Introduction
Fish freezing is a process of preservation in which the temperature of fish and fishery products is lowered at -40°C or below with most of the water inside of fish tissue turning into ice. Icing and chilling can keep the fish for a few weeks only. Fish is required to be kept for longer periods, for example to even out supplies during gluts, to enable fish to be distributed and sold in distant markets, and to stock pile products for lean seasons. If fish are properly frozen and stored at correct temperature, it is possible to provide a product which closely resembles fresh fish; in many cases consumers are unable to distinguish between a piece of fresh fish and a piece of frozen fish.
Some definition of freezing
Freezing rate (°c\h)
The Freezing rate of a product is defined as the difference between the initial and final temperature divided by the time taken for the freezing. It is calculated by the speed of movement of the ice formed through the product. The speed is faster near the surface and slow at the core or centre.
Some freezing codes are recommendations define freezing in terms of thickness of fish frozen in unit time. The freezing rate,however is always faster nearer the surface of the fish which is in contact with cooling medium, and slower in the centre. Freezing rates are therefore only average values. Following table shows some of the terms used in relation of freezing at different rates.
Terms used in relation of freezing rates-
Term used
Rate of freezing(mm\h)
Slow freezing
2
Quick freezing
5-30
Rapid freezing
50-100
Ultra rapid freezing
100-1000
The term freezing rate may be better applied to the time taken for the center of a product to reach certain temperature; this is often the case in commercial operations.
Sharp freezing
The term is often used when referring to the freezing of fish, but it has no precise definition. In practice, sharp freezing is often slow.
Deep freezing
Deep freezing is defined as a process whereby the average temperature of a product is reduced of 0°F (-17.8°C) and then kept at 0°F or lower.
This definition does not take into account the rate of freezing, and a product that has been deep frozen may not necessarily have been quick frozen.
Sashimi freezing
One exception to the general requirements for quick freezing is the freezing of fish (usually tuna) for the Japanese sashimi (raw fish) market. The tuna used for sashimi can be very large (60-100kg) and they have to be frozen gutted but in the round. The vessels which catch those fish operate blast freezers at -50°C to -60°C but heat extraction from the center of very large fish in still slow; it often takes 24h to freeze completely. The requirement for air blast freezing of tuna is one special case in which the general rules for quick freezing are impractical.
Thermal centre:
The Thermal centre of a product is the point within the product which has the warmest temperature at the end of the freezing process.
Adiabatic condition:
These are conditions at which no heat is either added or abstracted frpm the environment: they can be achieved by the use of an insulated container.
Blanching:
It is the process of subjecting the products to heat treatment for a short interval of time prior to processing for freezing or canning. The heat treatment, time and temperature vary for different products. Blanching is done at temperatures between85°C and 100°C. Blanching inactivates enzymes responsible for spoilage and time of blanching is related to heat sensitivity of the enzymes. For products like canned shrimp higher time and temperature are recommended and for frozen shrimp short intervals of times are used.
Figure: Blanching of products
A number of additives like citric acid, salt, sugar and polyphosphate are used in blanching water. Acidification of blanching water increases the thermal liability of enzymes and also helps to reduce the time of blanching. During blanching of shrimp for freezing 0.2% citric acid is sometimes added to water. Blanching also brings down the microbial load of the fish, particularly on the surface. Blanching has number of advantages and disadvantages. In freezing of shrimp, it improves the color as well as consumer acceptability. It can, however, case loss of some flavor, pigments, water soluble nutrients and proteins.
Glazing:
Figure: spraying with cold water
Glazing is the application of a layer of ice to the frozen fish as a surface coat. Dipping or spraying with cold water maintained at 0°C to 3°C immediately after freezing accomplishes immediately after freezing accomplishes it. Glazing is done to prevent freeze dehydration of the frozen products when kept in cool storage. During frozen storage water escapes from the surface of the frozen fish to atmosphere and the surface gets dehydrated and oxidized causing sever loss of quality. Glaze acts as a close-fitting wrapper. Good freezing techniques recommend hardening of glazed water by refreezing prior to storage. During extended cold storage glazing has to be done if there is loss of glaze. Double freezing:
Double freezing means freezing a product, thawing or partly thawing it, and then re-freezing it. This process is often used when making frozen fish products from raw material which has previously been bulk frozen. Cod frozen whole at sea in vertical plate freezers is often used for the production of items such as fish fingers or sticks. The whole frozen fish are defrosted and filleted; the fillets are then refrozen into blocks for sawing into uniform pieces for fish stick manufacture.
