Introduction:
Fish freezing is a process of preservation. In this method, inside water of fish tissue and fishery product is turned into ice at lower temperature. This method increases the shelf life of fish and fishery product. This method is used world wide for controlling the quality of fish and fishery product at the time of preservation.
Freezing Methods and Equipment
There are several ways in which fish can be frozen, but all require specialized equipment to effect the necessary rapid drop in temperature and reduce the core temperature sufficiently to ensure that the product can be safely placed into cold storage. If unfrozen fish are simply put into a cold store running at -300c, the temperature will be reduced sufficiently quickly and poor quality frozen fish will result; cold stores are designed to hold fish already frozen, at low temperatures. The types of freezer in common use can be divided into the following three categories:
a) Type 1: Those in which a stream of cold air absorbs heat energy from the product as it passes over them, thereby reducing their temperature. These are generally known as air blast freezers.
b) Type 2: Those in which heat is absorbed when the products are placed in contact with a refrigerated surface . These are generally known as contact freezer.
c) Type 3: Those in which heat is rapidly absorbed from products surrounded by a cold liquid or a spray of liquid. These are generally known as immersion freezers.
Type 1
Air blast freezers : Air blast freezers are generally small rooms or tunnels in which cold air is circulated by fans over and evaporator, colded by ammonia, brine or other refrigerants around the products to be frozen. The freezers are loaded at one time by rolling or pulling a rack of shelves of fish into the insulated rooms. Now a days conveyer have been used to move fisheries products through the rooms or tunnels. Most freezers of this type operate at air temperature of -400c. The velocity of air moving over the products generally varies between 5 m/s and 10 m/s to give most economic freezing.
Fig: Air blast freezer
The main advantage of air blast freezer is that they can be used to freeze different sizes and shapes of product without adaptation and are therefore versatile. They are economic in operation under full capacity utilization and lesser degree of handling of products is required during freezing. However, they tend to occupy more space then other freezers and consume more energy. They because of their versatility, they are often used in efficiently as well. The greater energy consumption can be attributed to their larger size, greater internal volume, heat transfer through the insulation, and air circulation system. As the products are often exposed, it may occur dehydration, freeze burn and desiccation of the products. Packaging of fish fillets often undergoes bulging. Sometimes discoloration of products occurs at low temperature and high velocity of air.
As the name suggest, air blast freezer operate by blasting refrigerated air over a product to remove heat. Air is usually blown by means of a fan through a cooling coil [evaporator] to produce cold air. This cold air then passes over the product which needs to be frozen, picks up heat from the product and the walls of the freezer and returns to the evaporator reduction of temperature. The ion needs to be efficient, and there are several factors which may affect the freezing rate of fish.
a) Air speed: Although a faster freezing rate is achieved with a faster air speed, a lot of energy is expended through convection.A compromise between high costs and low freezing rates is therefore necessary. Air speeds are usually about 5 m/s, but this can be increased to or even 15 m/s in continuous airs where size is a limiting factor.
b) Temperature: If the temperature of the air as it passes over the product is too high,the freezing rates of the more remote products between be reduced. Normal air temperatures are between -300c and -400c with an acceptable average rise of between 1 and 30c as the air passes over the product.
c) Air flow: Uniform air flow is essential for efficient operation. Fans should always be positioned so that they can blow air through the cooler.It is important to stack and layer the product correctly on the freezing trays or trolleys. Air will always take the easiest path .If a blast freezer is only partly filled, it is best to load it so that the air passes evenly over fill the product. Even spacing throughout the freezer will ensure even air flow and uniform freezing rates. If the freezer has a very small load it might be advisable to block of sum of the areas of air flow completely so that maximum use can be made of the air flow.
Air blast freezer can be divided into continuous and batch type, depending on their mode of operation.
Batch freezers
In this freezers, trolleys, shelves or plates are used for loading the products in batches. When fully loaded, they are placed in the freezer, the door is closed and the freezing is started. When fully frozen the first batch is removed for cold storage and another batch is loaded into the freezing compartment. Compared to continuous freezer, batch freezer have a very high refrigeration load at the beginning of the cycle because of their mode of operation.
Continuous air blast freezer
These can be divided into batch continuous freezers and continuous belt freezers.
