13. FREEZING AT SEA


13.1 Why freeze at sea?
13.2 Type of freezer vessel
13.3 How good are sea-frozen fish?
13.4 Freezers for use at sea
13.5 Handling fish before freezing
13.6 Handling frozen fish
13.7 Cold stores on freezer vessels
13.8 Unloading freezer vessels


13.1 Why Freeze at Sea?

The length of time a fishing boat can remain at sea depends on the time the fish can be kept so that they are still edible on reaching the consumer. Storage in ice or by other means which keep the fish chilled is adequate for periods not much in excess of two weeks.

Fish such as haddock and cod caught in the North Atlantic fisheries can be stored for up to 15 days in ice and then rapidly become inedible. It has been found that fish caught in tropical waters can remain edible for even longer periods when stored at chill temperatures. This may not be a general rule and the limitation of chilled storage must be established by local experience.

In practice, the time restriction for storage in ice often means that fishing vessels must return to their home port with the fish room partly empty. There is therefore a need for some means of preservation that will extend the storage life without substantially altering the nature of the raw material. Quick freezing and cold storage is an excellent way of doing this.

When newly caught, fish are frozen quickly and stored at a low temperature on board, so there is no limit imposed on the length of voyage due to spoilage of the catch. Fishing vessels can remain at the fishing grounds until the hold is full. This increases the proportion of time spent at the fishing ground and improves the economics of fishing. It also allows the fish to be distributed to a wider market even without the existence of an elaborate "cold chain ". Fish which have been frozen at sea are of very good quality when landed; therefore, more time is available for the fish to be distributed over a wider area and still be in good condition.

Freezing at sea has therefore an important role in world fisheries. A look at a map will show that large areas of ocean are far distant from any centres of population or even land, and many potential fisheries are therefore unexploitable without a method of preserving the fish for long sea voyages. Only quick freezing and low temperature storage has so far satisfied this need and, as traditional near water fisheries become overfished or are unable to satisfy the growing demands of an ever increasing population, freezing at sea will become more and more necessary.

13.2 Type of Freezer Vessel

Fish frozen at sea may be frozen whole, immediately after catching and when thawed on shore, can then be used in much the same way as fish traditionally preserved in ice. Alternatively, the fishing vessel can operate as a fish processing factory and the fish may then be filleted, packaged and frozen, and the waste products converted to fish meal and oil.

Freezing of the whole fish has the following advantage over processing before freezing. The number of crew required is not much greater that for a fishing vessel of comparable size preserves its catch in ice. Processing equipment and factory deck space, are a good deal less. The whole fish, when thawed after landing, are available for any form of traditional processing. The problems associated with freezing newly caught fish are less with whole fish than fillets. For the above reasons, it may therefore be advisable as a first step for a developing country to freeze whole fish and progress to a factory-freezer operation as the situation demands.

13.3 How Good. Are Sea-Frozen Fish?

Sea-frozen fish, properly handled between landing on deck and loading into the freezer, when thawed are almost undistinguishable, from fresh fish kept in ice for a few days.

The loss in quality as a result of freezing, cold storage and thawing is small when these treatments are properly applied. Thus, when very fresh fish are frozen at sea, the final product can be equal to the best on the market.

13.4 Freezers for Use at Sea

A number of conventional freezer units may be used at sea with little modification but may have to conform with national regulations and insurance requirements for fishing vessels. Many countries, for instance, do not allow the use of ammonia as refrigerant because of its toxicity and because there is a potential explosion hazard. The design and operation of the freezers and the refrigeration system must take into account the movement of the vessel, vibration, sea-water corrosion and the extra rough usage likely under the arduous conditions experienced at sea. Another factor that may influence the choice of type of freezer is the type and variety of fish species to be frozen. The freezer should be able to cope with the variation in fish sizes in fisheries where many different species are caught.

The VPF was specially designed for freezing whole fish at sea. In most applications, a 100mm spacing between plates was found to be adequate. This spacing allowed a very high percentage of the catch to be quick-frozen and reduced to the cold storage temperature of -30C in the recommended time of 4h. Oversized fish were normally frozen in a separate air blast freezer room.

