Effects of pre-slaughter stress and electrical stimulation intensity on meat tenderness : A thesis submitted in partial fulfilment of the requirements for the degree of Master of Agricultural Science - Meat Science at Lincoln University

Abstract

The experiments in this thesis were part of on-going research on the determinants of meat quality. The two main factors considered in this Thesis were (i) the amount of electrical stimulation applied to beef and lamb carcasses and (ii) the effect of pre-slaughter stress on lambs. In the first experiment, on 20 Angus cross steers, the level of electrical inputs immediately following stunning were varied. One group of carcasses received mild electrical stimulation (MS) for 20 seconds and the other group was intensely stimulated (IS) for 80 seconds. Meat quality characteristics of the m. longissimus (LD) were determined at 1 and 7 day(s) post-mortem. This intensely stimulated carcasses reached their ultimate pH after nine hours post-mortem. The level of electrical stimulation had no effect on the tenderness of the LD at 1 day post-mortem but, at 7 days post-mortem, the intensely stimulated carcasses were significantly tougher (p<0.05) compared to the mildly stimulated carcasses (6.96 kgF versus 5.55 kgF).

The results of the tenderness measurements could be explained by differences in post­mortem proteolysis and muscle contraction. At day 1 post-mortem, muscles from the intensely stimulated carcasses showed increased proteolysis compared to the mildly stimulated carcasses (MFI: 140 versus 99). The intensely stimulated muscles had a tendency to be more contracted with shorter sarcomeres of 1.80 µm compared to the mildly stimulated muscles which had longer sarcomeres of 1.85 µm (p= 0.09). The opposing effects of these two observations on meat tenderness may explain the lack of differences in meat tenderness between the two treatments at day 1 post-mortem. At day 7 post-mortem, however, muscle proteolysis for both treatments had progressed to a similar level with MFI at 194 and 193 for the intensely and mildly stimulated carcasses respectively. Thus, the toughness in the intensely stimulated muscles at day 7 may be explained by the effect of electrical stimulation on muscle contraction.

A second experiment was carried out on 30 beef carcasses with electrical inputs intermediate to those in the first experiment. Carcasses received either 44 seconds (method 1) or 55 seconds (method 2) of electrical input. The pH profiles of the LD as a result of electrical stimulation in the second experiment were similar to those observed for the carcasses that were intensely stimulated in the first experiment. There were no significant differences in meat tenderness between the two treatments in this experiment. Method 1 led to shear force values of 9.07 kgF and 6.21 kgF, whereas method 2 led to shear force values of 8.53 kgF and 6.64 kgF at 1 and 7 day(s) post-mortem, respectively. From these results, it can be inferred that 20 seconds of electrical stimulation during immobilization is sufficient stimulation for the carcasses to yield acceptable quality meat.

The interaction between pre-slaughter stress and electrical stimulation was investigated in a line of 80 lambs. Previous reports had suggested that stress affects meat tenderness independently of ultimate meat pH. Lambs were partitioned into a control group and a stress group. Stress was applied by mustering the animals at the slaughter house at a high pace with the assistance of dogs, and swim-washing 3 hours prior to slaughter. Both the control and stress group were divided .into two sub-groups; one received high voltage electrical stimulation (HVES) and the other no HVES. The meat quality characteristics for them. longissimus were evaluated at 2 and 42 days post-mortem. These evaluation periods represent the time it takes meat to enter the local consumer market (2 days) or overseas market (42 days). A higher number (32.5%) of the carcasses from the stressed lambs yielded LDs with an ultimate pH higher than 5.8 compared to 15% from the control group. Ten carcasses withpH below 5.8 were selected at random from each of the four groups (stressed/non­stressed lambs, electrically stimulated/non-electrically stimulate carcasses) for further evaluation. No significant differences in tenderness were observed between electrically stimulated carcasses irrespective of whether they were from stressed or non-stressed lambs. There were, however, differences in meat quality characteristics between the stimulated and non-stimulated carcasses. The stimulated carcasses yielded more tender meat at day 2 post-mortem (7.84 kgF) compared to non-stimulated carcasses (9.86 kgF), but after 42 days post-mortem storage there was no difference ( -3.0 kgF). The differences in meat tenderness between the stimulated and non-stimulated carcasses at 2 days post-mortem could be explained by the extent of muscle shortening. The LD from the non-stimulated carcasses had shorter sarcomeres (1.69 µm) than LD form the stimulated carcasses (l.78 µm). This probably accounts for the differences in shear force rather than the extent of proteolysis. It was also evident from the study that carcasses with ultimate pH above 5.8 yielded tougher meat. Most of the carcasses in this category were from the stressed lambs. The experiments in beef showed that there was an over use of electrical inputs in the processing plant. It was observed that even low electrical inputs adequately stimulated the carcasses. Therefore, it would be advisable to monitor the application of electrical stimulation to optimize its effects on meat quality. The third experiment has shown that stressing the lambs prior to slaughter should be avoided. It was evident from the results that most of the carcasses which had ultimate pH above 5.8 were from the stressed lambs and, that these carcasses yielded tougher meat compared to carcasses with ultimate pH less than 5.8.