Batch continuous freezers
These freezers are those in which fish packed on to trolleys or tracts move through the freezers on rail.The product move through the freezer in the opposite direction to the flow of air,i.e.,the coldest air passes over the coldest products. Loaded trolleys are pushed into the freezer at one end, and when the product is fully frozen, they are removed from the other end to make room for a fresh batch at the entry point.
There are also batch continuous freezers in which the transport of fish through the chamber is mechanized. Multishelved racks are suspended from and over head conveyor which moves the fish through and air freezing tunnel. The speed of transport through the tunnel can be varied to suit the type of product being frozen, i.e., small fish, fillets or prawn would go through the tunnel in about 1-2 h wheres large whole fish or pre-packed products would need much longer.
Continuous belt freezers
These are used for small individual items which can be frozen within 30 min; longer freezing items would require longer belts which would be cumbersome and costly. Double or treble belt may be installed if the product can be transferred easily between belts. Spiral belt freezers in which a long belt can be compacted into a relatively limited space are now available.
Belts are usually made from stainless steel or plastic interlocking mesh. Air flow may be either counter to the flow of the product, or across the belt. The points entry and exit for the product must be protected against loss of refrigerated air and entry of warm air. This is usually achieved with plastic or rubber curtains. The belt speed can be altered to cater for the different freezing times of different products. Belt freezers need to be fully loaded and used continuously as much as possible in order to be economic.
Fig: Air blast freezer
Merits of air blast freezer
I. Main advantage of air blast freezer is its versatility.
II. The freezer is very economical.
III. Only little handling of fish is involved during freezing.
Demerits
I. May occur dehydration, desiccation, shrinkage and freeze burn of the products;
II. Discolouration of product may happen at low temperature or high velocity of air.
Type 2
Contact freezers
Freezing by placing the products in direct contact or plate freezer are commonly used in the fishing industry for the production of blocks of fish or other seafood.The two main types of plate freezers are horizontal plate freezer and vertical Plate freezer. In both types, the products are brought into close contact with aluminium alloy plates which contain circulating refrigerant. All plate freezers have hydraulic system which move the plates closer or further apart. This allows the products to be compacted so they freeze more quickly by closer contact, and are released more quickly after freezing.
Horizontal plate freezers
In this type of freezer , the product is packed into freezing trays before freezing. The trays may either be lined with polyethylene sheet prior to packing, or cartoons may be used. Depending on the product, the aluminium trays may divided into compartments to give uniform block sizes, The trays, which may also have closely fitting lids , are then placed on the freezer shelves. The hydraulic system positions the plates in close contact with the top and bottom of the trays to ensure maximum heat exchange. Trays and cartons should be filled to the top to ensure good contact with both plate.
It is important that the plates are kept free of ice and frost in order to ensure good contact between the plate and the product. If water is spilt on the plates, it will freeze in lumps, cause poor contact, and prolong freezing . With careful loading and unloading, it should be possible to avoid ice build-up so that defrosting between freezing batches is unnecessary. With care, defrosts should only be required once or twice a day. Defrosted plates must be free of ice and must be dried before re-use.
Similarly, compression is important to ensure good contact between the food and the plates. However, excessive pressure will damage the food. It is essential that the plates should be fully loaded; if this is not possible, spacing bars should be used. If a plate is only partially loaded when the product is compressed, the plates may bend, causing poor contact and damage to the plates. Spacing bars should be made as per the specifications of the manufacturer for the thickness of product being frozen. Generally, spacers are slightly thinner than the packs to be frozen so that the blocks are slightly compressed and good contact is ensured. All the blocks of product to be frozen must be of the same thickness; this ensures that contact with both sides of the block is correct, all the blocks freeze at the same rate, and the plates are not subject to stress and distortion.
Horizontal plate freezers are used mainly in shore- based fish freezing plants, although they are sometimes installed on board ships for freezing prawns at sea. If the plant is installed within a processing line, the loading and unloading of the freezer is often done from opposite sides to speed up the process, increase efficiency, and reduce the time the doors of the freezer cabinet are open and thus, frosting of the plates.
