Pre or post mortem muscle activity in Atlantic salmon (Salmo salar). The effect on rigor mortis and the physical properties of flesh

To distinguish between stress, exercise and physical burst and their effect on flesh quality, 30 Atlantic salmon (Salmo salar) were on three occasions either killed, rested or acutely stressed. Of the 20 rested salmon killed, 10 of the carcasses were exercised electrically, applying pulsed direct current at 5 V, 5 Hz for 2 min. Results show that electrically stimulated fish had the fastest onset of rigor mortis (2-4 h), while the stressed fish had a later onset of rigor mortis (4-24 h). The control group had the latest onset of rigor (12-36 h), and the average rigor index was generally low. No difference could be detected in texture properties measured as gaping, shear force or drip loss between the electrically stimulated and the rested fish. However, the stressed fish had a significantly higher drip loss, gaping score and softer texture than the rested fish. In the scenario of stress, these results indicate that there are mechanisms other than energy metabolism and rigor mortis that are the source for activating proteases and thereby accelerating the post mortem softening of the flesh. The reason may lay in physical stress of the muscle fibrils and connective tissue as a direct result of bursts of physical activity in attempts to flight.