44
of the belt causes the system to occupy little space. In addition, spiral belt freezers
are used for the freezing of packaged amorphous products (Gokoglu 2002;
Cemeroglu et al. 2003; Rahman and Velez-Ruiz 2007).
Immersion Freezing
In this method, the product which is packaged or un-packed is immersed in a suitable liquid cooled to low degrees, or the liquid is sprayed onto the product. When
freezing the product without packaging, a rapid heat transfer between the product
and the cooler is ensured. In addition, many products which have not in a distinct
shape and as in particulate form can be successfully frozen by this method. The
number of refrigerants that can be used in this method is limited. The reason for this
is some of the features required in the freezing fluid. Freezing liquid should not
freeze at low temperatures, not have toxic effect, not contain foreign colour, taste,
and odour, not change the colour of the product, and have a sanitary quality. Most
importantly, the liquid to be used must be compatible with the sensory properties of
the product to be frozen. The fluids usually used are salt solutions (sodium chloride), sugar solutions and glycol and glycerol solutions. The most suitable liquid
used for freezing seafood is salt water (Gokoglu 2002).
Cryogenic Freezing
Liquefied gases with a very low boiling point are called cryogenic liquids. The most
used cryogenic liquids are liquid nitrogen and liquid carbon dioxide. Of these, the
liquid nitrogen boils at −196 °C and the liquid carbon dioxide at −145 °C. In this
method, direct contact between the product and the cryogenic liquid is achieved and
a very rapid freezing takes place. In cryogenic freezing, liquefied gases are in direct
contact with foods. The product is exposed to atmospheres below −60 °C by direct
contact with liquid nitrogen or liquid carbon dioxide or vapours. This is a very
quick-freezing method. Rapid formation of small ice crystals reduces damage
caused by cell rupture, preserves colour, texture, aroma, and nutritional value. Fast
freezing also reduces weight loss via evaporation, provides high product yield and
needs less space. The most used liquid nitrogen from cryogenic liquids is produced
by compressing the air into a liquid and then distilling the nitrogen through a special
valve to remove the nitrogen gas from the oxygen. The nitrogen gas produced is recompressed and liquid nitrogen gas is obtained. Liquid nitrogen gas remains liquid
at atmospheric pressure and only a small portion of it converts into nitrogen gas at
−196 °C. Liquid nitrogen is not toxic, does not react with the product, prevent oxidative reactions, does not need a separate cooling equipment and provides very fast
freezing and for these reasons it is preferred priority. Liquid carbon dioxide is
obtained by compressing the carbon dioxide gas under high pressure. The boiling
point is −145 °C. The product is applied by spraying on liquid carbon dioxide product as it passes through a tunnel. In some systems, solid carbon dioxide (dry ice) is
placed on the conveyor belt before the product is placed on it and then sprayed with
liquid carbon dioxide. It is possible to find carbon dioxide in any form at any time.
2 Crustacean Shellfish
of the belt causes the system to occupy little space. In addition, spiral belt freezers
are used for the freezing of packaged amorphous products (Gokoglu 2002;
Cemeroglu et al. 2003; Rahman and Velez-Ruiz 2007).
Immersion Freezing
In this method, the product which is packaged or un-packed is immersed in a suitable liquid cooled to low degrees, or the liquid is sprayed onto the product. When
freezing the product without packaging, a rapid heat transfer between the product
and the cooler is ensured. In addition, many products which have not in a distinct
shape and as in particulate form can be successfully frozen by this method. The
number of refrigerants that can be used in this method is limited. The reason for this
is some of the features required in the freezing fluid. Freezing liquid should not
freeze at low temperatures, not have toxic effect, not contain foreign colour, taste,
and odour, not change the colour of the product, and have a sanitary quality. Most
importantly, the liquid to be used must be compatible with the sensory properties of
the product to be frozen. The fluids usually used are salt solutions (sodium chloride), sugar solutions and glycol and glycerol solutions. The most suitable liquid
used for freezing seafood is salt water (Gokoglu 2002).
Cryogenic Freezing
Liquefied gases with a very low boiling point are called cryogenic liquids. The most
used cryogenic liquids are liquid nitrogen and liquid carbon dioxide. Of these, the
liquid nitrogen boils at −196 °C and the liquid carbon dioxide at −145 °C. In this
method, direct contact between the product and the cryogenic liquid is achieved and
a very rapid freezing takes place. In cryogenic freezing, liquefied gases are in direct
contact with foods. The product is exposed to atmospheres below −60 °C by direct
contact with liquid nitrogen or liquid carbon dioxide or vapours. This is a very
quick-freezing method. Rapid formation of small ice crystals reduces damage
caused by cell rupture, preserves colour, texture, aroma, and nutritional value. Fast
freezing also reduces weight loss via evaporation, provides high product yield and
needs less space. The most used liquid nitrogen from cryogenic liquids is produced
by compressing the air into a liquid and then distilling the nitrogen through a special
valve to remove the nitrogen gas from the oxygen. The nitrogen gas produced is recompressed and liquid nitrogen gas is obtained. Liquid nitrogen gas remains liquid
at atmospheric pressure and only a small portion of it converts into nitrogen gas at
−196 °C. Liquid nitrogen is not toxic, does not react with the product, prevent oxidative reactions, does not need a separate cooling equipment and provides very fast
freezing and for these reasons it is preferred priority. Liquid carbon dioxide is
obtained by compressing the carbon dioxide gas under high pressure. The boiling
point is −145 °C. The product is applied by spraying on liquid carbon dioxide product as it passes through a tunnel. In some systems, solid carbon dioxide (dry ice) is
placed on the conveyor belt before the product is placed on it and then sprayed with
liquid carbon dioxide. It is possible to find carbon dioxide in any form at any time.
2 Crustacean Shellfish