Vertical plate freezers
This type of freezer is particularly suitable for freezing fish at sea. It was originally designed for use on board trawlers for freezing cod as a whole gutted product. After freezing ,the fish are kept in a cold store for later on shore processing. The freezer consists of a series of vertical, aluminium alloy, refrigerated plates with spaces known as stations between them. The refrigerant passing through the plates reduces their surface temperature to between -300C and -400C when in operation. Warm fish are dropped between the plates until each station is full. The plates are then closed together to form the fish into blocks; the refrigerant flowing through the plates then freezes the fish. Fish of similar sizes should be packed into each block; mixed sizes will lead to uneven freezing rates and may damage smaller fish. As these freezers are often used on fishing vessels the fish to be loaded may be very fresh. Consequently, they may not yet have passed through rigor, or they may be in rigor when they are put in the freezer. If they are already in rigor, no attempt should be made to straighten them to make them fit between the plates as this will tear and damage the muscle structure. Freezing pre-rigor can cause serious problems on thawing. If fish are frozen pre-rigor and stored for only a short time[less than eight weeks at -180C for cod], they will not have come out of rigor when they are thawed, and a large amount of water, known as thaw drip, will be lost from the fish at this stage. This can be overcome by thawing the fish slowly at ambient temperature so that rigor is resolved before thawing is complete. If fillets are cut from a fish still in rigor, the meat will contract as it is released from a fish still in rigor, the meat will contract as it released from the bone; the fillets will then shrink and have a corrugated surface and a tougher texture when cooked and eaten.
Once frozen, the blocks are released from the stations by a partial defrost. They can either be removed from the side of the station by raising a plate at the bottom , or they may be dropped through the bottom directly into the fish room below. In contrast to the horizontal plate freezer, the plates must be above freezing point before loading, otherwise the fish will stock to them and prevent full loading. The blocks must be handled carefully on removal from the freezer so that they remain in one piece. It is not normal for water to be added to the block to assist in block formation, but if it is necessary, plastic film or paper liner should be placed in the station to hold the water in the block.
Fish loaded into a freezer on board a boat will usually be at ambient temperature. If a horizontal plate freezer is used, the fish are often chilled before loading. This means that the initial refrigeration load on a vertical plate freezer.
Advantages of contact freezers
I. Very rapid and effective cooling;
II. Suitable for block and packaged fishery products
III. No chance of dehydration, shrinkage or freeze burn
Demerits
I. Ice on plate and open space inside the block may loose contact and deform the products.
Type 3
Immersion and spray freezers
Immersion and spray freezers operate by maintaining direct contact between a very cold liquid,or gas from that liquid and the fish.
The various liquids which have been used can be divided into:
a. solutions of salts in water which are mechanically refrigerated to well below freezing point; and
b. pressurized liquids which boil at very low temperatures.
The first type relies on the mechanical refrigeration of the liquid and the circulation of that liquid through a tank containing the products to be frozen. In the fishing industry, the commonest type of freezer in this category uses sodium chloride brine which can be made to any strength to achieve the desired freezing point. However, saturated brine, which contains 26% salt, freezers at -210C and in practice, the operating temperature cannot be reduced below about -180C. This limits the use of this freezer as the temperature needs to be -300C or below for most freezing processes. However, even with brine it is possible to achieve rapid freezing because of the intimate contact between the product and the freezing medium. The rate of freezing depends on the circulation of the refrigerated brine throughout the tank, and this can be a critical factor in the efficiency of a brine freezing system. Another disadvantage of using brine freezing is that the fish will absorb salt. The amount of salt taken up depends on several factors including the temperature of the brine, the length of time of immersion, the fat content of the fish and the surface area.
The brine freezing process is not widely used because of the technical limitations. Its most common application is for on-board freezing of tuna destined for canning. Newly caught fish are chilled in a tank containing sea water refrigerated to about 00C. They are held in chill until the tank has been filled by the addition of subsequent catches, or for up to five days. Salt is then added to the tank to produce a freezing-brine and the temperature is reduced to freeze the fish. When they are frozen [which takes at least 24 h], the tank is pumped dry and used for storage until landing. The fish may thaw, or partially thaw on board the boat as it returns to base so that they are ready for immediate unloading. The sea water and brine is usually refrigerated by a series of pipe
Fig: Spray freezer.
girds on the tank sides which may be supplemented by an external heat exchanger.
