Freezing of shrimp
Shrimp freezing is the most vital of all freezing plants operating for processing of seafood. General freezing steps and methods are described below.
Pre-processing for freezing
1. Inspection for quality: The shrimps are inspected for freshness visually. There should not be any black spot formed. If any black spots are seen, it should be discarded.
2. Icing: The shrimps are iced immediately after capture and re-iced after coming to the factory. At no stage should the temperature of shrimp be above 5°C to maintain top quality of the final product.
3. Grading: grading is done as per the size of shrimp.
4. Weighing: The desired quality is weighed in bulk for processing.
5. Icing: The graded and weighed shrimp is iced again to maintain the temperature around 0°C.
6. Chill storage: The iced shrimp is kept in a chilled room maintained at an operational temperature of 4°C ± 2°C until further processing for freezing.
All these types of pre-processed shrimps are frozen either as block or individual quick frozen (IQF) product depending on foreign buyer's requirements.
Freezing method
Quality of frozen shrimp depends on the freshness of raw materials. Hence, it is imperative that the shrimp should be iced immediately after capture. The recommended storage temperature is 3°C till it is processed in the factory. The material should be stored in stainless steel or polypropylene/plastic containers in crushed ice- water maintained at 2 to 3°C.
For blanched products, the materials should be blanched in stainless steel container, heated electrically or by steam by holding the products in water with or without additives or exposing to steam. Blanching gives a good color to the meat but care should be taken that the product is not cooked. Best result can be obtained by immediate cooling in chilled water subsequent to blanching. After blanching, the shrimps are graded into different sizes. They are washed in potable water containing 10 mg/kg available chlorine. The material is then processed further for freezing either as blocks or individuals. For block freezing, the graded shrimps are weighed and packed in wax laminated cartons with a lining of water-proof material. They are then quick frozen in contact plate freezer at — 40°C. The time taken for the freezing operation varies from 1.5 to 3 h depending on the cooling efficiency of the freezer and the thickness of the slab. The quicker the freezing, the better the quality. Thickness of the block should be around 5 cm. The anterior core of the block should attain a temperature of about — 15°C to — 20°C.
For individual quick freezing (IQF), the blanched or graded shrimps are individually loaded into the conveyer of the IQF machine and frozen rapidly in the freezing chamber. Either spiral, tunnel or belt freezer can be used for individual quick freezing of shrimp. A common equipment for value-added IQF products of shrimp is spiral freezer. The contact time may vary from 5 to 15 min depending on the thickness of the shrimp.
In order to prevent freeze drying of the body surface, glazing is done with cold water. The glaze water should contain 5 to 10 mg/kg available chlorine. In block freezing glaze water can be added prior to freezing to produce uniform glaze. Addition of citric acid, sugar and chloride are permissible in glaze water at the levels 0.2, 0.5 and 0.5% respectively.
The products are packed in master cartons and kept stored at - 20°C. The storage of frozen fish is ideal at —30°C to have minimum quality changes and many countries have adopted —30°C as recommended storage temperature for frozen shrimp.
Freezing of prawn in Bangladesh
The raw material prawn, either headless on head on, is received through the receiving window (temperature 4 to 5°C) of the processing factory. The prawns are washed with potable chilled water, graded, called the factory grading and then weighed. Factory grading is done on the basis of the size of prawn, ranging from U5 to U16. US means five individuals in one pound. Prawns are washed again with chilled water and re-iced with the flake ice or crushed block ice produced by the factory itself. Prawn mixed with ice is taken on the panning table for dressing and preparing for the product as per the buyer's demand. Products of different categories, like tail on, tail off, peeled undeveined (PUD), peeled and deveined (P & D), shell on (S/O), etc. as shown in Table 10.3 are generally prepared. Prepared items are washed again followed by pressure washing through strong water jet by a motor. Items to be frozen through contact freezer are packed in small paper boxes according to export grade such as 2/4, 4/6, 6/8, 8/12, 12/15, 15/18, etc. Grade 2/4 means that there are two to four prawns in each pound in the packet. Generally, a two-pound pack is prepared. Prawn is also weighted in kg, if the buyer wants. The weight of the blocks are generally maintained at 2 Ibs, 4 Ibs, 1 kg, 1.8 kg or 2 kg, as dictated by the buyers. The packets are transferred to the contact plate freezer, but before that, very often, a 200 g water is added to each packet to fill the pack tightly. Water helps to freeze the prawn as a block but not to stick each other. Surrounding frozen water also protects the products from dehydration, desiccation, damage, abrasion, etc. Sutter packing is also done without water, but glazing is done after freezing. The packets are kept fixed to the plates of contact freezer. It generally takes 2 hours to freeze the whole block. The temperature at the core of the prawn must reach — 20° to — 25°C. The core temperature is checked with a special type of thermometer. After freezing, the prawns are glazed with little amount of water if water is not added during freezing. Frozen block is checked for filths, passed through a metal detector, packaged in polythene-polypropylene coated paper carton and finally in master carton (generally 6 blocks in case of shrimp). The product is kept in frozen storage until shipment.
