9. Chilling fish on land
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At the quayside
Lean fish. Where fish have been chilled to ice temperature at sea, they should be handled on shore so that this temperature is maintained, as far as possible, throughout the distribution chain. Once the fish are allowed to warm, it is extremely difficult to chill them again. Where fish are landed without ice, they should be chilled in ice as soon as possible after landing.
At large ports, where the catches from a number of vessels may be sold at the same time, discharge may have to begin several hours before sale time. Fish left for hours on a warm quayside without much ice on them can warm rapidly, particularly in tropical countries.
At smaller ports, or at landing places where no marketing facilities exist, the fish are sometimes landed and carried away immediately, but whenever there is a delay at the quayside the fish should be well iced. Where there is no covered market, the iced fish should be protected from the sun by covering the boxes in some way. A temporary awning or shelter can be rigged, an insulated blanket can be used, or at the very least a tarpaulin can be laid over the stack of boxes.
If adequate icing at the quayside is impracticable, delay in moving the fish away should be avoided. For example, some mechanical handling may be possible at larger ports and careful timing of the arrival of transport can prevent unnecessary warming of unprotected fish (Fig. 24).
Uniced fish may be as warm as 15°C on landing in temperate climates and perhaps 30 to 35°C in the tropics. Unless the fish can be cooled rapidly on shore, the catch will deteriorate in a very short time. Ice must be distributed throughout the fish to chill them effectively. Warm fish in boxes which are stacked together will remain warm when ice is sprinkled only over the top of the stack. Each box must have ice in it to chill the contents and, where it is possible only to ice the top of the box, a shallow box is better than a deep one.
Fig. 24. Handling fish at small ports
Chilling is equally important once the fish have been sold. They should be removed from the market as quickly as possible and kept iced until they are sold to the consumer, or used for processing.
Fatty fish. Because mackerel and other small fatty fish are usually caught in quantity over a fairly short period of time and often not far from port, they are usually of fairly uniform quality on landing. Sale can often be by sample, and the main bulk of the catch can be unloaded direct from fishing vessel to road vehicle without delays on the quayside. If the fish are going to a factory near the quay at a large port, it is sometimes practicable to move the fish from the vessel's fishroom to factory reception by pump or conveyor.
It is sometimes impracticable to ice small fatty fish adequately at sea, and in any case the voyage to port may not be long enough to allow the ice time to chill the fish properly. It is therefore all the more important to chill them once they are landed. Again, ice over the top of a large heap will be ineffective; the centre of the heap will remain warm for a long time, and fish at the bottom will be crushed. The catch must be split into small lots, and each lot iced separately, so that chilling can be rapid.
Alternatively, refrigerated seawater can be used to chill large quantities of small fatty fish, either at the quayside or, if they are going to a nearby processing plant, at the factory itself. Uniced herring, still warm on discharge from the ship, dispatched to a factory or market some distance from the port, can be completely spoiled at the end of a journey of only a few hours' duration.
Shellfish. In many cases shellfish are landed live a relatively short time after catching. During this period it is usually sufficient to cover the shellfish so that they are not exposed to direct sunlight and, with most species, it is also necessary to keep the shellfish moist. On landing, the shellfish should be iced as soon as possible and mixed intimately with the ice to ensure quick cooling. If the fishing trip is of longer duration the advantages of rapid chilling by icing are just as important as for fish. Since shellfish are usually more valuable than other types of fish, more generous quantities of ice are used in order to ensure that sufficient is available to maintain the fish at chill temperature in the event of any unforeseen delays.
At the port merchant's premises
When fish are sold at the quayside to a port merchant or processor they should be moved to his premises as quickly as possible. His main functions may be simply to pack and transport them to the main centres of population, or to process the fish in some way, perhaps filleting, smoking, salting or drying, or possibly quick freezing followed by cold storage for long term preservation. Whatever the ultimate outlet for the fish, the handling of the raw material is much the same.
As soon as the fish arrive at the merchant's premises from the quayside, they should be iced or re-iced if they are not to be processed immediately. It is not sufficient to put the fish in a chillroom without ice; cooling will be very slow because air is a poor conductor of heat. The fish should first be mixed with small pieces of ice and then put in a chillroom so that the job of the ice will be confined to chilling the fish and not the warm air outside. A chillroom can be used to keep fish that have already been chilled to ice temperature, but even then some ice is needed on top of exposed fish to stop them drying out.
