The effect of exercise stress, adrenaline injection and electrical stimulation on changes in quality attributes and proteins in Semimembranosus muscle of lamb.

Exercise pre-slaughter has previously been shown to increase drip loss and tenderness in the longissimus muscle of lambs. The mechanism causing higher water loss in post-mortem muscle of lambs is not well understood. This experiment examines the effects of exercise stress, adrenaline injection and electrical stimulation (ES) on meat tenderness and water holding capacity. Forty crossbred lambs (20±2 kg live weight) were used in this study. Meat quality measurements were carried out on the Semimembranosus muscle (SM). The ultimate pH was higher with exercise stress (pH 5.93) compared to controls (pH 5.55; P<0.01). As expected the glycogen levels 30 min and 24 h post-mortem in the exercise stressed (P<0.01) muscle were lower. Lactate concentration at 30 min post-mortem in the electrical stimulation (ES: P<0.05) treated muscle was higher than all other treatment combinations. Myofibrillar protein denaturation, measured by protein solubility, was higher (P<0.05) and the rate of proteolysis of muscle proteins titin and troponin-T, as observed with western blots, increased with exercise stress. However, there were no treatment effects on Warner-Braztler shear force (WBSF). Exercise stress and an interaction with adrenaline caused increased drip loss (P<0.05) and purge (P<0.01) from the muscle. Muscle drip from the exercise stress treatment group had lower Mg(2+) (P<0.01) and K(+) (P<0.01) than controls. The main findings were that exercise stress pre-slaughter increased water loss and the rate of titin breakdown but had no effect on tenderness measured by WBSF. Together the different rates of proteolysis, protein denaturation and ionic conditions may have caused the reduced capacity of the muscle proteins to hold water, despite their relatively high ultimate pH.