For IQF cooked product, dressed and prepared prawns are cooked through steaming or boiling at 90°C for variable time period as per buyer's specification. Salt or other chemicals may be added if the buyers demand so. After definite cooking, the prawns are immediately dipped into chilled water (4°C) to stop the effect of cooking. Cooked items are transferred to the belt of the spiral freezer and frozen at — 40°C for 30 to 40 minutes. Frozen prawns are glazed, Packed, weighed, checked for filth and stored in the frozen storage after master packaging. The temperature of frozen storage is maintained at 22°C Different prawn/shrimps like galda, bagda, chaga, tiger shrimp, brown shrimp (lailla or horina icha), etc. are processed as IQF products. The main IQF shrimp/prawn products manufactured in Bangladesh are beheaded IQF, raw IQF, blanched IQF, cooked IQF, black tiger tail-on black tiger tail-off, and deveined, etc. gold easy (butter fly), peeled IQF, horina PUD, horina P & D peeled and deveined etc.
Semi IQF of white fish
The white fish are not processed as IQF products. Some semi- IQF products of Whitefish like frozen hilsa, frozen ruhu, frozen catla, frozen Carps, frozen bairn, frozen aier, frozen chital, frozen bowl, etc. are Produced in the shrimp processing plants. After receiving in iced condition, the raw materials are dressed, gutted, washed with chilled chlorinated (10 to 20 ppm) water (4°C) and weighed. fish is, however, not gutted during processing for semi-IQF product. Adhered water of the washed fish are dried by fanning and then kept necked in air blast freezer at —40°C for 6 hours. The fishes are individually glazed and packed in 20 kg carton under the grade 3/5, 5/6, 6/8 according to buyers demand. A 3/5 grade means that there are three to five fish in a 20-kg package. Ilish and Calbasu are packed as 1000 g+, 1200 g+, etc. The carton is packed in durable poly bag and frozen stored at 25 C.
Storage of frozen products
Once fish have been frozen they need to be kept under suitable storage conditions to maintain quality. The recommended storage temperature for frozen products varies to —250c. At this temperature protein changes and denaturation are minimized and bacterial action is practically arrested. However, even at these law temperatures. other changes can occur which lead to loss of product quality. These include oxidation of fat, dehydration. color changes and possible protein deterioration under fluctuating storage conditions. Correct design and use of cold stores can reduce these problems .the most important design factors are
v low temperature
v uniform temperature (throughout the store)
v steady temperature (with minimal fluctuation)
v good air distribution (to maintain uniform conditions)
v minimum air circulation(to reduce dehydration losses)
v minimum air ingress (to minimize fluctuations).
Dehydration of the product is a major concern for the cold store operator. It occurs as a consequence of the different amounts of water which can be held as vapour in the air at different temperatures. Warm air at 100% relative humidity holds more g/litre of water than the same volume of air at a lower temperature. As air passes over the product in cold storage and circulates inside the cold store, it becomes warmer than it was when it left the evaporator. As it warms it becomes less saturated with water and is therefore able to pick up moisture from the surroundings. As a result, water is removed from the product in storage. This moisture is carried along by the air which is returning to the evaporator. The evaporator cools the air down, so that it becomes over-saturated (more than 100(c relative humidity). The water then precipitates out or tile air to form frost on the evaporator coils. Cold stores are therefore generally designed to minimize these problems.