The prawn are often frozen into 2kg(5 Ibs) blocks in horizontal plate freezers in their country of origin. The frozen blocks are exported to other countries where they are defrosted; the prawns are then separated and refrozen as individual quick frozen products, consumer packs, ready meals,etc.
In both examples, it is vitally important that only first class raw materials are used for freezing, that the freezing and thawing processes are strictly controlled, and that hygiene and quality control procedures are adhered to, if the final product standards are to be maintained.
Some important temperatures related to low temperature processing-
Process
Temperature(°C)
Chilling
0.6 to - 2.2
Icing
0±1.0
Supper chilling
-2.0 to -5.0
Refrigeration
4.0±1.0
Freezing
-8.0 to -40
Sharp freezing
-22.0 to -28.0(slow freezing)
Air blast freezing
-40.0
Contact freezing & IQF
-40.0(Quick freezing)
Immersion freezing(ordinary)
<-20.0
Immersion freezing(Shashimi)
-50.0 to -60.0
Frozen storage
-20.0 to -25.0
Frosting
During the freezing process, there is loss of water from the surface of the fish, mainly in the early stages of freezing. This moisture is deposited and frozen on to the freezing coils of the evaporator, called frosting. If this ice is allowed to build up it will restrict the air flow, so defrosting (melting of ice) of the evaporator will be necessary. If ice collects on the evaporator coils, the heat transfer will be slow and the air will be less effectively cooled. If the evaporator becomes fully iced up, there will be very little refrigeration effect. A well designed freezer should be able to operate for 8h before defrosting; a poor design may need defrosting every 2h.
Although the product must be left in the freezer long enough to ensure effective freezing, it is important that unpacked products are not left in the freezer too long. This would not only lead to increased dehydration and frosting on evaporator coil, but it is also wastage of energy and therefore money.
Conclusion
Freezing of fish is one kinds of preservation procedure of fish. It is better than chilling and icing. Because, in the freezing point total moisture of water in fish body convert into ice. So, oxidation and autolysis cannot occur easily in fish body. There are various types of freezing method available for fish processing and preservation. All those method are not suitable for all fish species, such as Individual Quick Freezing only for shrimp. Knowledge about freezing is most important for fish processing study
Fish freezing is a process of preservation in which the temperature of fish and fishery products is lowered at -40°C or below with most of the water inside of fish tissue turning into ice. Icing and chilling can keep the fish for a few weeks only. Fish is required to be kept for longer periods, for example to even out supplies during gluts, to enable fish to be distributed and sold in distant markets, and to stock pile products for lean seasons. If fish are properly frozen and stored at correct temperature, it is possible to provide a product which closely resembles fresh fish; in many cases consumers are unable to distinguish between a piece of fresh fish and a piece of frozen fish.
Some definition of freezing
Freezing rate (°c\h)
The Freezing rate of a product is defined as the difference between the initial and final temperature divided by the time taken for the freezing. It is calculated by the speed of movement of the ice formed through the product. The speed is faster near the surface and slow at the core or centre.
Some freezing codes are recommendations define freezing in terms of thickness of fish frozen in unit time. The freezing rate,however is always faster nearer the surface of the fish which is in contact with cooling medium, and slower in the centre. Freezing rates are therefore only average values. Following table shows some of the terms used in relation of freezing at different rates.
Terms used in relation of freezing rates-
Term used
Rate of freezing(mm\h)
Slow freezing
2
Quick freezing
5-30
Rapid freezing
50-100
Ultra rapid freezing
100-1000
The term freezing rate may be better applied to the time taken for the center of a product to reach certain temperature; this is often the case in commercial operations.
Sharp freezing
The term is often used when referring to the freezing of fish, but it has no precise definition. In practice, sharp freezing is often slow.
Deep freezing
Deep freezing is defined as a process whereby the average temperature of a product is reduced of 0°F (-17.8°C) and then kept at 0°F or lower.
This definition does not take into account the rate of freezing, and a product that has been deep frozen may not necessarily have been quick frozen.
Sashimi freezing
One exception to the general requirements for quick freezing is the freezing of fish (usually tuna) for the Japanese sashimi (raw fish) market. The tuna used for sashimi can be very large (60-100kg) and they have to be frozen gutted but in the round. The vessels which catch those fish operate blast freezers at -50°C to -60°C but heat extraction from the center of very large fish in still slow; it often takes 24h to freeze completely. The requirement for air blast freezing of tuna is one special case in which the general rules for quick freezing are impractical.