The second type of immersion freezer uses liquids with low boiling points such as liquid nitrogen, carbon dioxide or liquid air. The most common is liquid nitrogen. This is used for freezing individual filets and for prawns which sell for a relatively high price when sold as individual quick frozen product. Liquid nitrogen boils at -1960C. If the warm fish are brought into direct contact with the liquid nitrogen, the outer layers would freeze very rapidly; this would cause thermal stress and the break-up of the material.Therefore, the fish are normally sprayed first with nitrogen gas at about -500C, and then gradually reduced to much colder temperatures by spraying with liquid nitrogen.
The freezer consists of a stainless steel conveyor belt, which moves against the flow of gas and liquid spray in a freezing tunnel. Nitrogen freezers are small and freeze very quickly. They can be operated without the need for an on-site mechanical refrigeration system. However, in spite of the apparent advantages, the freezers are used only for specialty products. This is because first, operating costs are higher than air blast freezers, secondly, they can only freeze small products , and thirdly, they rely entirely on a regular and reliable supply of liquid nitrogen which requires special storage. These disadvantages severely limit their use in developing countries.
The other types of low boiling point liquid freezers are not so common. At one time, refrigerant 12 [dischlorodifluoromethane] in a specially purified form was used for immersion/ spray freezing. However, the high costs and concerns over the effects of R12 on ozone depletion in the atmosphere have led to a rapid decline in its use. Liquefied carbon dioxide [CO2]is used to ta a limited extent for freezing. The gas under pressure in cylinders is sprayed onto the product in much the same way as liquid nitrogen. The management of CO2 freezing is more difficult than liquid nitrogen and its use is less widespread. Normally, liquid CO2 is refrigerated before spraying.
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.
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.
Sharp freezing
The sharp freezer consists of an insulated room, usually maintained at -250C, containing a number of shelves, made from pipe coils through which cold brine or ammonia or other refrigerants circulates. Fish frozen by this method are placed directly on the shelves or aluminium pans/plates covering the pipe coils. Sharp freezing is largely out-dated and its use is limited. It is a slow freezing method.
Fig: Sharp freezer
Some other freezers
Due to modernization of freezing equipment, depending upon the nature and shape of the products, various other freezers are used in seafood processing industry. Some of them are as follows:
Band freezers
Band freezers are designed to freeze small layer of pastes or fish fillets whose thickness is around 15 mm. The product is allowed to press between two plates and freeze very rapidly. The plate is usually placed in a polythene bag to avoid sticking to the metal plates.
Spiral freezer
Spiral freezer is basically an air blast freezer, usually a round chamber in which cold refrigerant blows over wire mesh belt that moves spirally and carries individual items to be frozen. The freezer is used to freeze balanced or cooked IQF shrimp or fish products. Freezing temperature is usually maintained within the range of -300C to -400C,depending on the thickness of the items to be frozen. Spiral freezer is very effective for IQF products as it freezes very quickly, as quickly as between 15 and 60 minutes.
Fig: spiral freezer
Rotary freezer
Rotary freezer is a high capacity freezer that can handle about 10 to 15 tons of fish a day. It is mostly used in commercial seafood processing, where products of thickness up to 8 cm and length/width 80 50 cm be cooled down to -110F in two hours. The freezer applies pressure to the products being frozen and is fully automatic in operation.
Cryogenic freezer
Freezer that uses liquid nitrogen at -1960C [-320C] is called cryogenic freezer. This is essentially a single belt freezer and the refrigerant is sprayed over the items on the belt. The nitrogen evaporates and is allowed to escape into atmosphere after the vapours are used to cool the fish/ shrimp. Alternatively, the material can be dipped into liquid nitrogen, but this soften results in development of cracks on the surface. For freezing 1 kg of fish 1.0 to 1.5 kg of liquid nitrogen is needed. So, only cheaply available nitrogen can make this process economically viable. Solid carbon dioxide, dichloromethane and R12, with boiling point -220F can also be used to freeze fish in cryogenic freezer.