The cold stoic can affect the storage life of products by the ability or inability to maintain a low, uniform and steady temperature with the minimum of air circulation. The use or good thermal insulation during construction, the installation of adequate vapour barriers for the insulation, the construction of an air lock, air curtain or similar device to reduce air ingress, the proper management and organization of the stored products, are all factors which will help to produce optimum and efficient use of the store
Insulation of cold storage
Good thermal insulation is a prerequisite for cold store design; the type of insulation and its correct insulation are vital to efficient operation. An insulating material is one which slows down the passage of heat energy from higher to lower temperatures. The choice of insulator depends not only on its low thermal conductivity., but also on its-
v resistance to passage of water and water vapour
v resistance to rot
v resistance to attack by vermin and insects
v structural strength
v toxicity
v flammability
v price
Various traditional materials have been used for insulating cold store walIs including, cork, wood shavings sawdust and vermiculite. these have been replaced in recent years by foam and expanded plastic materials which have a number of advantages. The traditional, vegetable materials were not resistant to water had a tendency to rot and settle after installation, were not as efficient as the newer materials, and in sow cases such as cork, had become very expensive. The insulating plastics are expanded polystyrene and polyurethane foam.
Changes in frozen fish during storage
Sever changes are found in fish body in the muscle during the freezing and frozen storage of fish. These changes are, however, influenced by temperature of the medium, packaging and glazing of the products and relative humidity of the stores. The changes are as follows:
1. Physical changes
i. Crystallization of water in tissue increases the volume about 8-10% of the raw fish;
ii. Desiccation starts from the surface of the body;
iii. Discoloration occurs due to break down of hemoglobin;
iv. Toughness of muscle increases;
v. Rigidity increases as it becomes hard;
vi. Loses weight;
vii. Freeze-burn occurs;
viii. Trimming loss in association with the loss of nutrients occurs;
ix. Thaw-drip or water drip occurs when frozen fish are thawed slowly;
x. Slow freezing produces big ice crystals that ruptures the cell wall and causes physical damage:
2. Chemical changes
i. Chemical and enzymatic actions take place slowly;
ii. Proteins irreversibly dehydrated;
iii. Myoglobin oxidized;
Lipid oxidized and hydrolyzed
Shrimp freezing is the most vital of all freezing plants operating for processing of seafood. General freezing steps and methods are described below.
Pre-processing for freezing
1. Inspection for quality: The shrimps are inspected for freshness visually. There should not be any black spot formed. If any black spots are seen, it should be discarded.
2. Icing: The shrimps are iced immediately after capture and re-iced after coming to the factory. At no stage should the temperature of shrimp be above 5°C to maintain top quality of the final product.
3. Grading: grading is done as per the size of shrimp.
4. Weighing: The desired quality is weighed in bulk for processing.
5. Icing: The graded and weighed shrimp is iced again to maintain the temperature around 0°C.
6. Chill storage: The iced shrimp is kept in a chilled room maintained at an operational temperature of 4°C ± 2°C until further processing for freezing.
All these types of pre-processed shrimps are frozen either as block or individual quick frozen (IQF) product depending on foreign buyer's requirements.
Freezing method
Quality of frozen shrimp depends on the freshness of raw materials. Hence, it is imperative that the shrimp should be iced immediately after capture. The recommended storage temperature is 3°C till it is processed in the factory. The material should be stored in stainless steel or polypropylene/plastic containers in crushed ice- water maintained at 2 to 3°C.
For blanched products, the materials should be blanched in stainless steel container, heated electrically or by steam by holding the products in water with or without additives or exposing to steam. Blanching gives a good color to the meat but care should be taken that the product is not cooked. Best result can be obtained by immediate cooling in chilled water subsequent to blanching. After blanching, the shrimps are graded into different sizes. They are washed in potable water containing 10 mg/kg available chlorine. The material is then processed further for freezing either as blocks or individuals. For block freezing, the graded shrimps are weighed and packed in wax laminated cartons with a lining of water-proof material. They are then quick frozen in contact plate freezer at — 40°C. The time taken for the freezing operation varies from 1.5 to 3 h depending on the cooling efficiency of the freezer and the thickness of the slab. The quicker the freezing, the better the quality. Thickness of the block should be around 5 cm. The anterior core of the block should attain a temperature of about — 15°C to — 20°C.
