Effects of stress and high voltage electrical stimulation on tenderness of lamb m. longissimus

This study was conducted to investigate the reported effect of pre-slaughter stress on meat tenderness independent from its effect on ultimate pH, and its interaction with electrical stimulation. From a group of 80 Coopworth lamb, 40 were stressed by subjecting the animals to a swim wash 3 h before slaughter and the use of dogs to assemble the animals to the access ramp of the abbatoir. Half of the carcasses of each group was electrically stimulated within 30 min post mortem. Temperature and pH decline of the longissimus was monitored and shear force of the cooked muscle was determined at 2 days post mortem and after 6 weeks vacuum storage at 1°C. To investigate an effect of stress independent of ultimate pH, 10 muscles with an ultimate pH below 5.8 were selected from each group for detailed analysis. This analysis consisted of determination of calpastatin activity and sarcomere length, and immunoblotting of μ-calpain and calpain substrates. The stress treatment led to an increase in the number of muscles with an ultimate pH above 5.8 (32.5 vs 15%), and muscles with an ultimate pH above 5.8 were significantly tougher than muscles with an ultimate pH below 5.8 at 2 days post mortem. Electrical stimulation improved tenderness at two days post mortem. This effect could be attributed to an effect on muscle contraction, but not on post mortem proteolysis of calpain substrates. A large variation in tenderness at 2 days post mortem was observed and this was not reduced by electrical stimulation. Six weeks of vacuum storage resulted in a 6 kgF drop in mean shear force and a uniformly tender product. Despite the fact that the stress treatment was similar to those in earlier studies, we failed to observe an effect of stress independent of ultimate pH on tenderness. The reason for this is unclear, but differences in the response to stress between breeds may be responsible. The results of the present study underscore the importance of minimizing pre-slaughter stress and adequate post mortem storage for meat quality.