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The quality of fresh fish and seafood can be compromised quickly due to bacterial growth and enzymatic processes. This is caused by a high water activity and a neutral pH value (ideal for micro-organisms), as well as by the presence of enzymes, as a result of which both flavour and smell can quickly deteriorate. The decomposition of micro-organisms causes an unpleasant smell.
The oxidation of unsaturated fats in fatty fish such as salmon, herring and mackerel causes a vile flavour and smell. Fish types such as herring and trout can even become rancid even before the bacterial decay becomes measurable.
In order to maintain a high quality of fresh fish products, keeping the temperature around the freezing point is vital. The combination of the correct temperature and the appropriate gas mixture can prolong the shelf life of fish by a few days. Pre-condition is that the cold-chain is not interrupted.
Quality preservation with carbon dioxide
The activity of many common aerobic bacteria such as Pseudomonas, Acinetobacter and Moraxella can be strongly reduced by the presence of carbon dioxide (CO2). This bacterial inhibiting effect starts in CO2 concentrations in excess of 20% in sufficiently large gas volumes.
The CO2 content is normally 50%. Depending on the storage temperature (0–2°C), the shelf life of raw fish can be extended by 3 to 5 days. The fish is packed in dishes with a film cover and within a protective atmosphere. However, excessively high concentrations of protective gas can cause undesired side effects, such as excessive reduction of the moist content in the tissue or, in the event of crab and herring, a sourish taste.
Fish such as cod, flounder, plaice, haddock and whiting can be preserved up to twice as long in a modified atmosphere and a temperature of 0°C.
Colour retention with oxygen
In MAP packaging, oxygen can be applied to combat the fading of pigment and loss of colour. Oxygen can also be used to combat the growth of anaerobic micro-organisms, such as Clostridium. These types of bacteria can produce toxic substances. In the correct protective atmosphere, the risk of growth of Clostridium in fish with short shelf lives is negligible. The growth of Clostridium can be prevented in temperatures of below 3°C.
In order to prevent the fish from becoming rancid, no oxygen can be used when packing fatty fish. Nitrogen would be more suitable.