Immersion & spray freezer

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An Assignment on Immersion & Spray
Freezer
Introduction:
Freezing the most important thing in fish processing. Freezing is most effective way
to slow down spoilage of fish. Freezing extends shelf life of fish by 30-50%. There are
different type of freezing are used in processing industries. Inthis assignment we will
know about immersion freezer and spray freezer.
Importance of this study:
 To know about immersion freezer.
 To know about spray freezer in details.
 To know about the working principle of immersion and spray freezers.
 To know the various freezing types.
 To know the uses of immersion and spray freezer.
Freezing:
In the simplest terms refrigeration means removal of heat from the body or
atmosphere, which is desired to be cooled and its transfer to another medium. It is
usually accomplished by the evaporation of a liquid refrigerant, which extracts heat
from the medium to be cooled. The refrigeration cycle involves steps to remove the
heat from the evaporating refrigerant by again converting it into the liquid state in
order that it may be used repeatedly in a continuous process.

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Various types of freezers:
There are now many different types of freezer available for freezing fish, and freezer
operators are often uncertain about which type is best suited to their needs. Three
factors may be initially considered when selecting a freezer; financial, functional and
feasibility.
The three basic methods of freezing fish are:
1. Blowing a continuous stream of cold air over the fish - air blast freezers.
2. Direct contact between the fish and a refrigerated surface - contact or plate
freezers.
3. Immersion in or spraying with a refrigerated liquid - immersion or spray
freezers.
Immersion freezer:
In this method freezing is achieved by immersion in, or spraying with, a refrigerant
that remains liquid throughout the process. Refrigerated aqueous solutions of
propylene glycol, glycerol, sodium chloride, calcium chloride and mixtures of sugars
and salt can be used as medium for freezing. Immersion freezing allows for intimate
contact of every surface of the material with the freezing medium and thus ensures
very efficient heat transfer. Food products are directly immersed in a bath of liquid
nitrogen. By immersing, the product is very quickly crusted, which eases further
freezing of separate or individual products. Dependent on the nature of the product,
on the size and the desired result (either crust freezing or deep freezing) the time
varies from a few seconds up to one minute.
It is more efficient to use liquid instead of air as a means of removing heat from the
product. However, most liquid refrigerants, otherwise suitable for this use, are not
acceptable as they may introduce a health hazard. One liquid in general use is

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sodium chloride (common salt) brine. This is used for freezing large fish such as tuna.
The amount salt absorbed by the fish is small and may be allowed for when the
method of freezing is to totally immerse the fish in the brine until the fish is frozen.
This has limited application, but the next freezer could be said to be a modification
of this method and has a wider use.
Fig.01: Immersion freezer.
Merits of immersion freezer
i. Ideally suitable for freezing unpackaged fish and shrimp on vessel;
ii. Transfer of heat from fish to a surrounding cold liquid is better than from a
fish to cold air, as in blast freezer;
iii. No chance of freeze burn.
iv. Frozen samples handled under cold dry nitrogen atmosphere.
v. Frozen samples handled under cold dry nitrogen atmosphere.

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Disadvantages:
i. There is no standard design and vary considerably with each application.
ii. Products are difficult to handle inside the tank;
iii. Immersion solution will penetrate into the fish and may adversely affect
the quality if temperature of the medium and time of immersion are not
precisely maintained.
SPRAY FREEZERS (LIQUID REFRIGERANT FREEZERS)
This type of freezer uses a specially purified form of the refrigerant R.12 (dichloro-
difluoro-methane).
To use the freezer, fish is loaded onto an inlet conveyor. While on this conveyor, the
fish is sprayed with refrigerant. This hardens the fish by freezing the surface.
At the end of inlet conveyor, fish drops into a tank of liquid refrigerant. The fish is
then moved through the tank to the outlet conveyor. Any surplus refrigerant drains
off the frozen product while on the outlet conveyor, and returns to the tank. This is
shown in Figure.

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The heat from the fish gained by the refrigerant causes it to evaporate. The vapour
is then recovered by a refrigeration system and returned to the tank as liquid
refrigerant.
Though this system costs more to operate than air blast freezers it is cheaper than
using liquid nitrogen which we will look at later. There is of course some loss of
refrigerant which means the system has to be topped up during production. Some
of this loss is due to 'carry-over' with the product. This may cause a problem. If the
product is to be sprayed with water. Immediately after freezing, the water will
freeze when it touches the fish and produce a coating of ice. This will trap any
remaining refrigerant between the ice and the fish. When the fish defrosts, there
may be a froth formed as the refrigerant warms up and changes into vapour.
These are sometimes called cryogenic freezers because they work at very low
temperature. These are the most common liquefied gas freezers in use in the fish
industry. A typical freezer of this type is shown in Figure.

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The freezer is basically a long, tunnel shaped, insulated box. Moving through the
tunnel is a stainless steel conveyor on which fish is laid.
At the sprays, temperature can be as low as-196°C. If the fish were not pre-cooled
before reaching these very cold sprays it would be damaged. The fish would cool
and freeze so fast that it would crack. This would mean a poor quality product on
thawing.
In the spraying zone, the liquid nitrogen comes into contact with the fish. The liquid
nitrogen takes the remaining heat to be removed from the fish very, very quickly.
This heat causes the liquid nitrogen to boil and the nitrogen flashes off to become
a gas and is drawn down the tunnel towards the extraction flue.
Good Points for Spray freezer:
• Freezing is very fast;
• It is non-poisonous;
• The freezer is relatively small;
• The only moving part is the conveyor belt, so maintenance is minimal;
• The capital cost of the freezer is relatively low.
Possible problems with Spray Freezer:
• Liquid nitrogen is expensive and must be bought in so running costs are high;
• Space is required for a pressure vessel to store the liquid nitrogen.

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Conclusion:
In this assignment we have learnt about the important features of immersion
freezer and spray freezer used in fish plant. We have learnt about the cooling
system working mechanism of these freezers. We also have learnt about the merits
and demerits of immersion and spray freezer.

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References:
 Hall GM, (1997), Fish processing technology Springer, ISBN 978-0-7514-
0273-5.
 Luten JB, Jacobsen C and Bekaert K, (2006), Seafood research from fish to
dish: quality, safety and processing of wild and farmed fish Wageningen
Academic Publishers. ISBN 978-90-8686-005-0.
 United Nations Development Fund for Women (1993), Fish processing Food
Technology Source Book Series (UNIFEM) Series, ISBN 978-1-85339-137-8.
 Bremner HA (2003), Safety and Quality Issues in Fish Processing Woodhead
Publishing Limited, ISBN 978-1-85573-678-8.
 Cutting CL, L. Hill (1955), Fish saving; a history of fish processing from ancient
to modern times.
 Nowshad Alam AKM; Participatory Training of Trainers; (129-131).