Chilling of fish
Chilling is a process by which temperature of the fish is lowered to a point near freezing but not below it by means of heat withdrawal. The freezing point for different fish species varies between -0.6 and -2.2˚C and depends on the concentration of the cell fluids. It is usually taken as equal to -1˚C. Fish and fishery products processed in this way are called as chilled products.
Supper chilling
Supper chilling is a word used for fishing industry to describe the condition of the fish stored at temperature just below freezing point where the fish flesh begins to freeze. Supper chilled fish are held at even one degree lower than -2.2˚C. The optimal temperature for supper chilling is about -2.2˚C, but varies from -2.0 to -5.0˚C, at which point half of the water is frozen in fish. From the quality control stand point, supper chilled fish have a shelf life extending from 2 to 3 weeks beyond that of normal ice-stored fish.
Different chilling methods
1. Chilling of fish with ice
2. Cold air blown over fish
3. Fish immersed in chilled water-
a. Refrigerated sea water (RSW)
b. Chilled sea water (CSW)
Icing of fish
Icing is one kind of chilling method. High heat absorption capacity of ice makes it an ideal medium for chilling of fish. Icing of fish is very easy that does not involve sophistication or high level of silk. Ice is available almost everywhere in the country and the fish can be kept for couple of weeks (20-30 days) in acceptable quality if proper icing is done. However, due to lake of knowledge icing is not properly practiced in traditional fish handling and preservation. The use of ice can substantially reduce post-harvest losses and improve the quality of fish.
Advantages of icing
Ice protects the fish from spoilage by reducing the temperature. Besides, icing has the following advantages (Clucas and Ward, 1996);
· Ice melt-water helps to wash away surface bacteria and contaminants.
· Ice melt-water keeps the surface of fish wet that prevents dehydration and preserves the glossy appearance.
· Ice melt-water in contact with the fish is a good conductor of heat that facilitates cooling.
· Ice made from potable water is non-toxic and safe.
· Ice can be transported from place to place and is, in effect, a method of portable refrigeration.
· As ice melts at 0˚ it will not freeze the fish but automatically controls the temperature at the ideal chill level.
· Ice is relatively cheap compared with other means of preservation.
· Fish can be iced for some times in any container even if ice-box is not available.
· Fish can be iced anywhere. Fishermen, transporters, large traders, small-scale traders, consumers-everybody can ice fish.
Types of icing
Following types of ice are generally used in icing of fish:
1. Block ice: Block ice is the most common type of ice used to ice fish outside the processing plant in Bangladesh. Traditional ice plant makes the ice in cans which are submersed in tanks containing circulating sodium or calcium chloride brine. The dimension of the can and the temperature of the brine are usually selected to give a freezing period of between 8 and 24 hours. The block weight can vary from 12 to 150 kg depending on the requirement. A common size produced in Bangladesh is 2.5 x 1.5 x 1 feet weighing 70-80 kg .Due to inadequate freezing, the ice blocks often remain hollow inside.
2. Flake ice: This type of ice plant make a very thin ice, 2 to 3 mm thick on the surface of a cylinder or drum and the ice is harvested as dry sub-cooled flakes usually 100 to 1000 mm2 in area. Normal freezing temperature in a flake ice machine is -20 to -25˚C. Low temperature is necessary to produce a sub-cooled ice quickly. This type of flake ice is mainly produced and utilized in the fish\shrimp processing plants.
3. Tube ice: Tube ice is formed on the inner surface of vertical tubes and is produced in the form of small hollow cylinders of about 50 x 50 mm with a wall thickness of 10 to 12 mm. As ice drops from the tube a cutter chops the ice into suitable lengths, normally 50 mm. The usual operating temperature of this type of plant is -8 to -10˚C.
4. Plate ice: Plate ice is formed on one face of a vertical plate and released by running water on the other face to defrost it. Optimum ice thickness is 10 to 15 mm and particle size is variable.
Besides, there are many other types of ice used in fish preservation, like shell-ice, chip-ice, soft-ice.
Factors regulating the amount of ice required in ice box
When ice is placed in close contact with the fish to be chilled, heat is transferred from warm fish to the ice, thus lowering the temperature of the fish and melting the ice.
How much ice is required to chill a fish hold or ice box can be calculated (Clucas and Ward, 1996). We know that one kcal is the amount of heat required to raise the temperature of 1 kg water by 1˚C. More heat is required to warm water than almost any other substance. This capacity of substances to hold heat, when compared to water, is known as specific heat (SH). The SH of water is 1, for other substances it is less than 1. For eg. Ice about 0.5; wet fish-about 0.9, frozen fish-about 0.4, air about 0.15, most metals-about 0.1.
The SH can be used to calculate how much heat has to be removed to cool a substance, eg.
