Postmortem muscle metabolism and pork quality under different chilling regimes

The conversion of muscles to meat after an animal is exsanguinated involves a series of biochemical and biophysical changes that are initiated at the time of death. The physiology of the animal and postmortem handling of the carcass have a controlling influence on the metabolic events before, during and after rigor mortis. Consequently, these aspects influence the potential of meat quality for further processing, as well as its acceptability by consumers. After slaughter, meat must be chilled and maintained at an internal temperature of 6 to 8C to minimise bacterial growth. To achieve this temperature, carcasses are chilled at 0-4C in batch chillers over a 24- hour cycle. This prolonged chilling, however, causes a significant carcass weight loss as the carcass surface is slow to cool. Weight loss is critical in determining profitability of the meat industry. Currently, a two-stage chilling process is being developed to reduce the problem of weight loss during chilling. The first stage operates at sub-zero temperature to quickly lower the high surface temperature of the carcass. This is then followed by the conventional chiller. The rapid chilling, moreover, improves meat colour and waterholding capacity and also accelerates the shift of carcasses to processing. However, it can adversely affect meat tenderness. The experiment described in this thesis was undertaken to: 1) investigate the postmortem muscle metabolic changes in pork sides subjected to two-stage as compared to conventional chilling used in commercial application; 2) examine the effects of these metabolic changes on pork quality, and 3) determine the relationships among pork quality characteristics and postmortem metabolism. The experiment was carried out using two groups of 10 cross-bred finishing pigs slaughtered conventionally during two visits three weeks apart to the same abattoir. All the pigs were split lengthwise and the left sides (BLAST) were rapidly chilled in two stages (air at -20C for 2 hours followed by 0C for 22 hours) and the right sides (CONY ) were conventionally chilled (air at 0C for 24 hours). The BLAST chilling lowered the loin temperature significantly faster than the CONV chilling. The postmortem glycolysis of the longissimus dorsi muscles for BLAST and CONV proceeded in a similar manner and the development of rigor mortis occurred within 9 hours. The meat quality characteristics of loins; i.e. colour, soluble muscle protein concentration, tenderness, cooking loss and sarcomere length, were not different between chilling treatments. There were good relationships between the surface reflectance and pH at 9 hours or 23 hours and between the surface and internal reflectances. These results suggested that the two-stage chilling used in this study had no effect on either postmortem metabolism or meat quality of the longissimus dorsi muscle. The measurement of internal reflectance coupled with pH estimation on intact pork carcasses as soon as rigor mortis set in should enable accurate identification of carcass abnormality (PSE and DFD meat) at the earliest time