For individual quick freezing (IQF), the blanched or graded shrimps are individually loaded into the conveyer of the IQF machine and frozen rapidly in the freezing chamber. Either spiral, tunnel or belt freezer can be used for individual quick freezing of shrimp. A common equipment for value-added IQF products of shrimp is spiral freezer. The contact time may vary from 5 to 15 min depending on the thickness of the shrimp.
In order to prevent freeze drying of the body surface, glazing is done with cold water. The glaze water should contain 5 to 10 mg/kg available chlorine. In block freezing glaze water can be added prior to freezing to produce uniform glaze. Addition of citric acid, sugar and chloride are permissible in glaze water at the levels 0.2, 0.5 and 0.5% respectively.
The products are packed in master cartons and kept stored at - 20°C. The storage of frozen fish is ideal at —30°C to have minimum quality changes and many countries have adopted —30°C as recommended storage temperature for frozen shrimp.
Freezing of prawn in Bangladesh
The raw material prawn, either headless on head on, is received through the receiving window (temperature 4 to 5°C) of the processing factory. The prawns are washed with potable chilled water, graded, called the factory grading and then weighed. Factory grading is done on the basis of the size of prawn, ranging from U5 to U16. US means five individuals in one pound. Prawns are washed again with chilled water and re-iced with the flake ice or crushed block ice produced by the factory itself. Prawn mixed with ice is taken on the panning table for dressing and preparing for the product as per the buyer's demand. Products of different categories, like tail on, tail off, peeled undeveined (PUD), peeled and deveined (P & D), shell on (S/O), etc. as shown in Table 10.3 are generally prepared. Prepared items are washed again followed by pressure washing through strong water jet by a motor. Items to be frozen through contact freezer are packed in small paper boxes according to export grade such as 2/4, 4/6, 6/8, 8/12, 12/15, 15/18, etc. Grade 2/4 means that there are two to four prawns in each pound in the packet. Generally, a two-pound pack is prepared. Prawn is also weighted in kg, if the buyer wants. The weight of the blocks are generally maintained at 2 Ibs, 4 Ibs, 1 kg, 1.8 kg or 2 kg, as dictated by the buyers. The packets are transferred to the contact plate freezer, but before that, very often, a 200 g water is added to each packet to fill the pack tightly. Water helps to freeze the prawn as a block but not to stick each other. Surrounding frozen water also protects the products from dehydration, desiccation, damage, abrasion, etc. Sutter packing is also done without water, but glazing is done after freezing. The packets are kept fixed to the plates of contact freezer. It generally takes 2 hours to freeze the whole block. The temperature at the core of the prawn must reach — 20° to — 25°C. The core temperature is checked with a special type of thermometer. After freezing, the prawns are glazed with little amount of water if water is not added during freezing. Frozen block is checked for filths, passed through a metal detector, packaged in polythene-polypropylene coated paper carton and finally in master carton (generally 6 blocks in case of shrimp). The product is kept in frozen storage until shipment.
For IQF cooked product, dressed and prepared prawns are cooked through steaming or boiling at 90°C for variable time period as per buyer's specification. Salt or other chemicals may be added if the buyers demand so. After definite cooking, the prawns are immediately dipped into chilled water (4°C) to stop the effect of cooking. Cooked items are transferred to the belt of the spiral freezer and frozen at — 40°C for 30 to 40 minutes. Frozen prawns are glazed, Packed, weighed, checked for filth and stored in the frozen storage after master packaging. The temperature of frozen storage is maintained at 22°C Different prawn/shrimps like galda, bagda, chaga, tiger shrimp, brown shrimp (lailla or horina icha), etc. are processed as IQF products. The main IQF shrimp/prawn products manufactured in Bangladesh are beheaded IQF, raw IQF, blanched IQF, cooked IQF, black tiger tail-on black tiger tail-off, and deveined, etc. gold easy (butter fly), peeled IQF, horina PUD, horina P & D peeled and deveined etc.