The UK design considerations which led to 100mm spacing being used, were based on the freezing of gutted fish with heads on. Other countries' requirements may be for ungutted fish to be frozen or for fish to have the heads removed as well as guts before freezing. These considerations will have to be taken into account as well as the size, shape and variety of species to be frozen before a decision is made on the preferred plate spacing.

Another factor to be taken into account with any type of freezer is the overall size and weight of the frozen product. If the product has to be lifted and stacked in the fishing vessel cold store, care should be taken that the block weight is well within the physical capabilities of the crew. It has been possible to operate in the UK with 45 to 50 kg blocks measuring approximately 1060 x 520mm.

Many other types of freezer are also suitable for freezing fish at sea, and HPF, brine freezers and a variety of air blast freezers have been used. Most of these freezers have been described in Chapter 4 but for use at sea they have to satisfy some special requirements.

The following design and operational requirements for freezers to be used at sea will give the reader guidance on whether a freezer is suitable for this application.

  1. The freezer should be easily loaded and unloaded.
  2. Freezers with trolleys should have special arrangements to make them safe during rough weather.
  3. The freezer should be able to retain the product during the loading and unloading procedures; serious damage or injury can result from a dislodged frozen block or tray of fish.
  4. The freezer should be able to operate with part loads which may result from variations in the catching rate.
  5. The refrigeration system should not give rise to uneven freezing due to displacement of the refrigerant with the movement of the vessel.
  6. The freezer should be robust.
  7. The material used in the construction of the freezer should be resistant to seawater corrosion.
  8. The freezer should be constructed so that it can be cleaned by hosing with seawater.

The above list is not exhaustive but it is sufficient to indicate that many types of freezer would, in fact be unsuitable for use on a fishing vessel.

Space on a fishing vessel is limited especially the height available between decks. Freezer designs and layout should therefore be made to suit this space restriction. The quicker the freeze, the smaller is the size of a freezer for a given capacity. Freezers for use at sea should therefore be designed for short freezing times, taking into account both the refrigerant operating conditions and the product shape and size. Large fish like tuna are frozen individually. Brine immersion freezers have been used but there has been a recent trend toward air blast freezers for this purpose. Shell-on shrimp and other shellfish are also frozen individually, but apart from these few exceptions, freezers for IQF products are unlikely to be required on a fishing vessel.

Extra care has to be taken with the refrigeration equipment. Pipework should be secure and routed so that it is unlikely to be damaged. The use of secondary refrigerants for many shipboard systems has resulted in a system that can be maintained in a relatively leakfree condition. When a secondary refrigerant is used, the primary refrigerant is confined to the condensing unit and heat exchanger. Since secondary refrigerants are liquids at atmospheric pressure which only operate at pumping requirement pressures, there will be a greatly reduced incidence of refrigerant leaks. Calcium chloride brine is by far the most popular in use today. However there is controversy at the present concerning the corrosion inhibiter required for multi metal units. The existing chemical is alleged to be harmful to health and although there are alternatives proposed, the problem has, as yet, to be resolved. The major requirement for both the freezer and the refrigeration system on a fishing vessel is reliability.

13.5 Handling Fish Before Freezing

The layout of a stern trawler illustrates a good arrangement for handling fish before freezing. The fish are pulled up the stern ramp, poured from the net through hatches to the factory deck below and then moved forward as they go through the various stages of processing. Not all vessels can use this preferred arrangement and it would be impossible to cover all the potential layouts for the wide variety of vessels now used for freezing at sea.

The pre-freezing procedures described below are typical of a freezer trawler fishing in the North Atlantic. They may, however, have a more general application, and only minor modifications may be necessary to accommodate other vessels and their particular requirements. Fish must not be left lying on the upper deck exposed to direct sunlight but stored on a sheltered working deck immediately below. The fish should be kept as cool as possible immediately after catching and throughout the time they are awaiting freezing. Some means of cooling, such as a of seawater spray or chilled sea water (CSW) tanks, is therefore recommended when the fish are subject to any delay before gutting. Cooling of the fish not only helps to retard spoilage but also stops the blood from clotting too quickly. In tropical conditions, it will be necessary to provide a means of chilling the water used for this purpose.

