Fish Processing


The fish processing industry is very widespread and quite varied in terms of types of operation, scales of production and outputs. The species of fish processed include cod, tuna, herring, mackerel, pollock, hake,haddock, salmon, anchovy and pilchards. Marine fish account for more than 90% of fish production, with the remainder being fresh water fish and fish produced by aquaculture.

In general, fish processing operations are located close to commercial fishing areas. However in some cases catches may be transported long distances or exported for processing. The Northwest Pacific region is by far the most important fishing area in terms volumes caught and processed. China, Peru, Chile, Japan, the United States, the Russian Federation and Indonesia (in that order) are the top producing countries, together accounting for more than half of world fish production.

Approximately 75% of world fish production is used for human consumption and the remaining 25% is used to produce fish meal and oil. Fish meal is a commodity used as feed for livestock such as poultry, pigs and farmed fish and fish oil is used as an ingredient in paints and margarine.

Currently, only about 30% of fish produced for human consumption are marketed fresh. The supply of frozen fish fillets and fish, in the form of ready-to-eat meals and other convenience food products is growing in both developed and developing countries.

The end products from fish processing may be fresh, frozen or marinated fillets, canned fish, fish meal, fish oil or fish protein products, such as surimi. Surimi is an important fish product, with the majority of catches for some species used solely for its production.

Fish processing most commonly takes place at on-shore processing facilities. However some processing can take place at sea, on board fishing vessels—for example the gutting of oily fish. In some regions of the world, where large sea fleets operate, processing can also take place

on board fishing vessels. For some sea fleets, 100% utilisation of the catch may be required by legislation. This means that the entire processing operation, including fish meal and oil production for offal and fish waste, takes place on board the fishing vessels.


The length of voyage of fishing vessels storing the catch in ice may range from a few hours for small inshore vessels to about three weeks for the largest distant water trawlers. White fish, that is those species in which most of the fat is in the liver and the flesh is lean, are handled in much the same way on all sizes of vessel. The catch is released from the net on to the deck, gutted immediately, washed, and stowed with ice in boxes or compartments below deck.

Gutting of round fish like cod, haddock and whiting means slitting the belly from throat to vent, removing the liver and cutting out the guts to leave the belly cavity empty. This operation is traditionally done by hand with a knife, but gutting machines are coming into use on both large and small ships to make the task of the fisherman easier. Gutting helps to preserve the fish by removing the main source of spoilage bacteria and digestive juices which attack the flesh of the fish after death. On the larger fishing vessels the livers are cooked in steam boilers to extract the liver oil, but on small boats the livers are discarded with other offal.

The gutted fish are washed to remove traces of blood and debris, and to wash away most of the bacteria present on the skin and in the gills of the fish. The washing equipment on small boats may be simply a hose and an open mesh basket, but on large trawlers a more sophisticated washing tank with circulating water is in general use. In these washers the fish are discharged over a weir and down a chute to the fishroom below deck.

Fishrooms for iced fish are mainly of two types, either an undivided hold in which the catch is stowed in boxes, or a hold divided by partitions into a number of sections called pounds in which the catch is stowed on portable shelves. The principle of stowage is the same; the fish are in shallow layers completely surrounded by ice, whether on a shelf or in a box, so that they are cooled rapidly to ice temperature and kept close to 0°C throughout the voyage. About one part of ice to three parts of fish by weight is required to protect fish for up to 5 days; one part of ice to two of fish is needed for longer voyages. White fish, promptly gutted, washed, and stowed in ample ice, will keep in first class condition for 5-6 days, become stale after 10-12 days, and are unlikely to be edible after 15-16 days.

Boxed stowage is usual on smaller fishing vessels, and the practice of boxing is gradually being extended to larger ships, since the method has a number of advantages including delivery of the fish to the merchant undisturbed by rehandling at the port market, ease of identification of size, species and time in ice, and avoidance of damage and loss of weight during stowage.

Ice plants, particularly older ones at the larger ports, supply crushed block ice to fishing vessels, but more recently built plants, particularly at smaller ports where ice was not locally available in the past, usually deliver small, smooth pieces of ice known as flake ice. Flake ice is normally bulkier than crushed block ice, but weight for weight the cooling capacity of all types of ice, made by any method and from hard or soft fresh water, is the same.

