Texture Quality in Farmed Salmonids

 

 

Research technician Doug Singer examines the texture of salmon steaks using a portable fish texture tester.

 

 

 

The farmed salmonids of today are very different than the wild fish caught in the past. The potential control and efficiencies achievable through aquaculture create opportunities to produce fish with far superior quality and shelf life attributes. One of the largest differences between farmed and wild caught fish is perhaps muscle tone. Farmed fish do not have to swim long distances in search of food and have a tendency to accumulate much more fat, particularly along the connective tissue junctions which separate the "myotomes" or "flakes". Since this fatty tissue has little tensile strength, manipulation of the fish and the process of rigor mortis can lead to "gaping" or the undesirable separation of fillet segments. This problem can be exaggerated if the fish have been fed high energy (high fat) diets.

 

Changes in Fish After Harvest Affecting Texture and Implications for Processing

Upon death, the most important event is the loss of blood circulation and a rapid decrease in oxygen in the muscle tissues. In response, enzymes that are still active after death produce lactic acid and the muscle pH (a measure of acidity) drops from around a normal 6.8-7.0 reading to about 6.1-6.2. The chemistry of the protein molecules that make up the muscle is changed by the acid environment. The muscle proteins form chemical bonds and the muscle becomes rigid. This muscle contraction or state of rigidity is rigor mortis. Over time the contortion, stiffening and inextensibility of rigor mortis is "resolved", the muscle becomes pliable and processing can begin. With salmon, rigor mortis may not be fully resolved for 30-60 hours after death, depending upon many factors including temperature, fish size, pre-slaughter stress, degree of starvation before slaughter, genetics, etc.

Simple strategies for gauging the state of rigor have been devised. A simple and easy way is to hold a fish horizontally by the tail in one hand and look for droop. In rigor the fish is "straight as a ball bat" and as rigor resolves the fish gradually droops. Generally the fish flesh feels dry and sticky before rigor mortis, whereas after rigor is resolved the flesh feels and looks wetter.

Pre-rigor processing of salmonids may have costly implications because the fillets can shrink by as much as 10-15 percent. If not properly controlled the rigor process can itself lead to quality problems including gaping. Adequate chilling of farmed fish is critically important since the degree of shortening of muscle fibres is related to temperature; the extent of shortening is always lowest at the temperature of ice (~0 ºC).

Rigor mortis must be completely resolved prior to any processing (scaling, skinning, filleting, etc.) if texture damage is to be prevented. This is because fish in rigor are usually contorted (deformed) and forced straightening will cause unnecessary tearing of the tissues. The tearing most often takes place at the connective tissue junctions which divide the fillets into "flakes". This tissue damage (called gaping) may or may not be immediately evident but becomes progressively worse with time of chilled storage.

Effects of Handling After Death

Softening of farmed fish will occur eventually no matter what preventive measures are taken. These textural changes are caused by the proteolytic enzymes which are found in the various tissues in large amounts. These enzymes are normally present in living fish but are not activated until required. Such times may be spawning (enzymes used to help move muscle protein into eggs and milt), starvation, injury or death. Following death, the enzymes contained in the lysosomes (membrane bound packages in the muscle) are gradually released and degradation of the fish flesh ensues. The proteolytic softening can be retarded (but never stopped) by reducing the amount of handling and manipulation of the flesh, proper and complete bleeding and by storing the fish at the lowest possible temperature preferably near 0 ºC. Pressure or crushing of the flesh as resulting by packing iced fish in deep tote boxes has also been shown to cause extensive softening.

For additional information of this and other food quality and safety related topics please contact the seafood specialists at CIFT.