Fig: cryogenic freezer
Conclusion:
Knowledge about freezing method and equipments is very important for our fisheries sector .Because fish is a perishable food. Preservation is essential for increase fish and fishery products shelf life and sustainable food supply
Fish freezing is a process of preservation. In this method, inside water of fish tissue and fishery product is turned into ice at lower temperature. This method increases the shelf life of fish and fishery product. This method is used world wide for controlling the quality of fish and fishery product at the time of preservation.
Freezing Methods and Equipment
There are several ways in which fish can be frozen, but all require specialized equipment to effect the necessary rapid drop in temperature and reduce the core temperature sufficiently to ensure that the product can be safely placed into cold storage. If unfrozen fish are simply put into a cold store running at -300c, the temperature will be reduced sufficiently quickly and poor quality frozen fish will result; cold stores are designed to hold fish already frozen, at low temperatures. The types of freezer in common use can be divided into the following three categories:
a) Type 1: Those in which a stream of cold air absorbs heat energy from the product as it passes over them, thereby reducing their temperature. These are generally known as air blast freezers.
b) Type 2: Those in which heat is absorbed when the products are placed in contact with a refrigerated surface . These are generally known as contact freezer.
c) Type 3: Those in which heat is rapidly absorbed from products surrounded by a cold liquid or a spray of liquid. These are generally known as immersion freezers.
Type 1
Air blast freezers : Air blast freezers are generally small rooms or tunnels in which cold air is circulated by fans over and evaporator, colded by ammonia, brine or other refrigerants around the products to be frozen. The freezers are loaded at one time by rolling or pulling a rack of shelves of fish into the insulated rooms. Now a days conveyer have been used to move fisheries products through the rooms or tunnels. Most freezers of this type operate at air temperature of -400c. The velocity of air moving over the products generally varies between 5 m/s and 10 m/s to give most economic freezing.
Fig: Air blast freezer
The main advantage of air blast freezer is that they can be used to freeze different sizes and shapes of product without adaptation and are therefore versatile. They are economic in operation under full capacity utilization and lesser degree of handling of products is required during freezing. However, they tend to occupy more space then other freezers and consume more energy. They because of their versatility, they are often used in efficiently as well. The greater energy consumption can be attributed to their larger size, greater internal volume, heat transfer through the insulation, and air circulation system. As the products are often exposed, it may occur dehydration, freeze burn and desiccation of the products. Packaging of fish fillets often undergoes bulging. Sometimes discoloration of products occurs at low temperature and high velocity of air.
As the name suggest, air blast freezer operate by blasting refrigerated air over a product to remove heat. Air is usually blown by means of a fan through a cooling coil [evaporator] to produce cold air. This cold air then passes over the product which needs to be frozen, picks up heat from the product and the walls of the freezer and returns to the evaporator reduction of temperature. The ion needs to be efficient, and there are several factors which may affect the freezing rate of fish.
a) Air speed: Although a faster freezing rate is achieved with a faster air speed, a lot of energy is expended through convection.A compromise between high costs and low freezing rates is therefore necessary. Air speeds are usually about 5 m/s, but this can be increased to or even 15 m/s in continuous airs where size is a limiting factor.
b) Temperature: If the temperature of the air as it passes over the product is too high,the freezing rates of the more remote products between be reduced. Normal air temperatures are between -300c and -400c with an acceptable average rise of between 1 and 30c as the air passes over the product.
c) Air flow: Uniform air flow is essential for efficient operation. Fans should always be positioned so that they can blow air through the cooler.It is important to stack and layer the product correctly on the freezing trays or trolleys. Air will always take the easiest path .If a blast freezer is only partly filled, it is best to load it so that the air passes evenly over fill the product. Even spacing throughout the freezer will ensure even air flow and uniform freezing rates. If the freezer has a very small load it might be advisable to block of sum of the areas of air flow completely so that maximum use can be made of the air flow.
Air blast freezer can be divided into continuous and batch type, depending on their mode of operation.
Batch freezers
In this freezers, trolleys, shelves or plates are used for loading the products in batches. When fully loaded, they are placed in the freezer, the door is closed and the freezing is started. When fully frozen the first batch is removed for cold storage and another batch is loaded into the freezing compartment. Compared to continuous freezer, batch freezer have a very high refrigeration load at the beginning of the cycle because of their mode of operation.
Continuous air blast freezer
These can be divided into batch continuous freezers and continuous belt freezers.
