Ph.D. student Silje Ottestad at the Nofima food research institute has developed measurement methods to monitor the quality of salmon in storage in both the fresh and the frozen state. With the aid of spectroscopy, she has illuminated pieces of salmon to investigate how the spectrum of light changes.
The methods are non-destructive, which means that the fish can still be used after it has been measured on the conveyor belt. The method developed by Ottestad covers several different areas.
Controlling the quantity of ice is important
Superchilling is a method of freezing in which only a small percentage of the water content of the fish is turned to ice. This means that the fish will keep for longer than normal chilled fish, while the quality is better than thawed deep-frozen fish. In order to achieve the optimum quality with superchilled products, it is very important to be able to control the amount of ice in the product.
"I have developed a method for determining the amount and the distribution of ice. The correct amount of ice for maximum fish quantity is something that varies. It depends on the size of the fish, whether it is a whole fish or fillets and what type of fish it is. For example, there will be differences between cod and salmon because of the different fat content," explains Silje Ottestad.
The colour of salmon is affected by blood
Colour is an important quality parameter for salmon. The redder the flesh, the higher the price per kilo. With farmed salmon, the colour is derived from pigments in the feed, but this pigment is an expensive ingredient and there are other factors that affect the colour scale after slaughter. It is also important to be able to control these.
We have known for a long time that salmon can lose colour during storage. "We have found out that it is the blood in the fish that causes the colour to change. When fish is stored in air, the haemoglobin in the blood locks up the oxygen it comes into contact with. The blood then gives the fish a more golden-brown colour and the variation in colour can be quite large," says Ottestad. It is better to vacuum pack the fish - from the point of view of colour, storage in air is the worst thing.
She has used spectroscopic measurement methods to find out whether there is a large or small amount of blood, but also to investigate what chemical changes cause the colour alterations. The concentration of blood in the salmon is affected by several factors: how well the fish has been bled, whether the fish was subjected to stress just before slaughter and the slaughtering method itself. But even with a small amount of blood, colour effects can still be seen.
More robust measurement methods for pigment concentration
Spectroscopic measurement methods have been used for some time to determine the optimum pigment concentration, but the measurements are not always correct. Measurement errors are often connected with the texture of the fish, because this determines how deeply the light can penetrate the sample. The softer the texture, the further into the flesh the light penetrates, and the further it penetrates, the redder the colour appears to be.
Ottestad has been working on removing these differences in levels and has developed a model that is more robust when faced with differing muscle textures. This method can also be used to determine chemical concentrations in other products with varying textures. Ottestad will continue to work on this after she has completed her doctorate. At Nofima, she will be developing spectroscopic measurement methods for looking at texture and light scatter in other food products. "The aim is to also arrive at a more robust and cheaper method for foods such as meat, milk, fruit and vegetables. The method could be used for sorting by quality, for example," says Ottestad.
Her disputation is at the Norwegian University of Life Sciences (UMB) on 30 March. The title of the thesis is "Kvalitetsovervking av fersk og frossen laks ved bruk av synlig og nrinfrard spektroskopi" (Monitoring the quality of fresh and frozen salmon through the use of visible and near-infrared spectroscopy). The teaching supervisors are Professor Tomas Isaksson of IKBM-UMB, Senior Lecturer Elling-Olav Rukke, IKBM-UMB and Senior Research Scientist Jens Petter Wold of Nofima.
Ottestad previously majored in biophysical chemistry at the University of Bergen, specialising in protein chemistry and high-field NMR.
The research has been sponsored by the Research Council through the KMB Lnnsom Foredling (profitable processing) project and Nofima.