Ungutted fish may require to be gutted as a first procedure on shore, because the contents of the intestines rapidly decay and spoil the surrounding flesh. Further dressing, such as heading, filleting or splitting, will depend on market requirements. Any operation should be done in cool surroundings; during delays the raw material should be protected by the judicious use of ice and chillrooms.
When filleting is done by hand, the fish are usually kept in a tank or trough of water and taken out one by one. The water in the trough is often warmer than the fish, and the fish warm up. Where practicable ice should be added to the water in the filleting trough to chill it or, all process water can be put through a central chilling system. As soon as enough fillets have been cut to fill a box, the top of the box should be iced over, and the box removed to chill storage. Whatever other operations are carried out, whether by hand or machine, the same principles apply; keep delays between operations as small as possible. Wherever practicable, use ice to keep the product chilled at every stage.
Typically, fish warm considerably during handling and processing. Even in temperate climates, although the fish entered the premises close to 0°C, the fillets produced a few hours later may reach 1 0°C or more by the time they are to be packed. Such temperature excursions result in a measurable increase in the rate of spoilage or loss of quality.
Fish on display in a shop should be kept on a bed of ice. A further sprinkling of pieces on and around the fish will keep it chilled efficiently and enhance its appearance to the customer.
Insulation beneath the display counter will help to conserve the ice and it is possible to use a refrigerated display unit, provided the temperature is kept above ice melting point. The fish should not be displayed without ice in such a unit. As in the chillroom, uniced fish will dry out, become dull and unattractive, and may become partially frozen. Temperature control of refrigerated display units is sometimes difficult and inaccurate, but ice is its own thermostat.
A glass or transparent plastic shield round the display area will help to maintain a pool of moist air around and above the fish, and will restrict draughts of warm air which can dry the fish. Fish on display should be in thin layers so that they can be kept adequately chilled; fish piled high will warm up and remain warm. The display unit should be hygienically designed and have adequate drainage, so that the fish do not become waterlogged or contaminated by dirty melt-water (Fig. 25).
Smoked fish products should not be laid directly on ice, but they can be displayed in the same unit as fresh fish simply by putting the fish on trays laid on the bed of ice. As with fresh fish, the main stock of smoked fish should be kept in a chillroom, and only small amounts displayed for sale at one time.
Finally, it should be remembered that fish remains fresh only for a limited period even when surrounded by liberal quantities of ice. Stock should be replenished at frequent intervals and, if there is any doubt about the freshness of fish, it should not be sold. If in doubt, throw it out!
Fig. 25. Retail display of fresh fish
Icing practice for transportation
Once the fish have been dressed or filleted to suit the needs of the market, they are then packed in containers for onward distribution from the port. All too often, insufficient ice is used for the journey, and put in the wrong places.
An interesting method used in East Asia to pack medium or large size gutted fish is shown in Fig 26. The plastic foil used reduces heating up of the fish by protecting the content of the basket from heat gains, which would be due to air convection and diffusion of moisture during transport.
The ice packed in a box of fish is intended to do two things, firstly to chill the fish to 0°C and secondly to keep it at that temperature, despite the heat entering the box from its surroundings. Fresh fish is rather a poor conductor of heat, that is to say the heat takes a long time to pass through the fish. It is common practice in some fisheries to pack fillets about 10 cm deep in a box with a layer of ice 2 to 3 cm thick on top; it takes about 24 h to cool fillets from 1 0°C to 0°C in this way. The time taken for a fish or fillet to cool depends on its distance from the ice layer, as explained in Chapter 2, so that fillets at the bottom of the box cool very slowly indeed; even when there is ice remaining on top of the fish at the end of the journey, there may still be fillets at°5 C or above in the box. Ideally, the fish or fillets should be cooled close to 0°C before packaging.
It is thus very important to put ice in the right places in the box. If ice is placed only at the ends of the box, for example, it may take several days to chill the fish in the middle of the box, if they are ever chilled at all.
In other words, ice correctly and check temperatures whenever possible as well as observing the presence or absence of ice during transportation.
Fig. 26. A manner of icing gutted fish
Fig. 27. How to ice a box of fish or fillets
The best practice for boxes of fillets is to place a layer of ice in the bottom of the box and a layer at the top (Fig. 27). Provided enough ice is used in the first place, all the fish should then reach their destination after a journey of several hours with temperatures close to 0°C. The fish in the centre are the slowest to cool, and the thicker the layer of fish, the longer cooling takes. Table 15 shows that after 18 h with a layer of ice only on top, a 7.5 cm layer of fish initially at 1 0°C has cooled only to 4°C. When the fish are iced top and bottom, it can be seen from Table 16 that it takes four times as long to chill a layer 15 cm thick as it does to chill a layer 7.5 cm thick. Although the initial temperature of the fish has some effect on the time taken to cool it, the thickness of the layer has much more effect, as shown in Table 16.