Semi IQF of white fish
The white fish are not processed as IQF products. Some semi- IQF products of Whitefish like frozen hilsa, frozen ruhu, frozen catla, frozen Carps, frozen bairn, frozen aier, frozen chital, frozen bowl, etc. are Produced in the shrimp processing plants. After receiving in iced condition, the raw materials are dressed, gutted, washed with chilled chlorinated (10 to 20 ppm) water (4°C) and weighed. fish is, however, not gutted during processing for semi-IQF product. Adhered water of the washed fish are dried by fanning and then kept necked in air blast freezer at —40°C for 6 hours. The fishes are individually glazed and packed in 20 kg carton under the grade 3/5, 5/6, 6/8 according to buyers demand. A 3/5 grade means that there are three to five fish in a 20-kg package. Ilish and Calbasu are packed as 1000 g+, 1200 g+, etc. The carton is packed in durable poly bag and frozen stored at 25 C.
Storage of frozen products
Once fish have been frozen they need to be kept under suitable storage conditions to maintain quality. The recommended storage temperature for frozen products varies to —250c. At this temperature protein changes and denaturation are minimized and bacterial action is practically arrested. However, even at these law temperatures. other changes can occur which lead to loss of product quality. These include oxidation of fat, dehydration. color changes and possible protein deterioration under fluctuating storage conditions. Correct design and use of cold stores can reduce these problems .the most important design factors are
v low temperature
v uniform temperature (throughout the store)
v steady temperature (with minimal fluctuation)
v good air distribution (to maintain uniform conditions)
v minimum air circulation(to reduce dehydration losses)
v minimum air ingress (to minimize fluctuations).
Dehydration of the product is a major concern for the cold store operator. It occurs as a consequence of the different amounts of water which can be held as vapour in the air at different temperatures. Warm air at 100% relative humidity holds more g/litre of water than the same volume of air at a lower temperature. As air passes over the product in cold storage and circulates inside the cold store, it becomes warmer than it was when it left the evaporator. As it warms it becomes less saturated with water and is therefore able to pick up moisture from the surroundings. As a result, water is removed from the product in storage. This moisture is carried along by the air which is returning to the evaporator. The evaporator cools the air down, so that it becomes over-saturated (more than 100(c relative humidity). The water then precipitates out or tile air to form frost on the evaporator coils. Cold stores are therefore generally designed to minimize these problems.
The cold stoic can affect the storage life of products by the ability or inability to maintain a low, uniform and steady temperature with the minimum of air circulation. The use or good thermal insulation during construction, the installation of adequate vapour barriers for the insulation, the construction of an air lock, air curtain or similar device to reduce air ingress, the proper management and organization of the stored products, are all factors which will help to produce optimum and efficient use of the store
Insulation of cold storage
Good thermal insulation is a prerequisite for cold store design; the type of insulation and its correct insulation are vital to efficient operation. An insulating material is one which slows down the passage of heat energy from higher to lower temperatures. The choice of insulator depends not only on its low thermal conductivity., but also on its-
v resistance to passage of water and water vapour
v resistance to rot
v resistance to attack by vermin and insects
v structural strength
v toxicity
v flammability
v price
Various traditional materials have been used for insulating cold store walIs including, cork, wood shavings sawdust and vermiculite. these have been replaced in recent years by foam and expanded plastic materials which have a number of advantages. The traditional, vegetable materials were not resistant to water had a tendency to rot and settle after installation, were not as efficient as the newer materials, and in sow cases such as cork, had become very expensive. The insulating plastics are expanded polystyrene and polyurethane foam.
Changes in frozen fish during storage
Sever changes are found in fish body in the muscle during the freezing and frozen storage of fish. These changes are, however, influenced by temperature of the medium, packaging and glazing of the products and relative humidity of the stores. The changes are as follows:
1. Physical changes
i. Crystallization of water in tissue increases the volume about 8-10% of the raw fish;
ii. Desiccation starts from the surface of the body;
iii. Discoloration occurs due to break down of hemoglobin;
iv. Toughness of muscle increases;
v. Rigidity increases as it becomes hard;
vi. Loses weight;
vii. Freeze-burn occurs;
viii. Trimming loss in association with the loss of nutrients occurs;
ix. Thaw-drip or water drip occurs when frozen fish are thawed slowly;
x. Slow freezing produces big ice crystals that ruptures the cell wall and causes physical damage:
2. Chemical changes
i. Chemical and enzymatic actions take place slowly;
ii. Proteins irreversibly dehydrated;
iii. Myoglobin oxidized;
Lipid oxidized and hydrolyzed