Fresh fish and seafood rapidly lose their original quality as a result of microbial growth and enzymatic processes. This is due to their high water activity, neutral pH (where microorganisms thrive) and the presence of enzymes, which rapidly undermine both taste and smell. The breakdown of proteins by microorganisms gives rise to unpleasant odours.
The oxidation of unsaturated fats in oily fish such as salmon, herring and mackerel also results in an unappetizing taste and smell. Fish such as herring and trout can turn rancid even before microbial deterioration is detectable.
In order to maintain the high quality of fresh fish products, it is absolutely essential that the temperature is kept as close to 0°C as possible. The right gas mixture, combined with proper temperature control, can extend the shelf life of fish by a few crucial days, assuming of course, that the refrigeration chain is unbroken.
Carbon Dioxide Preserves Quality
The presence of carbon dioxide (CO2) has a strong inhibiting effect on common aerobic bacteria such as Pseudomonas, Acinetobacter and Moraxella. At CO2 levels above 20% in sufficiently large package volumes, it does this by lowering the pH level of the tissue surface of the fish.
The CO2 concentration is normally 50%. Depending on the storage temperature (0–2°C), Modified Atmosphere Packaging prolongs the shelf life of raw fish in a tray with film wrap by 3 to 5 days. However, excessively high concentrations can produce undesirable side effects in the form of lost tissue liquid or in the case of crabs, an acidic or sour taste.
Cod, flounder, plaice, haddock and whiting are examples of fish that can be stored twice as long in a modified atmosphere as in air at a temperature of 0°C.
Oxygen Preserves Colour
Oxygen can be used within a modified atmosphere mix to avoid discolouration and pigment fading in fish and seafood. Oxygen is also used to prevent the growth of anaerobic microorganisms such as clostridium, which can produce toxins.
The risk of clostridium growth in fish packaged in the correct modified atmosphere with a short shelf life is negligible. If the temperature is kept below +3°C (+37.4ºF), the growth of clostridium can be prevented.
To avoid rancidity, oxygen should not be used to package oily fish. Nitrogen is more suitable in this case.