prior to slaughter. This experiment has demonstrated that mixing of unfamiliar pigs during lairage can have a major influencer on pork quality, increase the incidence of DFD pork and reduce the incidence of PSE . 4. The effect of negative handling pre-slaughter and delays in carcass processing post-slaughter on pork quality. Forty-eight male crossbred pigs were used in a 2x2 factorial design which consisted of 2 abattoir handling treatments (minimal and negative handling just prior to slaughter) and 2 carcass processing durations (normal - 45 min and delayed - 70 min processing ). Pigs negatively handled prior to slaughter had lower muscle glycogen concentrations at slaughter, higher surface exudate and higher incidence of PSE pork compared to pigs minimally handled just prior to slaughter. Delays in carcass processing resulted in paler pork in the leg although the effect was minimal. The results from this experiment suggest that the quality of preslaughter handling had a greater effect on the incidence of PSE compared to delays in carcass processing. 5. The effect of dietary magnesium aspartate on pork quality. Forty-eight male crossbred pigs were used in a 2x2 factorial design which consisted of 2 diet treatments (control diet and diet supplemented with magnesium aspartate (MgAsp)) and 2 abattoir handling treatments. Pigs fed the MgAsp supplemented diet had higher plasma magnesium concentrations and lower plasma noradrenaline concentrations at slaughter compared to pigs fed the control diet. Pigs fed the control diet also had higher % drip loss, paler pork and a higher incidence of PSE compared to pigs fed the MgAsp supplemented diet. Dietary MgAsp supplementation reduced the incidence of PSE to zero irrespective of the handling treatment prior to slaughter. Pigs fed the control diet and negatively handled at the abattoir had lower muscle glycogen at slaughter and higher % drip loss compared to pigs fed the control diet and minimally handled prior to slaughter. The results from this experiment indicate that dietary MgAsp supplementation is effective in reducing the effects of stress, improving pork quality and reducing the incidence of PSE pork. 6. The effect of different dietary magnesium supplements on pork quality. Fortyeight male crossbred pigs were used in this experiment and consisted of 4 diet treatments (control diet, magnesium aspartate (MgAsp) supplemented diet, magnesium sulphate (MgSO4) supplemented diet and magnesium chloride (MgC12) supplemented diet. The MgAsp and MgSO4 but not the MgC12 supplemented diet increased plasma magnesium concentrations in comparison with the control diet however no reduction in plasma catecholamine concentrations were observed. Pigs fed the magnesium diets (MgAsp, MgSO4 and MgCl2) had higher muscle glycogen concentrations at slaughter and lower % drip loss in comparison with pigs fed the control diet. The results further highlight the benefit of dietary magnesium supplementation in improving pork quality. Thus MgSO4 and MgC12, which are cheaper and more readily available sources of magnesium can be as effective as MgAsp in improving pork quality. 7. The effect of dose and duration of dietary magnesium aspartate supplementation on plasma metabolites in pigs. Sixteen crossbred males which were cannulated via the cephalic vein prior to the start of the supplementation period were used in this experiment. The pigs were allocated to one of four diets (control diet, 20g MgAsp, 40g MgAsp and 60g MgAsp respectively). Pigs fed the magnesium diets exhibited increases in plasma magnesium concentration in the first two days. Also the highest increases in plasma magnesium concentrations corresponded to the pigs fed the lowest magnesium supplemented diet. The lowest magnesium dose maintained plasma concentrations throughout the supplementation period whereas the other diets declined to pre-treatment concentrations. The results hence indicate, that a lower dose of dietary magnesium for a shorter duration is more appropriate to maintain increases in plasma magnesium concentration. The results in this thesis demonstrate the importance of positive on-farm handling, management of pigs during the lairage period, handling just prior to slaughter and carcass processing to consistently produce quality pork. The propensity to develop PSE may be already evident prior to transport from the farm, however, results indicate that the factor which determines whether the pork carcass will be PSE is the nature of handling the pigs receives in the last few minutes prior to slaughter. Finally the results from this thesis highlight the effectiveness of dietary magnesium supplementation in improving pork quality and reducing the incidence of PSE pork