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Ice blocks used for preserving fish
Ice blocks used for preserving fish

Many different techniques have been used to preserve fish quality and to increase their shelf life. They are designed to inhibit or reduce the metabolic changes that lead to fish spoilage by controlling specific parameters of the fish and/or its environment. These techniques can be classified as follows:

Techniques based on temperature control

These encompass a wide array of technologies used to decrease the fish temperature to levels where metabolic activities - catalyzed by autolytic or microbial enzymes - are reduced or completely stopped. This is possible by refrigeration or freezing where the fish temperature is reduced, respectively, to approximately 0 °C or < - 18°C.

Fish refrigeration can use cool air circulating around the fish (mechanical refrigeration) or icing. Fish icing and boxing on-board fishing vessels is not always possible in the case of small pelagics that are caught in large quantities. These are chilled using refrigerated seawater (RSW) or chilled seawater (CSW). Chilled or frozen fish products require additional cooling in cold store to avoid an increase in temperature.

The design (size, insulation, palletization) and management of cold stores are key for fish quality and energy saving. A major environmental issue relates to the development of alternative refrigerants to replace the chlorofluorocarbons (CFCs) which are damaging to ozone layers.

Techniques based on the control of water activity

Water activity (aw) is a parameter that measures the availability of water in fish flesh. It is expressed as the ratio of water vapour pressure in fish/vapour pressure of pure water at the same temperature and pressure. Aw varies from 0 to 1.

Chorkor oven for smoking fish
Chorkor oven for smoking fish
FAO/FIIU Photo Library

Water is necessary for microbial and enzymatic reactions and several preservation techniques have been developed to tie up this water (or remove it) and thus reduce the aw. These include drying, salting, smoking, freeze-drying, the use of water binding humectants and a combination of these. Some of these techniques, such as drying, salting and hot smoking, have been used for thousands of years. They can be implemented very simply, e.g. by salting, solar drying, or using fully automated equipment with temperature control, relative humidity, etc.

Techniques based on the physical control of microbial fish loads, its chemical and enzymatic activity

These physical methods use heat (cooking, blanching, pasteurizing, sterilizing), ionizing irradiation (for pasteurization or sterilization) or microwave heating. Cooking or pasteurizing are processes that do not allow complete inactivation of microorganisms and thus often need to be combined with refrigeration to preserve fish products and increase their shelf life. This is not the case of sterilised products and which are stable at ambient temperatures (< 40°C). These require packaging in metal cans or retortable pouches before the heat treatment, thus the term "canning".

Techniques based on the chemical control of microbial activity and loads

These techniques are designed to add anti-microbial agents or decrease the fish muscle pH to levels that are inhibitory to microbial growth and proliferation. Most bacteria stop multiplying at pH < 4.5. The decrease of pH is obtained by fermentation, marinades or by adding acids (acetic, citric, lactic, etc.) to fish products.

Drying fish
Drying fish
FAO/FIIU Photo Library

In addition to the decrease in fish pH, fish fermenting lactic bacteria also produce anti-microbial compounds such as nisin, which improve preservation. This technique is often referred to as bio-preservation. Other preservatives include nitrites, sulphites, sorbates, benzoates or natural ones such as essential oils.

Techniques based on the control of the oxydo-reduction potential

Some spoilage bacteria and lipid oxidation require oxygen. Reducing the oxygen around fish will increase its shelf life. This is possible by vacuum packaging or by controlling or modifying the atmosphere around the fish. Specific combinations of CO2, O2 and N2 characterize controlled (CA) or modified atmosphere (MA). Vacuum packaging, CA and MA storage are often combined with refrigeration for fish preservation

Combination of several preservation techniques

Two or more of the above-described techniques can be combined to improve preservation efficiency while reducing undesirable effects such as the denaturation of nutrients by severe heat treatments. Combinations already in use include pasteurization-refrigeration, CA (or MA)-refrigeration, salting-drying, salting-smoking, drying-smoking and salting-marinating. Other process combinations are currently being developed along the "multiple hurdle theory".

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