Thermal centre:
The Thermal centre of a product is the point within the product which has the warmest temperature at the end of the freezing process.
Adiabatic condition:
These are conditions at which no heat is either added or abstracted frpm the environment: they can be achieved by the use of an insulated container.
Blanching:
It is the process of subjecting the products to heat treatment for a short interval of time prior to processing for freezing or canning. The heat treatment, time and temperature vary for different products. Blanching is done at temperatures between85°C and 100°C. Blanching inactivates enzymes responsible for spoilage and time of blanching is related to heat sensitivity of the enzymes. For products like canned shrimp higher time and temperature are recommended and for frozen shrimp short intervals of times are used.
Figure: Blanching of products
A number of additives like citric acid, salt, sugar and polyphosphate are used in blanching water. Acidification of blanching water increases the thermal liability of enzymes and also helps to reduce the time of blanching. During blanching of shrimp for freezing 0.2% citric acid is sometimes added to water. Blanching also brings down the microbial load of the fish, particularly on the surface. Blanching has number of advantages and disadvantages. In freezing of shrimp, it improves the color as well as consumer acceptability. It can, however, case loss of some flavor, pigments, water soluble nutrients and proteins.
Glazing:
Figure: spraying with cold water
Glazing is the application of a layer of ice to the frozen fish as a surface coat. Dipping or spraying with cold water maintained at 0°C to 3°C immediately after freezing accomplishes immediately after freezing accomplishes it. Glazing is done to prevent freeze dehydration of the frozen products when kept in cool storage. During frozen storage water escapes from the surface of the frozen fish to atmosphere and the surface gets dehydrated and oxidized causing sever loss of quality. Glaze acts as a close-fitting wrapper. Good freezing techniques recommend hardening of glazed water by refreezing prior to storage. During extended cold storage glazing has to be done if there is loss of glaze. Double freezing:
Double freezing means freezing a product, thawing or partly thawing it, and then re-freezing it. This process is often used when making frozen fish products from raw material which has previously been bulk frozen. Cod frozen whole at sea in vertical plate freezers is often used for the production of items such as fish fingers or sticks. The whole frozen fish are defrosted and filleted; the fillets are then refrozen into blocks for sawing into uniform pieces for fish stick manufacture.
The prawn are often frozen into 2kg(5 Ibs) blocks in horizontal plate freezers in their country of origin. The frozen blocks are exported to other countries where they are defrosted; the prawns are then separated and refrozen as individual quick frozen products, consumer packs, ready meals,etc.
In both examples, it is vitally important that only first class raw materials are used for freezing, that the freezing and thawing processes are strictly controlled, and that hygiene and quality control procedures are adhered to, if the final product standards are to be maintained.
Some important temperatures related to low temperature processing-
Process
Temperature(°C)
Chilling
0.6 to - 2.2
Icing
0±1.0
Supper chilling
-2.0 to -5.0
Refrigeration
4.0±1.0
Freezing
-8.0 to -40
Sharp freezing
-22.0 to -28.0(slow freezing)
Air blast freezing
-40.0
Contact freezing & IQF
-40.0(Quick freezing)
Immersion freezing(ordinary)
<-20.0
Immersion freezing(Shashimi)
-50.0 to -60.0
Frozen storage
-20.0 to -25.0
Frosting
During the freezing process, there is loss of water from the surface of the fish, mainly in the early stages of freezing. This moisture is deposited and frozen on to the freezing coils of the evaporator, called frosting. If this ice is allowed to build up it will restrict the air flow, so defrosting (melting of ice) of the evaporator will be necessary. If ice collects on the evaporator coils, the heat transfer will be slow and the air will be less effectively cooled. If the evaporator becomes fully iced up, there will be very little refrigeration effect. A well designed freezer should be able to operate for 8h before defrosting; a poor design may need defrosting every 2h.
Although the product must be left in the freezer long enough to ensure effective freezing, it is important that unpacked products are not left in the freezer too long. This would not only lead to increased dehydration and frosting on evaporator coil, but it is also wastage of energy and therefore money.
Conclusion
Freezing of fish is one kinds of preservation procedure of fish. It is better than chilling and icing. Because, in the freezing point total moisture of water in fish body convert into ice. So, oxidation and autolysis cannot occur easily in fish body. There are various types of freezing method available for fish processing and preservation. All those method are not suitable for all fish species, such as Individual Quick Freezing only for shrimp. Knowledge about freezing is most important for fish processing study