The fish are cooled when heat is absorbed by the surrounding ice, which is thus melted. Further cooling is obtained when the cold meltwater trickles down between the fish. The fishroom temperature is ideally kept slightly above 0°C in order to allow the ice to melt slowly, but is not kept so high as to waste the ice. To this end, most fishrooms on large vessels are completely insulated, and fishrooms on small boats often have partial insulation. Mechanical refrigeration plants are installed on one or two inshore vessels and on some, but by no means all, larger ones; their main purpose is to conserve ice on the outward voyage, and to keep the fishroom air cool during fishing; they have little or no direct effect on the stowed fish, which depend for cooling almost entirely on the surrounding ice.

Fatty fish, like herring, sprats, mackerel and pilchards, i.e. those containing a good deal of fat or oil, are not normally gutted at sea because their small size and the large numbers in which they are caught make this impracticable in the time available on typically short voyages to grounds not far from the port of landing. They are usually put below straight from the net, and iced in boxes. The keeping time of fatty fish in ice is much less than for white fish; the attack by bacteria and digestive juices is much more rapid because the fish are ungutted, and the fat absorbs oxygen to produce rancid flavours and odours. Herring for example are normally required to be in the hands of the port processors within 1-2 days after catching to give a first class product, although for some outlets it is possible to keep herring with a low fat content for 4-5 days in ice. Stowage at sea in refrigerated sea water is a possible alternative to ice as a means of rapidly cooling large quantities of small fatty fish.


Fish is smoked nowadays mainly to give it a pleasant flavour rather than to preserve it. Present day products are therefore only lightly salted and smoked and will not remain edible for much more than a week at ordinary temperatures. The smoking process consists of passing wood smoke over the surface of the fish, in a kiln. Most British products are cold smoked, that is the fish remains uncooked and the kiln temperature does not rise above 30°C. Typical smoked products are the finnan haddock, smoked cod fillet, the golden cutlet and the kipper. Fish that are hot smoked are cooked during the process; the kiln temperature may be as high as 80°C and the fish temperature may reach 60°C. Some hot smoked products in this country are sprats, eels, trout, buckling made from herring, and Arbroath smokies made from small haddocks.

Two types of smoking kiln are in general use, the traditional chimney kiln and the Torry mechanical kiln. It is estimated that more than half of the smoked fish made in Britain is now produced in mechanical kilns, and the proportion is continually increasing.

Before smoking, the fish are immersed in a brine solution. This assists in removing some of the water in the fish, thus tending to firm the flesh. The salt imparts a flavour to the product, but concentration and purity of the salt are extremely important and require to be carefully controlled. A 70 to 80 per cent saturated brine is used in most modern smoke cures.

Following the salting treatment, pre-drying of the fish is required in order to remove some of the moisture prior to smoking. For this purpose, the fish are hung to drip on open racks.

The source of smoke is almost universally a smouldering fire of hardwood chips and sawdust; although more sophisticated smoke producers have been made from time to time, and are used for smoking other foods, these have so far made little impact on the fish trade. In the traditional chimney kiln, the open fires are at the base of a tall, brick-built structure in which the fish are hung on rails of various types called banjoes, speats or tenters, and thus exposed to the rising smoke and warm air. The repositioning of the fish during smoking and eventual removal of the finished products are slow hand operations which require the services of a skilled craftsman in order to produce a satisfactory article.

In the mechanical kiln, the fires are contained in separate fireboxes, and the smoke is blown horizontally through trolleys holding fish in the kiln; the fish may be hung on rails or laid on trays, either of which are supported in the trolley. The temperature and speed of the mixture of smoke and air is carefully controlled to give a uniform product throughout the kiln in a much shorter time than is possible in the chimney kiln. Fish handling is much reduced using the mechanical kiln which can also readily be incorporated in the factory production line. Partial drying as well as smoke deposition is an essential part of the smoke curing process; typically a kipper which should lose about 14 per cent in weight during smoking will require 6-12 hours in a traditional kiln, but only 4 hours in a mechanical one.

Most cold smoked fish products are only lightly coloured by the smoke, and so a permitted dye is normally added to the brine bath through which the fish pass before going into the kiln, in order to enhance the appearance of the finished product.


Freezing Fish

Heat is removed from the fish in the freezing process either by surrounding the fish with a stream of cold air, by placing the fish in contact with a cold surface or by spraying with certain liquid refrigerants. Three main types of freezing plant are used that employ these techniques, the air blast freezer, the plate freezer and the immersion freezer.

The air blast freezer is essentially a tunnel in which a fast-moving stream of very cold air is blown over the fish, which are placed on trolleys or on a moving belt. The air is usually at a temperature of -30 to -40°C and moving at about 5 m/s. The air blast freezer is most suitable for a wide range of sizes of fish, and for products of irregular shape.