Table 15. Time to cool a layer of fish iced only on top
from 10°C to 4°C
from 10°C to 2°C
Table 16. Time to chill a layer of fish, well iced top and bottom, from various starting temperatures
at centre of box
2 C at the centre
|15||15||2.5°C after 10|
Number of boxes required
It is relatively easy to calculate the number of boxes required for each load of fish if the proportions of fish to ice are known. However, if the fish have to be graded either by species, size, source or other criteria, the box requirement will invariably increase, since all the boxes may not be fully utilised. In these situations a contingency factor will require to be applied, which may be based on previous experience or a reasoned estimate.
When considering the box requirement, it is also necessary to examine the total operation and determine likely box movements and locations. The best way to do this is by plotting, step by step, the box movements over a few days to establish both the normal pattern and, also, the consequences of stoppages at weekends and at other times. The box requirement is then worked out to ensure that there will be no delays in handling the fish, and a further factor applied to allow for losses and damage.
Box movements can be plotted as a line diagram showing the various routes, and operations. The example shown in Fig. 28 is based on a fish collection and processing operation where the following conditions apply:
1. Small boats supply fish to a collecting centre where the
nominal daily maximum catch is iced in 150 boxes ready for
transfer to the processing factory.
2. A trawler also supplies the collection centre with 150 boxes of fish daily.
3. The trawler's catch is boxed at sea and this may be reboxed at the collection centre or transferred to the factory in the same box.
4. All fishing craft leave at 0500 h and land their catch at 1100 h daily.
5. The work at the collection centres is completed by 1800 h and the fish are transported to the factory to arrive at 2400 h.
6. Boxes are stored at the factory overnight.
7. The boxes are emptied and washed next day and are ready for returning to the collection centre by 1200 h.
The procedure used to arrive at the final box distribution diagram, shown in Figure 28, is sometimes one of trial and error. The main requirement is that boxes must be available at each stage for stowing the fish and, in this case, the starting point was the need to have 300 filled boxes at the collection centre ready for transporting fish to the factory at 1800 h. In addition, if the trawler fish was reboxed, there would be 150 dirty boxes to go for washing. The daily pattern clearly shows that 900 boxes are required to sustain the fish collection system and, with a contingency factor of about 10%, 1000 boxes should be purchased. If it is anticipated that the pattern will change during the week, the movement of boxes will need to be determined over a longer period.
In the above example, other containers were used on the small fishing boats, and between the landing places and collection centre. If this is also an integrated boxing system, a similar box location diagram will be required to determine the number of boxes for this operation.
In order to avoid contamination of the finished product, fish are washed prior to processing, and boxes used for transportation are also kept separate from the boxes used in the factory. Again, it will be necessary to prepare a box location diagram for the factory operation in order to determine the number of boxes required.
Fig. 28. Box distribution diagram
Air shipment of chilled fish
Seafood packaged for transport by air usually has to conform to exacting standards laid down by individual airlines. These standards are mainly set to avoid the possibility of leakage from the packages which in the past has resulted in corrosion of airframes and contamination of other goods. Standards are also set for package weights and dimensions and these may depend on the type of aircraft.
The following information on packaging for air shipment is based on current practice in both the USA and Australia. The methods described will conform with the standards set by most carriers. It is essential that shippers of chilled seafood check with all airlines involved to ensure that their proposed method of packaging is acceptable.
The product should be prechilled before packaging, and prechilling the packages and containers will also be helpful. Prechilling reduces the quantity of coolant required and thereby reduces the weight and space requirements. Quick prechilling is also good practice since it eliminates the possibility of a long cooling period during which spoilage proceeds at a faster rate.
An early consideration is the selection of suitable packaging materials. Different products may have different requirements in order to achieve durability and water tightness and also provide the necessary insulation properties at as low a cost as possible. Local considerations may also influence the choice of materials.
Both the coolant and the product may be required to be packed in water- tight sealed bags. Polyethylene is suitable for this purpose and the thickness should be suitable for the type of product. Thicker material, for instance, may be required for shellfish if there is a greater likelihood of the bag being punctured. Absorbent material is often placed within the container even when there is no fluid loss from either the product or the coolant. Under certain conditions, water vapour in the surrounding air may condense on the cold outer surfaces of the packages and this should not be allowed to leak or be absorbed and thereby weaken the material used for the outer container.