Gutting of fish should commence as soon as possible after being caught not only to ensure the continuity of supply to the freezers, but also to reduce the rate of spoilage. The removal of the gut releases blood from the fish and must be followed by immediate washing in chilled water. Blood which is not released soon enough, clots within the tissues resulting in a permanent pink or red discoloration of the flesh which detracts from the appearance of the fillet. The liver must also be removed as it contains a fat which is highly perishable and could become rancid even at low temperatures. A time of 15 to 30 min in cold water is usually required for bleeding to be complete. However, in practice it is often difficult to ensure that all fish are given time to bleed properly. One solution is to make washing a two stage process. The gutted fish are first put into an open tank where they can bleed while they are kept cool by chilled water sprays, and then they are conveyed to an automatic fish washer where they are given a final rinse before freezing. The need for delay for proper bleeding of the fish before freezing must seem an added encumbrance. However, where it is desirable to gut before freezing, time must be allowed for the blood to be released from the fish to ensure they have good appearance. If appearance is not important in the final product, this delay for bleeding may not be necessary. Traditionally, some fish may only be marketable ungutted and in this case a special effort should be made to handle the fish quickly. They should be kept chilled and rough handling should be avoided.

Fish are usually sorted before freezing so that each block or package contains only one species. With some species, further subdivision into size grades may also be necessary. This extra handling for sorting and grading is viable when a premium is paid for graded fish. Some sorting is required before processing to reject unwanted and undersize fish and any trash material. Whether it is feasible at this time to sort the fish to be frozen into species and size grades will depend on individual requirements.

Heading of the fish may be desirable in some cases. Headed fish make a more compact block and heading allows larger fish to be frozen in some freezers. Removing the heads also means that the freezers can be used more efficiently and the proportion edible fish stored is also increased. One disadvantage of heading is that a small but not insignificant amount of edible fish is removed with the head. The cut surfaces may also become discoloured in time and trimming may be necessary.

Small pelagic fish such as herring are traditionally landed in the whole ungutted state and should also be kept cool since spoilage rate will be higher than for larger fish that have been gutted. Freezing if required should be done as soon as possible.

Another major problem, which usually only applies when fish are frozen at sea, is due to the effects of rigor mortis. Very often fish are bent prior to rigor. This should be avoided as far as possible since the flesh of the fish on the outside curve will be put under a strain and, when rigor sets in, the extra forces involved will pull the fish apart. This results in gaping of the fillets. The fillet in the inside of the curve, on the other hand, will shrink and contract so that two different looking fillets will be obtained form the same fish. One will be elongated with much gaping and the other will be short and compact. Freezing will of course maintain the fish in this bent condition, and will no doubt be blamed for the phenomenon. If bent fish in rigor are straightened before freezing, gaping of the short compact fillet will result. The onset of rigor mortis is quicker at higher temperatures and may occur only 10 to 20 min after death at temperatures near 30C. It is therefore essential that the fish be chilled quickly if problems due to rigor mortis are to be avoided during freezing.

If the fish are to be filleted at sea and frozen as fillets, it is even more important that chilled conditions exist along the entire production line. When a fish goes into rigor, there is a gradual increase in the tension of the muscle fibres and, as long as the muscle remains attached to the skeleton of the fish, shrinkage is restricted. However, once a fillet is cut from the fish, this restraint is removed and, if the onset of rigor mortis is not complete, the fillet will shrink. This contraction gives the fillet a corrugated appearance and a distorted shape. Temperature has an important bearing on this process, the higher the temperature, the faster the shrinkage and hence, the greater the effect in a given time. Fillets that are allowed to shrink at a high temperature before freezing can lose greater quantities of drip on thawing. In addition, excessive flexing or contact with water can increase the amount of shrinkage of a pre-rigor cut fillet.

A fillet taken from a fish before rigor has set in will, after freezing and thawing, have a dull appearance. This absence of gloss is probably due to the cut ends of muscle cells projecting upward. The fillet has a velvety feel and will not, for instance, produce an attractive smoked product. There is no known solution to this problem so far, apart from delaying filleting until after the onset of rigor mortis.