Heat to be removed =Weight of substance x temperature change x SH
If we want to cool 10 kg fish from 25˚C to 0˚C, we would need to remove-
10 x (25-0)˚C x 0.9=10 x 25 x 0.9=225 kcal
Thus the weight of ice required=225\80=2.81 kg
The present simple calculation shows that unless at least 2.81 kg ice is melted, it is impossible to chill 10 kg fish from 25˚C to 0˚C.
But this theoretical calculation can not take into consideration of some other important factors. Because, in practice, when fish is iced, say in box, some ice will be used in cooling the box and more will be melted on the journey by heat coming in through the lid and wall of the box.
Therefore, in quantifying the amount of ice the following points should be considered.
* Insulation of ice box: Ice absorbs the heat from the fish body and thus melts. Ice is also melted by surrounding air unless fish and ice is protected with insulating materials. Therefore, quantity of ice varies depending on the condition of insulation of the box.
* Cooling the box itself: Ice is needed for cooling of the box or container\insulation of the box.
* Nature of packing in ice: All the ice are not in direct contact with all fish. If ice crystals are small and homogeneous, packing is good, cooling of fish is prompt and ice will last long.
* Time for keeping fish in chilled condition\length of journey.
* How quickly the fish are chilled.
* Thickness of the fish.
* Ambient temperature.
Therefore, depending on the situation, it is wise to use 10-20% more ice than the calculated value. In tropical climate, outside heat consumes more ice than heat from fish. Therefore, insulation is particularly important in Bangladesh.
Nature of water to be used in ice
· Clean and contamination free water should be used for the manufacture of ice.
· Municipal water should not be used directly in ice production. It can be used after necessary cleaning and chlorinating (5-10 ppm).
· Water from ponds, lakes, ditches, rivers, canals or sea-shore should not be used. These natural waters contain mud, dirt, debris, bacteria and other contaminants that, if used in ice, may serve as a basis of fresh contamination in fish and lead to quick spoilage.
· Sea water contains salt and transformed into ice at much lower temperature than the usual temperature required in ice formation (0˚C). Therefore, the quality of ice from sea water is not good. To form ice, seawater is sometimes used in ice production if freshwater is scare, as in case of big trawler or factory ship in the sea or in desert states.
· Quality of ice will be good if adequately treated tube well or deep tube well water is used.
· Large tanks\reservoirs where water is preserved in ice plant should be cleaned regularly .
· Antibiotics in water at prescribed dose may increase shelf life of iced fish but this should strictly follow local and international regulations.
Chilling is a process by which temperature of the fish is lowered to a point near freezing but not below it by means of heat withdrawal. The freezing point for different fish species varies between -0.6 and -2.2˚C and depends on the concentration of the cell fluids. It is usually taken as equal to -1˚C. Fish and fishery products processed in this way are called as chilled products.
Supper chilling
Supper chilling is a word used for fishing industry to describe the condition of the fish stored at temperature just below freezing point where the fish flesh begins to freeze. Supper chilled fish are held at even one degree lower than -2.2˚C. The optimal temperature for supper chilling is about -2.2˚C, but varies from -2.0 to -5.0˚C, at which point half of the water is frozen in fish. From the quality control stand point, supper chilled fish have a shelf life extending from 2 to 3 weeks beyond that of normal ice-stored fish.
Different chilling methods
1. Chilling of fish with ice
2. Cold air blown over fish
3. Fish immersed in chilled water-
a. Refrigerated sea water (RSW)
b. Chilled sea water (CSW)
Icing of fish
Icing is one kind of chilling method. High heat absorption capacity of ice makes it an ideal medium for chilling of fish. Icing of fish is very easy that does not involve sophistication or high level of silk. Ice is available almost everywhere in the country and the fish can be kept for couple of weeks (20-30 days) in acceptable quality if proper icing is done. However, due to lake of knowledge icing is not properly practiced in traditional fish handling and preservation. The use of ice can substantially reduce post-harvest losses and improve the quality of fish.
Advantages of icing
Ice protects the fish from spoilage by reducing the temperature. Besides, icing has the following advantages (Clucas and Ward, 1996);
· Ice melt-water helps to wash away surface bacteria and contaminants.
· Ice melt-water keeps the surface of fish wet that prevents dehydration and preserves the glossy appearance.
· Ice melt-water in contact with the fish is a good conductor of heat that facilitates cooling.
· Ice made from potable water is non-toxic and safe.
· Ice can be transported from place to place and is, in effect, a method of portable refrigeration.
· As ice melts at 0˚ it will not freeze the fish but automatically controls the temperature at the ideal chill level.
· Ice is relatively cheap compared with other means of preservation.
· Fish can be iced for some times in any container even if ice-box is not available.
· Fish can be iced anywhere. Fishermen, transporters, large traders, small-scale traders, consumers-everybody can ice fish.
