Stress lowers salmon quality
Monday, 20 February 2012 16:02

Salmon that become stressed before slaughter have shorter shelf lifes because the flesh suffers more bacterial growth and develops undesirable flavours and odours more quickly. The negative consequences of stress are greater for raw than for cooked salmon.

“There are a number of issues that influence how long fish stays fresh. In this study, we have concentrated on what effect stress just before slaughter has on the quality of the flesh,” explains research scientist Anlaug Ådland Hansen of Nofima Mat, the Norwegian Institute of Food, Fisheries and Aquaculture Research.

The salmon were divided into three groups, subjected to varying periods of stress before slaughter. One group was handled as carefully as possible before slaughter, without crowding the fish in the cage before taking them out of the water. The salmon in the second group were crowded together for 20 minutes before slaughter, while those in the third group were crowded for about 20 hours. Feed, fish size and other factors that can affect quality were constant for all the fish.

After slaughtering, the salmon were immediately filleted, cut into portion sizes, packed in a modified atmosphere (MAP) and stored at 0.3°C. Regular quality analyses were made over the course of 22 days. It was found that both bacterial growth and undesirable sensory properties increased most rapidly in the salmon that had been subjected to the longest stress.

“Sensory analysis showed that undesirable odours especially became more obvious as a result of longer stress”, says researcher Marit Rødbotten. “We also found that the differences in quality became more marked when the fish was raw.”

An increasing proportion of Norwegian farmed salmon is filleted within a few hours of slaughter before rigor mortis occurs. Early filleting means more jobs in Norway and keeps the fish fresher and firmer when it reaches the customer. It is known that slaughter stress brings forward the time at which rigor mortis begins. Filleting salmon during rigor mortis is not recommended, which means that the filleting window is shorter for salmon that are stressed before slaughter. It is also important for fish welfare to handle the fish as carefully as possible.

“In our study, both the shorter and longer period of crowding accelerated the onset of rigor mortis, but it was the long-term crowding that really gave quality problems, shortening shelf life by three days,” said research scientist Turid Mørkøre, who was responsible for the study carried out at Nofima’s marine facility at Averøya. “Crowding the fish had no negative consequences for fillet colour, firmness or drip loss during storage.”


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