Before freezing, the fish may be stored in bins of an equivalent volume and adjacent to the freezers. This ensures that only the correct quantity of fish is available to fill the freezers and none are left on conveyors. The working space adjacent to the freezers should be kept cool to ensure that the fish do not warm up at this stage and the bins should be emptied in strict rotation so that no fish lie longer than necessary. Final sorting can be made at the freezer and a facility e provided for the storage or return of rejects.

Any chilling of the fish prior to freezing will not be an extra refrigeration requirement since any heat removed before entering the freezer means a reduction in the subsequent refrigeration loading. Chilling will be a low cost process with considerable benefits in improved fish quality.

13.6 Handling Frozen Fish

Fish should be transferred to the cold store immediately they are removed from the freezer. Even large blocks of fish warm up rapidly at ambient temperatures particularly in tropical climates. Heat added to the fish at this stage means that it has to be removed in the cold store and this will mean some loss in quality and an additional cold store load. Mechanical aids, such as chutes or conveyors, should be used since the less handling the frozen fish receives, the less chance there will be of the fish being damaged. Damaged blocks require more storage space and extra handling in the cold store. Damaged fish should be kept separate in the store since they may require special discharge arrangements.

Unfrozen or partially frozen fish should never be placed in the cold store as it is not designed for freezing. Partially frozen fish are also more easily damaged during handling. When fish are graded before freezing, the blocks or packages should be clearly labelled and, if possible, the different grades kept separate within the cold store. Labels placed on the surface of frozen fish and brushed over with clean water will adhere to the surface and this method of marking can be used if the fish are unwrapped. If wrappers are used for the fish, they should be suitable for marking so that the fish can be identified and handled more quickly at the time of discharge or later.

13.7 Cold Stores on Freezer Vessels

The cold store on a fishing vessel should operate at the temperature and on the same principles as recommended for shore based cold stores. Even if the storage time on the vessel is to be relatively short, it must be remembered that poor practice at any stage of handling and processing will have a cumulative effect which may become obvious by the time the fish reaches the consumer.

Loading and unloading of a fishing vessel's cold store is usually done through hatches at roof level. This is a good arrangement since there will be little exchange of air between the store and outside when the hatches are open. One distinct disadvantage of this hold arrangement is that even small refrigerant leaks can result in an accumulation of refrigerant in the cold store and, although the refrigerant may not be toxic, the resulting low oxygen level may be dangerous. Shipboard cold stores must therefore have an effective alarm system and the crew disciplined to use it and obey all other safety rules and regulations.

Frozen fish in the cold store of a fishing vessel may have to be stacked with a retaining system to prevent movement of the product. A structure similar to that used to form partitions and shelves in the fish room of a wet fish trawler has been used for this purpose. The following figures should only be used as a rough guide since the finer details of store construction and layout, the shape and size of the frozen product and the method of packing can mean considerable differences in storage densities for apparently similar installations.

Table 27 Stowage rates for frozen fish in the cold store of freezer vessels

  (m3/t)
Large blocks of cod (including allowance for support structure) 2.0
Large blocks of cod (open stowage with no support structure 1.4 to 1.7
large blocks of fillets (including allowance for support structure) 1.2 to 1.5
Frozen cod stowed as single fish 2.2 to 2.6

Insulation of a cold store on board a fishing vessel creates some special problems. The cold store insulation is usually attached directly to the ship's side; therefore, the rib structure of the vessel will penetrate into the insulation for some distance. Although the thickness of the insulation need not be increased to more than would be necessary for corresponding store on shore, the design should ensure that there is an effective thickness of insulation at all points in the store (Figure 44). Any internal structure within the cold store space should be attached to the main framework of the vessel with an effective heat barrier. Metal or any other material with a high thermal conductivity should not be used for this purpose.

v3630e44.gif (14839 byte)

Foamed-in-place polyurethane insulation and polyurethane slabs have been used for the insulation of shipboard cold stores. The application of this type of insulation is difficult and requires skilled operators and special equipment. Loose fill insulation may however be used in combination with others for instance for packing awkwardly-shaped areas where cutting of slabs would be difficult. Another desirable property of an insulation for a steel fishing vessel is that it should be reasonably heat-resistant to enable welding or other heat treatment to made to the outer hull. Unfortunately, none of the insulations that are likely to be used can completely satisfy all the requirements but some are significantly better than others. Insulation choice and method of application can only be made after consulting the relevant codes of practice or legislation for shipboard insulation for the country in which the vessel is registered or insured.

