If nitrogen is replaced by high oxygen when packaging fresh cod, the fish retains its quality longer.

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  Oddny Johnsen, Krysspress

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Scientist Anlaug Ådland Hansen and her colleagues at Nofima have identified the bacteria that are present in fresh-packed cod, and determined how the growth of these can be prevented in the best possible manner by the choice of packaging method and packaging material.

By taking samples of fresh cod fillets, they have found three types of bacteria that are known to be particularly detrimental to cod when it comes to the development of odour and taste. The three bacteria are Photobacterium, Pseudomonas and Shewanella, and all occur naturally in the fish. The amounts of the three bacteria that are included with the fish in the packaging before the fish is sent to the shops differ, depending on the quality of the raw material.

The production environment

“The amount that accompanies the fish into the packaging depends on the level of cleanliness during production. The production environment affects without doubt the bacteria we find,” says Hansen.

The task of the scientist is to ensure that the various bacteria are given as poor growth conditions as possible when the fish is placed in the packaging.

After identifying the bacteria, Anlaug Ådland Hansen has conducted experiments with different packaging gases in order to determine the growth of the various bacteria in fresh cod. It has previously been shown that the best method of getting rid of Photobacterium from cod is by freezing. Hansen has now shown that a mixture of gases that differs from the normal packaging gas used for meat and fish can give equally good results for this bacterium in particular.

“The more we know about the enemy, the easier it is to combat it,” she points out.

The new knowledge has been gained in a large research project about live storage, known as “CATCH”. The programme is to last for four years and is financed with NOK 29 million from the “Sustainable Innovation in Food and Bio-based Industries (BIONAER)” programme from the Research Council of Norway.

High oxygen

The most commonly used packaging gas in Norway today consists of 60% carbon dioxide and 40% nitrogen.

“Packaging the fish with the normal packaging gas does have a certain effect on Photobacterium, but the growth of this bacterium in particular is more strongly inhibited by replacing nitrogen with high oxygen. The packaging gas mixture also has an inhibitory effect on the other bacteria we found in fish, and this means that the industry should now focus on using high oxygen in the packaging process,” says Hansen.

She estimates that the shelf-life in the shops can be increased by up to 20%, if all other conditions are optimal for the storage.

“The levels of bacteria in the raw material, the choice of packaging method and the storage temperature are all crucial to obtaining a long shelf-life. The lower the initial levels of bacteria in the product, the longer the shelf-life that can be achieved. In addition to this, choosing the best packaging method ensures the best possible quality and longest possible shelf-life.”

Hansen is somewhat sceptical to the extension of the shelf-life of fresh cod to 10 days, using the current packaging methods.

“The knowledge we have gained from this project leads me to believe that the current practice of stating a shelf-life of 10 days is somewhat exaggerated. In contrast, the use of high oxygen has the potential to increase shelf-life. In the best case, with low initial levels of bacteria in the raw material and packaging using high oxygen, a shelf-life of 12 days can be achieved,” says Hansen.

The choice of packaging material is also important.

“Many factors play a role in determining shelf-life. The barrier properties of the packaging, for example, are affected by the storage temperature. Packaging in a Thermoform machine changes the properties of the packaging somewhat. It is, therefore, important to know the barrier properties of the final packaging in the actual storage conditions,” explains Hansen.

Working with two of the partners of the CATCH project, Tommen Gram, who manufactures plastic film, and Multivac, who produces packaging machines, Nofima scientist Anlaug Ådland Hansen is coming ever closer to optimal packaging, by measuring how much gas escapes from the plastic package at the recommended storage temperature for fresh cod. The longest possible shelf-life for fresh cod on the market will in this way be ensured.

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