The plate freezer is more compact than the air blast freezer, and is most useful for handling fish products that are uniform in thickness and that have a reasonably flat surface which can make good contact with the cold plates. Two versions of the plate freezer are in general commercial use, the horizontal and the vertical types. The horizontal plate freezer, used mainly in land installations, handles many of the catering and retail fish products that are already packed in cartons prior to freezing. Trays of packs are slid between pairs of horizontal plates, the plates are closed tightly on to the packs by hydraulic pressure to make good contact, and a cold refrigerant is circulated through serpentine passages within the plates; a retail pack 3 cm thick takes about an hour to freeze.

The vertical plate freezer, which was originally designed in the 1950s for use on fishing vessels, is employed for freezing large blocks of whole fish. The fish are packed between pairs of plates, usually without any wrappings, and the plates moved slightly towards each other to compact the block and ensure good contact. Liquid refrigerant is circulated through the serpentine passages within the plates until the fish are frozen; the complete process for a block of whole cod 10 cm thick takes about 4 hours including loading and unloading time, with refrigerant at -40°C.

The immersion freezer is not used as much as the plate and blast freezers in the British fish industry; the main difficulty is the limited choice of liquids that are both good refrigerants and suitable for use in direct contact with foods. Brines are sometimes used, but the fish may take up too much salt. Liquid nitrogen is used successfully in one type of immersion freezer, which subjects the product to a spray of liquid nitrogen as it passes through a tunnel.

The freezing process cannot improve the quality of fish; therefore the best frozen products are those made from first class raw material. This applies particularly to whole fish which, after thawing, are likely to be subjected to further processing. Whole iced cod, for example, when frozen not later than 3 days after catching can on thawing be treated in the same way as very fresh fish, but cod that has been delayed longer than this, or has been kept uniced, is unlikely after freezing and thawing to yield fillets of a high quality. Whilst some species, flatfish for example, can be kept in ice a little longer than cod before freezing without impairing the quality of the thawed product, others like haddock and hake do not keep so well. Fillets of most white fish species can be taken from whole fish 5-6 days in ice and frozen to give a high quality thawed product.

The freezing process should always be completed as rapidly as possible, not only to increase output of the equipment, but also to reduce the time in which bacteria and digestive juices are still able to attack the fish; bacterial action ceases below about -10°C, and the activity of enzymes is reduced as the temperature falls. There are marked changes in texture and flavour when fish is frozen very slowly at temperatures only a little below 0°C. The final temperature of fish being frozen should be that at which it is to be stored, namely -30°C; this ensures that the frozen product imposes no extra heat load on the cold store since this is designed only to keep the product cold and not to freeze it.

Cold Storage of Fish

Temperature of storage is the most important single factor affecting the storage life of frozen fish. Almost all cold stores for fish that have been built in recent years are designed to operate at -30°C, at which temperature the products will keep in first class condition for several months. Lean fish, such as cod and haddock, when stored for long periods at too high a temperature or in fluctuating temperatures can have marked changes in texture and flavour when thawed out. The thawed flesh may feel rubbery and appear dense white instead of translucent. After cooking they are found to be tough and fibrous or stringy.

Other causes of change in cold stored fish are dehydration and oxidation. Dehydration is kept to a minimum either by glazing unwrapped frozen fish before storing them, that is covering the surface with a skin of ice by dipping them quickly in cold water, or by packaging the fish in a material that is a good barrier against the passage of water vapour, for example polyethylene film.

Fatty fish like herring are particularly prone to absorb oxygen from the air and so become rancid; these are therefore wrapped in a material that forms a good oxygen barrier. In addition the space between the package and the contents may be evacuated to reduce even further the risk of rancidity. The ideal packaging material for fish products is often a laminated film combining the desired properties of two or more plastics.

The cold storage chain is maintained in distribution by the use of insulated, refrigerated vehicles or containers, both of which operate at about -20°C, and frozen food cabinets, again at -20°C, in shops and catering premises.

Thawing Frozen Fish

The growth of quick freezing in the fish industry, and particularly the production of large blocks of whole fish for subsequent processing, has made necessary the development of thawing plant. There are two main types of equipment, those in which the fish are heated in a warm air stream or in warm water, and those which directly use heat generated by electricity. The method most used for large blocks of sea frozen fish is air blast thawing; the fish are conveyed through a stream of moist moving air at about 20°C until they are thawed enough to permit filleting or other processing. The fish temperature should never exceed 20°C.