Batch continuous freezers
These freezers are those in which fish packed on to trolleys or tracts move through the freezers on rail.The product move through the freezer in the opposite direction to the flow of air,i.e.,the coldest air passes over the coldest products. Loaded trolleys are pushed into the freezer at one end, and when the product is fully frozen, they are removed from the other end to make room for a fresh batch at the entry point.
There are also batch continuous freezers in which the transport of fish through the chamber is mechanized. Multishelved racks are suspended from and over head conveyor which moves the fish through and air freezing tunnel. The speed of transport through the tunnel can be varied to suit the type of product being frozen, i.e., small fish, fillets or prawn would go through the tunnel in about 1-2 h wheres large whole fish or pre-packed products would need much longer.
Continuous belt freezers
These are used for small individual items which can be frozen within 30 min; longer freezing items would require longer belts which would be cumbersome and costly. Double or treble belt may be installed if the product can be transferred easily between belts. Spiral belt freezers in which a long belt can be compacted into a relatively limited space are now available.
Belts are usually made from stainless steel or plastic interlocking mesh. Air flow may be either counter to the flow of the product, or across the belt. The points entry and exit for the product must be protected against loss of refrigerated air and entry of warm air. This is usually achieved with plastic or rubber curtains. The belt speed can be altered to cater for the different freezing times of different products. Belt freezers need to be fully loaded and used continuously as much as possible in order to be economic.
Fig: Air blast freezer
Merits of air blast freezer
I. Main advantage of air blast freezer is its versatility.
II. The freezer is very economical.
III. Only little handling of fish is involved during freezing.
Demerits
I. May occur dehydration, desiccation, shrinkage and freeze burn of the products;
II. Discolouration of product may happen at low temperature or high velocity of air.
Type 2
Contact freezers
Freezing by placing the products in direct contact or plate freezer are commonly used in the fishing industry for the production of blocks of fish or other seafood.The two main types of plate freezers are horizontal plate freezer and vertical Plate freezer. In both types, the products are brought into close contact with aluminium alloy plates which contain circulating refrigerant. All plate freezers have hydraulic system which move the plates closer or further apart. This allows the products to be compacted so they freeze more quickly by closer contact, and are released more quickly after freezing.
Horizontal plate freezers
In this type of freezer , the product is packed into freezing trays before freezing. The trays may either be lined with polyethylene sheet prior to packing, or cartoons may be used. Depending on the product, the aluminium trays may divided into compartments to give uniform block sizes, The trays, which may also have closely fitting lids , are then placed on the freezer shelves. The hydraulic system positions the plates in close contact with the top and bottom of the trays to ensure maximum heat exchange. Trays and cartons should be filled to the top to ensure good contact with both plate.
It is important that the plates are kept free of ice and frost in order to ensure good contact between the plate and the product. If water is spilt on the plates, it will freeze in lumps, cause poor contact, and prolong freezing . With careful loading and unloading, it should be possible to avoid ice build-up so that defrosting between freezing batches is unnecessary. With care, defrosts should only be required once or twice a day. Defrosted plates must be free of ice and must be dried before re-use.
Similarly, compression is important to ensure good contact between the food and the plates. However, excessive pressure will damage the food. It is essential that the plates should be fully loaded; if this is not possible, spacing bars should be used. If a plate is only partially loaded when the product is compressed, the plates may bend, causing poor contact and damage to the plates. Spacing bars should be made as per the specifications of the manufacturer for the thickness of product being frozen. Generally, spacers are slightly thinner than the packs to be frozen so that the blocks are slightly compressed and good contact is ensured. All the blocks of product to be frozen must be of the same thickness; this ensures that contact with both sides of the block is correct, all the blocks freeze at the same rate, and the plates are not subject to stress and distortion.
Horizontal plate freezers are used mainly in shore- based fish freezing plants, although they are sometimes installed on board ships for freezing prawns at sea. If the plant is installed within a processing line, the loading and unloading of the freezer is often done from opposite sides to speed up the process, increase efficiency, and reduce the time the doors of the freezer cabinet are open and thus, frosting of the plates.
