Biochemical basis of the effect of pre-slaughter stress and post-slaughter processing conditions on meat tenderness

Abstract

Previous research has shown that pre-slaughter stress of cattle reduces beef quality because of its effect on post mortem muscle pH. Spray washing in the cattle yard is a hygiene requirement of the New Zealand Ministry of Agriculture and Fisheries (MAF) in order to reduce the contamination of carcasses. However, the washing stresses the animal. This study investigated the effect of spray-washing cattle on beef quality. Spray washing of cattle resulted in considerably more variation in the ultimate pH (pHu) of the meat (5.52 to 6.48) compared to meat from dry animals (5.50 to 5.79). The increased variability in pHu was reflected in the meat shear force values from the spray-washed animals, which ranged from 5.2 to 14.5 kgF, while that from the dry animals ranged between 4.8 and 8.6 kgF. There was a tendency for meat from spray-washed animals to be tougher than meat from dry animals (p = 0.089). The effect of spray washing on calpain I autolysis, desmin and troponin T degradation during post mortem period was determined. Results showed that spray washing tended to cause an increase (p = 0.07) in post mortem degradation of desmin but had no significant effect on troponin T degradation or calpain I autolysis.

The second part of the research investigated the effect of post-slaughter processing conditions on turkey breast meat tenderness, and the biochemical basis for the toughening of turkey breast meat. Anecdotal evidence showed that consumers perceived turkey breast meat as being tough. The preliminary results of this research indicated that the tenderness of turkey breast meat was variable and the mean shear force was higher than would be acceptable to consumers. The observed toughening of turkey breast meat may be a result of post-slaughter treatments of turkey carcasses, such as the rate of carcass cooling, stunning method and carcass deboning time. The effect of these factors on turkey breast meat tenderness were investigated in this research. Post-slaughter temperature and pH of turkey breast muscle was measured at different times. The carcass temperature ranged from 32°C to 36°C at 15 minutes post slaughter. Immersion chilling (1st water bath and 2nd ice slush bath) rapidly reduced the internal breast muscle temperature to about 10°C by 75 min after stunning. In all three experiments (2, 3 & 4), cold blast treatment significantly decreased the internal breast muscle temperature compared to the breast muscle that was chilled only. The breast muscle temperature immediately after a cold blast treatment (about six hours post-slaughter) ranged from 2-6°C. In this study, the mean pH of turkey breast muscle at 15 min post-slaughter was 6.52, indicating that turkey breast muscle exhibited accelerated rigor mortis compared to lamb or beef. The pH of turkey breast muscle at six hours post-slaughter was close to the ultimate pH, which ranged from 5.80 to 5.96.

Results of this research showed that turkey processing conditions have a toughening effect on the resulting breast meat. Deboning of turkey breast muscle three hours post-slaughter caused between a 80% and 90% increase in toughness of the meat compared to muscle aged on the bone for 24 hours. The use of a blast freezer to rapidly cool the breast also toughened the meat by an average of 30%. The mechanisms responsible for the meat toughening were both muscle contraction and a reduction of proteolysis. The sarcomere length of muscles deboned three hours post mortem was significantly shorter than muscles deboned at 24 hours post mortem. The myofibrillar fragmentation index (MFI), which is a measure of proteolysis, was significantly higher for muscles aged on bone for at least 24 hours compared to muscles deboned three hours post-slaughter. It is known that meat tenderness improves during post mortem ageing. Results of experiment 4 showed that six days extended ageing of pre-rigor or post-rigor deboned turkey breast muscle significantly improved the meat tenderness. In both muscles deboned at 3 hand 24 h post mortem, there was a 25% decrease in the shear force values after the 6-day ageing period. During the ageing period, MFI increased significantly both for muscles deboned at three hours post-slaughter (p = 0.025) and for the muscles aged on bone for 24 hours prior to deboning (p = 0.018).

Activities of calpain I, II and calpastatin were measured at three different times during post-slaughter ageing (0, 3 and 24 hours) in muscles aged on bone for 24 hours and muscles deboned three hours post-slaughter and then, aged for 24 hours. The activity of calpain I rapidly declined with increasing post mortem time to the extent that only trace levels were detected at three hours, and no calpain activity was detected at 24 hours post mortem. In contrast, there was no loss in calpain II activity over the post mortem period. Calpastatin retained only 35% of its at-death activity in turkey breast muscle at three hours post-slaughter and there was no calpastatin activity at 24 hours post-slaughter. Furthermore, deboning time had no significant effect on the activity of calpain I, II and calpastatin at 24 hours post mortem.

Based on these results, it can be concluded that the observed toughening of turkey breast meat originates from the rapid cooling of turkey carcasses and the three hours post slaughter deboning time. The mechanism of the toughening of turkey breast muscle deboned three hours post mortem is likely to be a direct result of cold shortening rather than reduced calpain activity. This suggests that the reduced proteolysis as measured by MFI is a result of reduced accessibility of the cal pain to myofibrillar protein because of the shortened sarcomere length of muscles deboned at three hours post slaughter and then aged for 24 hours.