Effect of pre- and post-mortem temperature on rigor in Atlantic salmon muscle as measured by four different techniques

The effects of ante- and post-mortem temperature regimes on the timing and strength of the rigor process in muscle of Atlantic salmon (Salmo salar) were measured by four frequently used methods for assessment of rigor. The methods were isometric tension (IT, Newton) in excised muscle strips measured by a Rigotech® meat analyzer, whole fillet contraction (WFC, percentage shrinkage) by automatic image analysis, changes in muscle hardness (H, Newton) by compression with a spherical probe and stiffness (S, percentage of full bend) measured by tail bending (also known as “Rigor index”). The fish were moved into experimental temperature tanks 10 days prior to slaughter. The temperature was either kept constant at 4 or 12 °C, or changed from 12 to 4 °C 2 h before slaughter. Storage (post-mortem) temperature was set to 4, 12 or 20 °C. Maximum IT, H and S decreased in response to higher storage temperature (p < 0.0001), while WFC increased (p < 0.0001). The occurrence in time of maximum value differed between methods, with chronological succession; IT→WFC→S→H. The rigor process was always delayed when storage temperature was reduced (p < 0.0001). The effect of ante-mortem temperature was more complex. At the same storage temperature (4 °C), fish that had been moved from 12 to 4 °C 2 h before slaughter had a significantly more rapid rigor process than fish that were kept at a constant temperature before slaughter (p < 0.0001), possibly indicating an effect of stress when changing water temperature. General agreement between treatments and relative response was observed among the four methods. Even so, significant differences were seen, especially in the resolution power of treatment effects, with IT > WFC > H > S.