Vertical plate freezers
This type of freezer is particularly suitable for freezing fish at sea. It was originally designed for use on board trawlers for freezing cod as a whole gutted product. After freezing ,the fish are kept in a cold store for later on shore processing. The freezer consists of a series of vertical, aluminium alloy, refrigerated plates with spaces known as stations between them. The refrigerant passing through the plates reduces their surface temperature to between -300C and -400C when in operation. Warm fish are dropped between the plates until each station is full. The plates are then closed together to form the fish into blocks; the refrigerant flowing through the plates then freezes the fish. Fish of similar sizes should be packed into each block; mixed sizes will lead to uneven freezing rates and may damage smaller fish. As these freezers are often used on fishing vessels the fish to be loaded may be very fresh. Consequently, they may not yet have passed through rigor, or they may be in rigor when they are put in the freezer. If they are already in rigor, no attempt should be made to straighten them to make them fit between the plates as this will tear and damage the muscle structure. Freezing pre-rigor can cause serious problems on thawing. If fish are frozen pre-rigor and stored for only a short time[less than eight weeks at -180C for cod], they will not have come out of rigor when they are thawed, and a large amount of water, known as thaw drip, will be lost from the fish at this stage. This can be overcome by thawing the fish slowly at ambient temperature so that rigor is resolved before thawing is complete. If fillets are cut from a fish still in rigor, the meat will contract as it is released from a fish still in rigor, the meat will contract as it released from the bone; the fillets will then shrink and have a corrugated surface and a tougher texture when cooked and eaten.
Once frozen, the blocks are released from the stations by a partial defrost. They can either be removed from the side of the station by raising a plate at the bottom , or they may be dropped through the bottom directly into the fish room below. In contrast to the horizontal plate freezer, the plates must be above freezing point before loading, otherwise the fish will stock to them and prevent full loading. The blocks must be handled carefully on removal from the freezer so that they remain in one piece. It is not normal for water to be added to the block to assist in block formation, but if it is necessary, plastic film or paper liner should be placed in the station to hold the water in the block.
Fish loaded into a freezer on board a boat will usually be at ambient temperature. If a horizontal plate freezer is used, the fish are often chilled before loading. This means that the initial refrigeration load on a vertical plate freezer.
Advantages of contact freezers
I. Very rapid and effective cooling;
II. Suitable for block and packaged fishery products
III. No chance of dehydration, shrinkage or freeze burn
Demerits
I. Ice on plate and open space inside the block may loose contact and deform the products.
Type 3
Immersion and spray freezers
Immersion and spray freezers operate by maintaining direct contact between a very cold liquid,or gas from that liquid and the fish.
The various liquids which have been used can be divided into:
a. solutions of salts in water which are mechanically refrigerated to well below freezing point; and
b. pressurized liquids which boil at very low temperatures.
The first type relies on the mechanical refrigeration of the liquid and the circulation of that liquid through a tank containing the products to be frozen. In the fishing industry, the commonest type of freezer in this category uses sodium chloride brine which can be made to any strength to achieve the desired freezing point. However, saturated brine, which contains 26% salt, freezers at -210C and in practice, the operating temperature cannot be reduced below about -180C. This limits the use of this freezer as the temperature needs to be -300C or below for most freezing processes. However, even with brine it is possible to achieve rapid freezing because of the intimate contact between the product and the freezing medium. The rate of freezing depends on the circulation of the refrigerated brine throughout the tank, and this can be a critical factor in the efficiency of a brine freezing system. Another disadvantage of using brine freezing is that the fish will absorb salt. The amount of salt taken up depends on several factors including the temperature of the brine, the length of time of immersion, the fat content of the fish and the surface area.
The brine freezing process is not widely used because of the technical limitations. Its most common application is for on-board freezing of tuna destined for canning. Newly caught fish are chilled in a tank containing sea water refrigerated to about 00C. They are held in chill until the tank has been filled by the addition of subsequent catches, or for up to five days. Salt is then added to the tank to produce a freezing-brine and the temperature is reduced to freeze the fish. When they are frozen [which takes at least 24 h], the tank is pumped dry and used for storage until landing. The fish may thaw, or partially thaw on board the boat as it returns to base so that they are ready for immediate unloading. The sea water and brine is usually refrigerated by a series of pipe
Fig: Spray freezer.
girds on the tank sides which may be supplemented by an external heat exchanger.
