PROCESSING OF MARINE
DEPARTMENT OF FOOD AND DAIRY TECHNOLOGY
• The freezing process alone is not a method of preservation. It is merely the means of preparing the
fish for storage at a suitably low temperature. In order to produce a good product, freezing must be
accomplished quickly. A freezer requires to be specially designed for this purpose and thus freezing
is a separate process from low temperature storage.
• As with any type of food it is important to handle seafood safely to reduce the risk
of foodborne illness, often called “food poisoning.” Follow these safe handling tips
for buying, preparing, and storing fish and shellfish – and you and your family
can safely enjoy the fine taste and good nutrition of seafood.
FREEZING OF STORAGE SEAFOOD
• Only buy fish that is refrigerated or displayed on a
thick bed of fresh ice (preferably in a case or under
some type of cover). Because the color of a fish can be
affected by several factors including diet, environment,
treatment with a color fixative such as carbon
monoxide or other packaging processes, color alone is
not an indicator of freshness. The following tips can
help you when making purchasing decisions:
• Fish should smell fresh and mild, not fishy, sour, or
• A fish’s eyes should be clear and shiny.
•Whole fish should have firm flesh and red gills with no odor. Fresh
fillets should have firm flesh and red blood lines, or red flesh if fresh
tuna. The flesh should spring back when pressed
•Fish fillets should display no discoloration, darkening, or drying
around the edges.
•Shrimp, scallop, and lobster flesh should be clear with a pearl-like
color and little or no odor.
•Some refrigerated seafood may have time/temperature indicators
on their packaging, which show if the product has been stored at the
proper temperature. Always check the indicators when they are
present and only buy the seafood if the indicator shows that the
product is safe to eat.
•Fresh fish and fish fillets sold as “Previously Frozen” may not have
all the characteristics of fresh fish (e.g., bright eyes, firm flesh, red
gills, flesh, or bloodlines), however, they should still smell fresh and
mild, not fishy, sour, or rancid.
GUIDELINES ON FREEZING
• Look for the label: Look for tags on sacks
or containers of live shellfish (in the shell)
and labels on containers or packages of
shucked shellfish. These tags and labels
contain specific information about the
product, including the processor’s
• Discard Cracked/Broken Ones: Throw
away clams, oysters, and mussels if their
shells are cracked or broken.
• Do a “Tap Test”: Live clams, oysters, and mussels will close when the shell is tapped. If they
don’t close when tapped, do not select them.
• Check for Leg Movement: Live crabs and lobsters should show some leg movement. They
spoil rapidly after death, so only live crabs and lobsters should be selected and prepared.
Fish is largely water, normally 60-80 percent depending on the species, and the freezing process converts most of
this water into ice.
Freezing requires the removal of heat, and fish from which heat is removed falls in temperature in the manner shown
in Figure 1. During the first stage of cooling, the temperature falls fairly rapidly to just below 0°C, the freezing point of
water. As more heat requires to be extracted during the second stage, in order to turn the bulk of the water to ice, the
temperature changes by a few degrees and this stage is known as the period of "thermal arrest". When about 55% of
the water is turned to ice, the temperature again begins to fall rapidly and during this third stage most of the
remaining water freezes. A comparatively small amount of heat has to be removed during this third stage.
FROZEN OF SEAFOOD
• Frozen seafood can spoil if the fish thaws during transport and is
left at warm temperatures for too long before cooking.
• Don’t buy frozen seafood if its package is open, torn, or crushed
on the edges.
• Avoid packages with signs of frost or ice crystals, which may
mean the fish has been stored a long time or thawed and
• Avoid packages where the “frozen” fish flesh is not hard. The fish
should not be bendable.
Put seafood on ice or in the refrigerator or freezer soon after buying it. If seafood will be used within
2 days after purchase, store it in a clean refrigerator at a temperature of 40°F or below. Use a
refrigerator thermometer to check! Otherwise, wrap it tightly in plastic, foil, or moisture-proof paper
and store it in the freezer.
frozen storage of fish can give a storage life of more than one year, if properly carried out. It has
enabled fishing vessels to rem ain at sea for long periods, and allowed the stockpiling of fish during
periods of good fishing and high catching rates, as well as widened the market for fish products of
The mechanism by which frozen fish deteriorates is somewhat different from that causing spoilage of
chilled fish. Provided the temperature is low enough - below -10°C bacterial action will be stopped by
the freezing process. Chemical, biochemical and physical processes leading to irreversible changes
will still occur, but at a very slow rate. Deterioration during frozen storage is inevitable, and in order
to obtain satisfactory results, fish for freezing must be of good quality.
The rate of oxidation can be reduced by reducing the exposure to
oxygen. This can be achieved by introducing a barrier at the surface of
the fish. Thus fish in a block keep better than fish frozen individually,
and the addition of an ice glaze is beneficial. Glazing is carried out after
freezing by brushing or spraying chilled water onto the surface of the
fish or by dipping in cold water. Packaging materials, impermeable to
moisture and oxygen can be effective, especially if vacuum packaging
Some transfer of moisture from the product is unavoidable during
freezing and frozen storage, which leads to dehydration of the fish.
Good operating conditions are essential in order to keep dehydration to
It has been clearly established that fluctuating cold store temperatures
are a major cause of dehydration. In practice the more severe cases of
drying occur during frozen storage rather than during freezing. In
extreme dehydration the frozen fish acquires a dry wrinkled look, tends
to become pale or white in colour and the flesh become spongy. This
characteristic appearance is called, inappropriately, 'freezerburn'.
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