Fresh fish rapidly loses the original freshness under the influence of bacterial growth and fermentative processes. Fish and seafood are particularly favorable environment for bacterial growth, which has high activity in water. Fish and seafood have a neutral pH (which promotes the growth of bacteria) and fermentative substances which are rapidly changing the taste and colour of the product. Decomposition of protein caused by bacteriums leads to the occurrence of odor. Oxidation of fats in fatty fish such as tuna, herring, mackerel, causes the unpleasant taste and odor. Varieties of fish such as herring and trout are subjected to spoiling faster than it can be detected by microbiological analysis.
It is required a low-temperature regime - around 0 ˚ C in order to achieve high quality during storage of fish and seafood. In combination with properly chosen gas mixture storage period may be extended for several days. These types of fish like cod, flounder and hake at 0 ˚ C have a shelf life in a protective atmosphere twice higher than in air.
Carbon dioxide - the main component of the gas mixture
Carbon dioxide is necessary for packaging of fish and seafood to prevent the growth of aerobic bacteria such as Pseudomonas, Acinobacter and Moraxella. Carbon dioxide dissolves in the surface layer and lowers the pH, thereby stopping bacteria growth. The most commonly used concentration of CO2 - 50%.
Depending on the temperature (0-2 ˚ C) protective atmosphere increases the shelf life from 3 to 5 days compared with fish, packed in cellophane. The high concentration of carbon dioxide may lead to negative effects such as loss of liquid or sour taste and smell.
Oxygen - an important element of the gas mixture
Oxygen is used for packaging of fish and seafood in a protective atmosphere in order to prevent discoloration of the product and reduce pigmentation. Also, this gas is used to prevent the development of toxic anaerobic bacteria such as Clostridium Butulimun. To prevent the rancidity, oxygen is not used for packaging of oil-rich fish.