Although high quality insulation such as polyurethane may be expensive it may be more economical to use. Better quality insulation will reduce the coolant requirement or, if the same insulation value is maintained, the better insulation properties of the polyurethane will reduce the insulation thickness and thereby save space. Expanded polystyrene is another popular insulation material. Although this material is waterproof and light, it has poor impact resistance. Consequently, when polystyrene boxes or containers are used they should be placed inside a strong waterproofed cardboard carton as protection against damage. When containers are made from materials such as cardboard or fibreboard, they should be waterproofed by applying a polyethylene coating or by wax impregnation.
Coolants such as gel refrigerants, dry ice (solid carbon dioxide) and water ice are all used to cool fish transported by air. Gel refrigerants have the advantage that they do not produce any gaseous or liquid effluent during the storage period. If they are used on a non-return basis they will prove to be more costly than the other coolants. When dry ice is used, gaseous carbon dioxide is evolved. This displaces oxygen in confined spaces, so dry ice is classified by airlines as a potentially dangerous substance and is subject to special regulations with respect to how and where it can be used. One precaution to be made is that the carbon dioxide gas must be vented to prevent rupture of the package or container. There may also be regulations to limit the quantity of dry ice which can be loaded into the hold of an aircraft. Therefore each package should be labelled showing the initial weight of dry ice used. The cargo handling office of the airline will advise on the packaging and labelling instructions to be followed. Water ice has less cooling capacity for a given weight than the other coolants but, providing it is retained within the container, it is harmless and relatively cheap. Water ice also controls the temperature at a desirable 0°C, whereas, with the other coolants, care has to be taken to ensure that there is no partial freezing of the product. The coolants should be arranged within the package to ensure uniform cooling (Fig. 29).
Container storage of chilled fish
Generally, boxes are containers which can be lifted easily by one or two persons. A storage container can be classified as a unit which is bigger than a box but is not a fixture in a fishing vessel, chillroom or transport vehicle. Containers may be insulated or uninsulated, but generally when this type of storage is used in tropical climates the containers are insulated. The one advantage of using larger containers rather than smaller boxes is that it is more economic to insulate one larger container. Containers are therefore more likely to find a use in tropical countries than in cold or temperate countries where ice meltage rate is lower.
The arguments against deep bulk storage and consequent crushing of the fish may also apply to containers. The optimum size of container usually means a storage depth of about one metre, but it is recommended that storage depths of fish should be limited to about 300 mm. However, a case can be made for the use of containers, and they are now available in a wide range of sizes, shapes, materials and insulation properties.
Insulated containers and chill stores
Uninsulated boxes may be stored in large insulated containers or insulated stores to achieve savings in the icing requirement. The likely magnitudes of these savings of ice are shown in the Table 17, which compares the relative ice meltage rates for a number of iced storage methods.
Figure 29 Packaging for air shipment
Table 17. Relative ice meltage rates
surface area (m²)
per unit weight of fish
|Insulated room||2400 (80 boxes)||34.5||1|
As can be seen from Table 17, insulation of a fish box significantly reduces the ice meltage rate during storage. Increasing the unit size to an insulated container also reduces the ice requirement. Storage in a larger insulated room, even if the boxes are uninsulated, will result in a further reduction in the ice meltage rate.
Ice usage can be reduced to an almost negligible amount by storing the fish boxes in a refrigerated space. In most cases, the additional refrigeration cost can be more than offset by the savings in ice. This system is often used where it is important to save storage space as well as reduce ice meltage. The use of uninsulated boxes and the absence of large quantities of ice allow a greater payload of fish to be carried. Typical applications (of refrigerated systems) are on larger fishing vessels and during transportation.
Some of the benefits described above in terms of reduced ice meltage are based on simple relationships determined for a single box or container which do not truly reflect commercial conditions. For instance, fish boxes are usually stacked and under these conditions ice meltage for a load of fish will be considerably reduced. However, it is reasonable to consider only one box or container as a guide. Icing practice must be suitable for all eventualities. Boxes may either be stored singly or be located on the outside of a stack where meltage will be much the same as for individual units.
The choice of an insulated box will not only depend on the potential ice savings achieved during normal storage but also on the conditions during the entire handling and storage period. For instance, if boxes are exposed to high ambient temperatures, even for a relatively short time, it may be necessary to have them insulated since all the benefits of using insulated or refrigerated rooms for most of the rest of the storage period may be lost. The type of container and the choice of materials used in its construction not only depend on potential ice savings but also on other costs, hygiene, availability of materials and handling aspects.
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