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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.
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. 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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