The second type of immersion freezer uses liquids with low boiling points such as liquid nitrogen, carbon dioxide or liquid air. The most common is liquid nitrogen. This is used for freezing individual filets and for prawns which sell for a relatively high price when sold as individual quick frozen product. Liquid nitrogen boils at -1960C. If the warm fish are brought into direct contact with the liquid nitrogen, the outer layers would freeze very rapidly; this would cause thermal stress and the break-up of the material.Therefore, the fish are normally sprayed first with nitrogen gas at about -500C, and then gradually reduced to much colder temperatures by spraying with liquid nitrogen.
The freezer consists of a stainless steel conveyor belt, which moves against the flow of gas and liquid spray in a freezing tunnel. Nitrogen freezers are small and freeze very quickly. They can be operated without the need for an on-site mechanical refrigeration system. However, in spite of the apparent advantages, the freezers are used only for specialty products. This is because first, operating costs are higher than air blast freezers, secondly, they can only freeze small products , and thirdly, they rely entirely on a regular and reliable supply of liquid nitrogen which requires special storage. These disadvantages severely limit their use in developing countries.
The other types of low boiling point liquid freezers are not so common. At one time, refrigerant 12 [dischlorodifluoromethane] in a specially purified form was used for immersion/ spray freezing. However, the high costs and concerns over the effects of R12 on ozone depletion in the atmosphere have led to a rapid decline in its use. Liquefied carbon dioxide [CO2]is used to ta a limited extent for freezing. The gas under pressure in cylinders is sprayed onto the product in much the same way as liquid nitrogen. The management of CO2 freezing is more difficult than liquid nitrogen and its use is less widespread. Normally, liquid CO2 is refrigerated before spraying.
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.
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.
Sharp freezing
The sharp freezer consists of an insulated room, usually maintained at -250C, containing a number of shelves, made from pipe coils through which cold brine or ammonia or other refrigerants circulates. Fish frozen by this method are placed directly on the shelves or aluminium pans/plates covering the pipe coils. Sharp freezing is largely out-dated and its use is limited. It is a slow freezing method.
Fig: Sharp freezer
Some other freezers
Due to modernization of freezing equipment, depending upon the nature and shape of the products, various other freezers are used in seafood processing industry. Some of them are as follows:
Band freezers
Band freezers are designed to freeze small layer of pastes or fish fillets whose thickness is around 15 mm. The product is allowed to press between two plates and freeze very rapidly. The plate is usually placed in a polythene bag to avoid sticking to the metal plates.
Spiral freezer
Spiral freezer is basically an air blast freezer, usually a round chamber in which cold refrigerant blows over wire mesh belt that moves spirally and carries individual items to be frozen. The freezer is used to freeze balanced or cooked IQF shrimp or fish products. Freezing temperature is usually maintained within the range of -300C to -400C,depending on the thickness of the items to be frozen. Spiral freezer is very effective for IQF products as it freezes very quickly, as quickly as between 15 and 60 minutes.
Fig: spiral freezer
Rotary freezer
Rotary freezer is a high capacity freezer that can handle about 10 to 15 tons of fish a day. It is mostly used in commercial seafood processing, where products of thickness up to 8 cm and length/width 80 50 cm be cooled down to -110F in two hours. The freezer applies pressure to the products being frozen and is fully automatic in operation.
Cryogenic freezer
Freezer that uses liquid nitrogen at -1960C [-320C] is called cryogenic freezer. This is essentially a single belt freezer and the refrigerant is sprayed over the items on the belt. The nitrogen evaporates and is allowed to escape into atmosphere after the vapours are used to cool the fish/ shrimp. Alternatively, the material can be dipped into liquid nitrogen, but this soften results in development of cracks on the surface. For freezing 1 kg of fish 1.0 to 1.5 kg of liquid nitrogen is needed. So, only cheaply available nitrogen can make this process economically viable. Solid carbon dioxide, dichloromethane and R12, with boiling point -220F can also be used to freeze fish in cryogenic freezer.
Fig: cryogenic freezer
Conclusion:
Knowledge about freezing method and equipments is very important for our fisheries sector .Because fish is a perishable food. Preservation is essential for increase fish and fishery products shelf life and sustainable food supply