Types of icing
Following types of ice are generally used in icing of fish:
1. Block ice: Block ice is the most common type of ice used to ice fish outside the processing plant in Bangladesh. Traditional ice plant makes the ice in cans which are submersed in tanks containing circulating sodium or calcium chloride brine. The dimension of the can and the temperature of the brine are usually selected to give a freezing period of between 8 and 24 hours. The block weight can vary from 12 to 150 kg depending on the requirement. A common size produced in Bangladesh is 2.5 x 1.5 x 1 feet weighing 70-80 kg .Due to inadequate freezing, the ice blocks often remain hollow inside.
2. Flake ice: This type of ice plant make a very thin ice, 2 to 3 mm thick on the surface of a cylinder or drum and the ice is harvested as dry sub-cooled flakes usually 100 to 1000 mm2 in area. Normal freezing temperature in a flake ice machine is -20 to -25˚C. Low temperature is necessary to produce a sub-cooled ice quickly. This type of flake ice is mainly produced and utilized in the fish\shrimp processing plants.
3. Tube ice: Tube ice is formed on the inner surface of vertical tubes and is produced in the form of small hollow cylinders of about 50 x 50 mm with a wall thickness of 10 to 12 mm. As ice drops from the tube a cutter chops the ice into suitable lengths, normally 50 mm. The usual operating temperature of this type of plant is -8 to -10˚C.
4. Plate ice: Plate ice is formed on one face of a vertical plate and released by running water on the other face to defrost it. Optimum ice thickness is 10 to 15 mm and particle size is variable.
Besides, there are many other types of ice used in fish preservation, like shell-ice, chip-ice, soft-ice.
Factors regulating the amount of ice required in ice box
When ice is placed in close contact with the fish to be chilled, heat is transferred from warm fish to the ice, thus lowering the temperature of the fish and melting the ice.
How much ice is required to chill a fish hold or ice box can be calculated (Clucas and Ward, 1996). We know that one kcal is the amount of heat required to raise the temperature of 1 kg water by 1˚C. More heat is required to warm water than almost any other substance. This capacity of substances to hold heat, when compared to water, is known as specific heat (SH). The SH of water is 1, for other substances it is less than 1. For eg. Ice about 0.5; wet fish-about 0.9, frozen fish-about 0.4, air about 0.15, most metals-about 0.1.
The SH can be used to calculate how much heat has to be removed to cool a substance, eg.
Heat to be removed =Weight of substance x temperature change x SH
If we want to cool 10 kg fish from 25˚C to 0˚C, we would need to remove-
10 x (25-0)˚C x 0.9=10 x 25 x 0.9=225 kcal
Thus the weight of ice required=225\80=2.81 kg
The present simple calculation shows that unless at least 2.81 kg ice is melted, it is impossible to chill 10 kg fish from 25˚C to 0˚C.
But this theoretical calculation can not take into consideration of some other important factors. Because, in practice, when fish is iced, say in box, some ice will be used in cooling the box and more will be melted on the journey by heat coming in through the lid and wall of the box.
Therefore, in quantifying the amount of ice the following points should be considered.
* Insulation of ice box: Ice absorbs the heat from the fish body and thus melts. Ice is also melted by surrounding air unless fish and ice is protected with insulating materials. Therefore, quantity of ice varies depending on the condition of insulation of the box.
* Cooling the box itself: Ice is needed for cooling of the box or container\insulation of the box.
* Nature of packing in ice: All the ice are not in direct contact with all fish. If ice crystals are small and homogeneous, packing is good, cooling of fish is prompt and ice will last long.
* Time for keeping fish in chilled condition\length of journey.
* How quickly the fish are chilled.
* Thickness of the fish.
* Ambient temperature.
Therefore, depending on the situation, it is wise to use 10-20% more ice than the calculated value. In tropical climate, outside heat consumes more ice than heat from fish. Therefore, insulation is particularly important in Bangladesh.
Nature of water to be used in ice
· Clean and contamination free water should be used for the manufacture of ice.
· Municipal water should not be used directly in ice production. It can be used after necessary cleaning and chlorinating (5-10 ppm).
· Water from ponds, lakes, ditches, rivers, canals or sea-shore should not be used. These natural waters contain mud, dirt, debris, bacteria and other contaminants that, if used in ice, may serve as a basis of fresh contamination in fish and lead to quick spoilage.
· Sea water contains salt and transformed into ice at much lower temperature than the usual temperature required in ice formation (0˚C). Therefore, the quality of ice from sea water is not good. To form ice, seawater is sometimes used in ice production if freshwater is scare, as in case of big trawler or factory ship in the sea or in desert states.
· Quality of ice will be good if adequately treated tube well or deep tube well water is used.
· Large tanks\reservoirs where water is preserved in ice plant should be cleaned regularly .
· Antibiotics in water at prescribed dose may increase shelf life of iced fish but this should strictly follow local and international regulations.