We are eating more and more fish. And fresh fish and fillet products account for the biggest increase in consumption. This presents both challenges and opportunities when it comes to shelf life and the packaging of fresh fish. Today, the use of the bacteria-inhibiting gas carbon dioxide (CO2) seem to be the best solution for the freshness, smell and taste of salmon fillets.

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Anne-May Johansen  

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In the project “High-quality salmon products” Nofima’s researchers have been studying packaging methods, including ways of packaging Frøyas brand of fillet Products.

Through experiments we have seen that improved packaging methods can contribute to better product quality and longer shelf life for a range of products.

“Here, we are talking about improved packaging using CO2 emitters, which when used produce bacteria inhibiting CO2 gas”, explains senior researcher Marit Kvalvåg Pettersen

CO2-emitters are pads that look like ordinary liquid absorbers, and which produce CO2 after packaging.

Scientists Anlaug Ådland Hansen and Marit Kvalvåg Pettersen.

Optimal packaging

More fresh fish was sold on the Norwegian market in 2014 than frozen fish. There was also an increase in fillet sales compared to whole fish and minced fish products. From 2013 to 2015, sales of packaged fish rose significantly. For researchers working on optimal packaging, this offers interesting challenges.

Researcher Anlaug Ådland Hansen believes that increased processing of whole fish to fresh fillet products calls for more attention on both microbiological quality (occurrence and growth of quality-reducing bacteria), sensory qualities (taste and smell) and packaging technology.

“Relatively long distribution chains, in the Norwegian and European markets alike, present challenges relating to adequate shelf life, while at the same time the product must have a high quality and be perceived as fresh”, says Ådland Hansen.

Correct product shelf life requires that we know what is perceived as a change in quality and the causes of quality change in salmon fillets.

To achieve good product quality, the raw material quality and the production process are important. The quality of the raw material to be packed must be good, the packaging must be adapted to the product, and, of course, the storage temperature must be correct.

Shelf life and food waste

However, there is difference in the shelf life of products that are to be eaten raw compared to those to be heat-treated through cooking or frying before being served. The researchers believe that this must also be taken into account when the product is being packaged for the consumer market – so no food gets thrown away unnecessarily. Marit Kvalvåg Pettersen explains:

“Frøyas is a fillet product that can be eaten raw, as sushi or sashimi. Today, vacuum packaging is often used for such products, and the shelf life is, as it is for similar products, about 10 days. Since the product can be used raw, the “acceptable” bacterial levels are significantly lower than for products to be heat treated. Many consumers choose this product for purposes other than sushi – and a limitation of shelf life for a raw product can cause the product to be discarded before its shelf life expires. There is a correlation between shelf life and food waste.

More transport economical

It is known that CO2 influences and inhibits the growth of a range of quality reducing bacteria. Modified atmosphere packaging (MAP), also referred to as protective atmosphere packaging, is therefore quite common, where CO2 is an important part of the gas used, often in combination with nitrogen in the gas fill mixture. To achieve the optimal effect of the CO2, and the best possible quality, a substantial gas volume around the product is necessary, which is not positive from a transport economics, environmental or financial point of view. Vacuum packaging is a packaging method that is more transport economical than MAP, but which also results in more rapid quality deterioration and a shorter shelf life.

By using CO2 emitters one can exploit the effect of CO2, while the packaging itself looks just like normal vacuum packaging.

Graf 1
Figure 1: Total number of bacteria (measured as log cfu/g) in Frøyas fillets packed in vacuum and vacuum CO2 emitters stored at 4 °C with analysis after 7 , 11 and 14 days.

Growth of bacteria inhibited by CO2

One cannot exclusively determine the shelf life of fish on the basis of the total number of bacteria. Some bacteria may be present in considerably smaller volumes, but can still contribute to the product perishing and smelling and tasting bad. As a starting point, one can consider that a number of 100,000 colonies of bacteria per gram of fish are on the borderline of what is acceptable for raw consumption. For products that are normally heat treated, the acceptable level may be 10 to 100 times higher.

The use of CO2 emitters, combined with vacuum packaging of Frøyas, resulted in the unacceptable level of total bacteria not being reached until 14 days after packaging, compared to 9-10 days without CO2 emitters, with storage at 4°C.

“So we are talking about a difference of about four days”, Anlaug Ådland Hansen points out.

Sensory quality

Graf 2
Figure 2: Acidic odor (positive characteristic) in Frøyas fillets packed in vacuum and vacuum With CO2 emitters stored at 4 °C with analyzed after 7 , 11 and 14 days of Storage

In this project, a sensory panel also assessed the quality of the raw fish. A so-called descriptive test was carried out, where trained sensory assessors assessed the level of different smells and tastes that had been determined in advance.

The acidic flavour and odour, which is related to the fresh taste and is considered a positive characteristic, was clearly reduced in salmon packed with the current packaging method (“Reference” – vacuum without CO2 emitters). In salmon packed with CO2 emitters, however, there was only a slight insignificant decrease during the storage period.

A fermented sour taste and smell are considered negative characteristics. Also here we see a distinct difference between the packaging methods: salmon packed with CO2 emitters gave better results. In this case, there was no significant increase or change in fermented sour smell or taste from start to 14 days of storage, while in the case of vacuum packaging these increased significantly.

Similar trends and results were also observed for pungent odour and taste.

What is the shelf life of the product?

Graf 3
Figure 3: Fermented sour odor (negative property) in Frøyas fillets packed in vacuum and vacuum with CO2 emitters stored at 4 °C with analyzed after 7 , 11 and 14 days of storage .

“Based on the tests that were carried out, we can conclude that vacuum packaging of Frøyas salmon fillet + CO2 emitters preserves the quality better than just vacuum packaging, and results in a longer shelf life both microbiologically and sensorial. It is therefore possible to extend the shelf life of this product by at least 2 to 3 days – from 10 to 12 – 13 days”, the two packaging researchers conclude.

This will have a positive impact on several links in the value chain, such as production, distribution, sales and in the hands of the consumer, and will probably contribute to a reduction in food Waste.


The project “High-quality salmon products” is a user-driven innovation project with SalMar as project owner, with packaging supplier Tommen Gram and machine supplier Multivac as partners of Nofima, the institute for applied research within the fields of fisheries, aquaculture and food.

The expertise needed to develop optimal and product customised packaging concepts and knowledge of materials and packaging technology and how different packaging methods affect product quality and shelf life have been developed over many years through Nofima’s strategic programme for packaging, funded by the Foundation for Research Levy on Agricultural Products. “Food Pack – innovative and sustainable packaging for optimal food quality” is the title of the current strategic programme. This is led by senior researcher Marit Kvalvåg Pettersen.

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