Less stress – better quality
Salmon that becomes stressed before slaughter has a shorter shelf life 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.
The negative effects are clearest in raw salmon
The salmon was divided into three groups. The only difference was the length of the period 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. We also found that the differences in quality became more marked when the fish was raw,” says Researcher and Sensory Scientist Marit Rødbotten.
Stress accelerates rigor mortis
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. Crowding the fish had no negative consequences for fillet colour, firmness or drip loss during storage,” says Research Scientist Turid Mørkøre, who was responsible for the study carried out at Nofima’s marine facility at Averøya.
Inter-disciplinary research collaboration
This work was part of a larger study financed by the Research Council of Norway. The main purpose was to investigate how different levels of stress before slaughter affect biochemical processes in the salmon muscles and thereby fillet quality. An inter-disciplinary team of researchers from Nofima was involved, but the work was also included in three doctorates at the Norwegian University of Life Sciences.
“Now that we know that stress has greater negative consequences for raw salmon than for cooked, more research is needed into what effects different types of aquaculture and fishing have on the end quality of raw fish. This is especially important now that raw fish, as sushi for example, is greatly increasing in popularity,” concludes Ådland Hansen.
This work has been described in a scientific article that is being published in the Journal of Food Science. The article is entitled “The effect of crowding stress on bacterial growth and sensory properties of chilled Atlantic salmon fillets”. The authors of the article are Anlaug Ådland Hansen, Marit Rødbotten, Thomas Eie, Per Lea, Knut Rudi and Turid Mørkøre.