Research for better durability in fish fillet

Janne Karin Brodin

The tenderizing (muscle degradation) that we want in meat, is not desirable in fish. Fish meat is already ”tender”, and degradation will only make the fish flaccid and soft. But in order to keep fish meat quality and reduce muscle degradation we need to learn more about what happens in fish muscle after slaughter

In this PhD thesis we have investigated this, and the results indicate that certain enzymes in the fish muscle are very important, and that we through breeding and proper handling of the fish both before and after slaughter time, may obtain a fish muscle that stays ”fresh” longer.

Diane Bahuaud Photo: Janne Brodin

Enzymes break down proteins in the muscle. The start-off point for PhD student Diane Bahuaud was to study a group of enzymes called cathepsines, which are found in the lysosomes of the fish muscle cells. The reason for this interest is that these enzymes are known to be important for muscle degradation in warm-blooded animals. In salmon, therefore, it may be important that we treat it so that these enzymes are kept at a low level of activity.

In her PhD work, Bahuaud has worked on four possible influences on cathepsine activity:

Super-chilling
Super-chilling is a process where the fish is kept for a short time in freezer rooms with large fans. Extreme cold chills the fish muscle very quickly down to -1 degree. Parts of the muscle will freeze.

Bahuaud’s focus was to see whether super-chilling had any effect on the degradation of fish muscle, and on cathepsine activity. The results showed that the technique needs to be developed further. Ice crystals were formed both inside and outside the cells, and the rate of degradation was quicker in the super-chilled group than in the control group of fish fillets which were not frozen.

Super-chilled salmom fillet with frozen fillet outer layer Photo: Norconserv. Forskningsrådet, Nytt fra HAVBRUK nr. 3. Nov. 2007

Even though the experiments so far showed reduced quality when super-chilling was used, the method will be researched further because it may have several advantages: among other things, ice is not necessary in this method. Transport volumes to foreign countries may therefore be significantly reduced.

The importance of feeds
Bahuaud looked at various types of feed and their influence on the occurrence of cathepsines. Three types of feed were tested: feeds containing fish oil, rapeseed oil and high concentrations of the healty fatty acids EPA and DHA. It turned out that there was no difference in the occurrence of cathepsines when using feeds with rapeseed oil and fish oil, whereas the experimental feeds containing extremely high levels of omega 3 fatty acids had a negative effect. Muscle degradation was quicker. As so often before, too much of a good thing is not good! The results showed that extreme feeds can influence the enzyme system negatively.

Stress influences fish
The third approach Bahuaud used in her PhD work was to look at the effect of short time stress (20 minutes crowding before slaughter) and long time stress (20 hours crowding before slaughter) on muscle degradation and onset of rigor.

There were clear effects from stress. Long time stress lead to accelerated decrease of muscle Ph. Most likely, stress leads to the breaking down of glycogen in the muscle. Pre-slaughter stress plays an important role, seeing that there was early activation of cathepsin with subsequent loss of fillet quality.

Genetic variation
Looking into the genetics, Bahuaud found that there were differences between families of Atlantic salmon in regard to fillet firmness measured two and five days after slaughter. The results showed that the higher the cathepsine activity, the softer the fillet became. Selection of Atlantic salmon for improved fillet texture may therefore possibly be done based on the level of cathepsines in the muscle.

Diane Bahuaud is 27 years old, and comes from Touluse in France. She took his Master degree at Graduate School of Life Sciences (ENSAT), Toulouse in 2004.

The trial lecture and the public defence took place Friday June 26 2009 at IHA. Title of the thesis: Atlantic salmon (Salmo salar L.) flesh quality – Role of lysosomes and cathepsins in muscle degradation. Prescribed subject of the trial lecture: Roles of lysosomes in live muscle physiology.

Bahuaud's main supervisor has been Professor Magny S. Thomassen, IHA. Professor Anders Kiessling, IHA og Dr. Ragni Ofstad, Nofima Mat have been assistent supervisors.

Updated: 05.07.09
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