Increased insulation thickness can be costly on a fishing vessel since only a few centimetres increase in thickness can mean a significant reduction in the storage space. A store holding about 100t of fish, for instance, would be reduced to about 95t if the insulation thickness is increased by only 5cm.

The choice of cooling systems available for the cold stores of fishing vessels is much the same as that for other stores. Pipe grids and forced circulation coolers have both been used successfully. When large blocks of fish are stored, the system selected should not be vulnerable to damage due to block handling or movement. Grids on the sides of the vessel require to be protected since a 45kg block of fish can damage pipework particularly since metal is a good deal more brittle at cold store temperature. Plain pipe grids on the roof only have been used but, in order to get the required heat transfer surface, finned grids are usually required and care must be taken to ensure that the frost on the pipes does not bridge the air space between the fins. If plain pipe grids are used, it will normally be necessary to continue the grids to at least halfway down the sides of the hold.

Attempts have been made to avoid using wall grids by having two or more rows of plain pipe grids on the roof. In terms of cold store quality, this is an inferior arrangement and also makes defrosting more difficult than when a single row only is used. Cooling grids together with any protective lining take up a good deal of space in a store but some of the space would normally be kept free as clearance around the produce.

As in some modern stores on shore, unit coolers may be used and they can be located outside the main storage area, for easier defrosting, and the cold air circulated uniformly within the room.

13.8 Unloading Freezer Vessels

Handling frozen fish is obviously different from handling iced fish and cannot be unloaded in the same manner. Any unloading system should handle the fish quickly between the vessel's cold store and the cold store on shore. Delays at this stage, particularly in warm climates, can result in partial thawing of the fish with a resultant loss in product quality.

There should be no delays on the quayside and any grading of the frozen fish according to species or size should be left until the produce is in the cold store or a least under cover. Ideally, cold stores should be adjacent to the landing place and fish can then be moved quickly, preferably by conveyor, into cold storage. Alternatively, if the store is reasonably near to the quay, unrefrigerated vehicles may be used for transporting the fish provided there are no delays. Vehicles however, should be of the enclosed type and they should be loaded under a canopy so that fish are not exposed to direct sunlight. Large capacity vehicles should not be used since extended loading times will result in a good deal of heat being added to the fish. Even when smaller vehicles are used, if delays are unavoidable the vehicle should be despatched to the coldstore with a partial load rather than wait to be fully loaded. In some countries, labour may be relatively cheap and mechanical handling may be considered an expensive luxury. In these cases, it may therefore be more economic and quicker to manhandle the fish. The rate of unloading of frozen fish from a vessel will depend on the size of blocks or packages or the containers used for loose fish. It will also depend on the facilities provided on the vessel, such as number and size of hatches and also the degree of accessibility and hence the number of men that can be employed at one time. With a mechanical unloading system and a skilled crew, unloading rates of 1 to 1.5 t/man hour can be achieved. Mobile cranes and ship's derricks have also been used to unload frozen fish into enclosed containers in the hold which are then transferred to the quayside. This operation should ensure that there is no delay on the quayside in order to transfer and sort fish. Storing the fish in containers in the ship's hold has been suggested but unless the vessel is specially designed for this purpose, up to 30 per cent of the available storage space could be lost due to the presence of the pallets and the need for squaring off the hold. This would mean that a fishhold capable of storing 600t of unpalleted frozen fish in open storage would only be able to hold 400t if the fish were graded and stored in containers. Only if a freezer vessel reached the proportions of a cargo ship would palletization be achieved without a significant loss since a vessel of that size would have parallel sides for a good length of the hold. The stowage rates given in Table 26 give some guidance for calculating the likely hold capacity.