Differential response to air exposure in crowded and uncrowded Atlantic cod (Gadus morhua): Consequences for fillet quality

Previous recommendations on bleeding of Atlantic cod (Gadus morhua) to secure proper blood drainage and good muscle quality are based on studies done on un-stressed fish. Therefore, the aim of this study was to investigate the effects of stress by crowding in a net, followed by air exposure before and after slaughter on blood parameters and fillet quality in Atlantic cod. Live fish were either directly or after 4 h of crowding exposed to air for 0, 15 or 30 min prior to or after killing by a blow to the head and bleeding. Blood clotting time, pH, lactate, glucose, and muscle pH were measured. Also, concentrations of haemoglobin in the muscles were measured using Vis/NIR hyperspectral imaging to indicate residual blood. Stress from crowding and air exposure before and after slaughter resulted in increased concentration of haemoglobin in the fillets, with a faster increase in fish crowded and slaughtered after air exposure. Blood clotting time was shorter after 15 min of air exposure, and decreased further with crowding. Blood and muscle pH, and lactate levels were mainly affected by air exposure time. Overall, air exposure had a negative effect on fillet quality, and this effect was strongest and fastest if fish were crowded prior to the air exposure. However, by slaughtering the fish before air exposure, quality can be improved as this delays the increase in the amount of residual blood.