Bacteria and moulds generally cannot grow in the absence of water and hence drying can be used as a means of preservation. Salt, if present in sufficient strength, will slow down or prevent bacterial spoilage of fish. Drying or salting, or a combination of both, have been used in the fish industry for centuries but nowadays only a very small proportion of the catch in Britain is processed by these methods.

A few companies still make dried salted fish from cod and related species, by heading and splitting the fish, removing most of the backbone, and stacking the fish in piles with layers of salt between them. The juices withdrawn from the fish by the salt are allowed to run away and, after frequent restocking over a period of months, the water content is further reduced by drying the fish in a heated chamber until the moisture content is somewhere between 10 and 30 per cent.

Some herring are pickle cured, particularly in north-east Scotland and the Shetlands, although this export trade is a mere shadow of what it was earlier this century. The whole herring, having been lightly sprinkled with salt while awaiting processing, are first gibbed by hand or machine, that is the gills, long gut and stomach are removed. They are then packed in barrels with a layer of salt on each layer of herring until the barrels are full. After a day or two, when the herring have shrunk appreciably, the barrels are topped up with further layers of herring and salt, the lids are fitted and the barrels then laid on their sides for 8-10 days. After this period the barrels are up-ended, the lids removed, and the blood pickle from the upper half drained off through the bung-hole. To make ready for storage the barrels are finally topped up with fish, the lids replaced and blood pickle poured through the bung-holes until all spaces are filled.

Klondyking is the name given to another method of preserving ungutted herring using salt. The name is thought to originate from a method developed about the same time as the famous Gold Rush of 1897.


 The canned fish industry in Britain is a small one, and the range of products is confined mainly to herring, sprats and pilchards packed in either tomato sauce or vegetable oil.

The process for herring in tomato sauce is typical. First the fish are nobbed by machine, that is the head and gut are removed. They are then immersed in saturated brine for up to 30 minutes and packed by hand into oval cans holding 200 g of fish. The tomato sauce is added, the can lids are lightly clipped on and the cans exhausted in steam for 10-15 minutes. The object of exhausting is to produce a partial vacuum in the headspace of the can which is not filled either with solids or liquids. The cans are then sealed, washed, and heat processed in steam at 115ºC for 55 minutes. After cooling, the cans are stored for about a month, labelled and packed in outer cartons for dispatch. The heat processing stage is critical, and is designed to inactivate all bacteria and enzymes present and in particular to destroy any harmful organisms.

There are a number of imported canned fish products that could be manufactured in this country from British-caught fish, and the canning industry is examining the possibilities of expanding their range.


Quality of seafood is based on many attributes. Details of workmanship are built into specifications between processors and their customers but basic requirements relate to raw material quality, safety, hygiene and identity.

Specifications for quality standards are agreed between processors and their customers in every sector of the industry. The specifications include details for all raw material characteristics, workmanship in processing, packaging, labelling and storage and for product quality in terms of sensory attributes and bacteriological factors. In-house quality assurance managers are responsible for compliance with these standards.

Sensory assessment is the basis for quality control of raw material and finished products in the freezing industry, saltfish processing, fresh fish production and aquaculture. Appearance and odour of the raw fish and flavour of the cooked products are important parameters. Interestingly, each saltfish is subjected to assessment of many attributes for appearance and this is the basis of the final quality grading. In the fishmeal industry, however, every lot is analysed chemically and bacteriologically and sold on basis of its chemical composition. Increasingly, fishmeal is also sold on the basis of in-vivo digestibility.

Finally, exporting companies for frozen seafood and saltfish operate their own quality control by inspection of products and premises and their subsidiary companies and further processing factories operate a comprehensive system for the final control of products


Recent years have seen the rise in popularity of quick frozen foods, including fish.Freezing is a form of preservation that has the advantage of presenting the foodstuff to the consumer in an apparently unchanged form.Canning, however, has sufficient unique advantages to withstand the encroachmnet of this new industry.Freezing requires elaborate plant and a chain of cold stores between the freezer and the consumer.A can of fish presents food ready-cooked in a form that can be easily transported and stored over a considerable period of time without regard to outside conditions.

Because of the uncertainty of the fishing during the past few years, the quantity of suitable fish available to the canners has not often been sufficient to allow them to meet their normal commitments.This shortage also prevents them from extending their range of products, which would put them in a beter position to complete in the considerable market enjoyed in this country by foreign canned fish products.

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