2. RAW MATERIAL


2.1 Fish
2.2 Fish Handling
2.3 Preparation of Raw Material

2.3.1 Gutting machines
2.3.2 Scaling machine
2.3.3 Heading machine
2.3.4 Filleting machines
2.3.5 Skinning machines
2.3.6 Bone separator or mincing machines
2.3.7 Battering and breading machines
2.3.8 Forming machine
2.3.9 Automatic shrimp-peeling machine
2.3.10 Weighing and grading machines

2.4 Ingredients

2.4.1 General
2.4.2 Additives

2.5 Products

2.5.1 General
2.5.2 Small pelagic
2.5.3 Whole demersal
2.5.4 Tuna
2.5.5 Frozen blocks
2.5.6 Shrimp/prawn
2.5.7 Fillets
2.5.8 Minced and/or formed frozen fish products


2.1 Fish

All fish are suitable for freezing in one form or another but, in practice, relatively few of the many species available are marketed in this way.

In developing countries freezing and cold storage are mainly used for fish intended for export, since in most of these countries the necessary "cold chain" for marketing frozen fish internally has yet to be developed.

Fish exported from developing countries tends to be the higher priced products since the foreign exchange earned is more beneficial to the home economy than if the products were used for internal consumption. Many types of shellfish, both crustaceans and molluscs, come into this category with shrimp/prawn being by far the most important. Other shellfish such as lobster, crayfish, crab, abalone, scallops and the cephalopods such as squid, octopus and cuttlefish are also in demand as frozen imports.

There is also an active international trade in some of the higher priced and more popular fin-fish species such as cod, haddock, pollock, pomfret, snapper, salmon and sole, both as whole fish and, for some of the species, processed or semi-processed fish products.

Tuna and other species which are caught seasonally are also in demand as frozen products in order to provide continuity of supplies throughout the year at processing establishments.

The above opportunities for the production and marketing of frozen fish products already exist for most countries and as internal cold chains develop, marketing opportunities for a wider range of frozen fish products will result.

In addition to their preference for certain species only, markets for frozen fish demand that other standards have to be met, with quality and size grading being two of the most important.

Quality standards may be imposed by the customer, by authoritative codes of practice or regulations in the country of origin or consignment or initially by the producer. Fish quality may be measured in many ways and standards changed from time to time, therefore, it is imperative that a full investigation should be made of the current requirements and whether they can be met before a freezing operation is initiated. More information on this aspect is given in 5.2.3 and in a number of the references given in Section 8.

Size grading standards are more uniform and more widely applied than other quality standards, probably because they are more readily defined, attained in commercial practice and checked by the customer. Size grading can be either by weight, measurement of fish length or other dimension or by a count of units in a standard package or weight.

Examples of grades applied to frozen shellfish products are given in Tables 1 and 2. It can be seen from these tables that market grades do not necessarily follow the grading recommendations set down by international codes, therefore, potential customers should always be consulted on this subject at an early stage of planning to ascertain the standards they are likely to require.

Table 1 Size grading of frozen shrimp

Headless shrimp
Japanese market
(count/kg)

Peeled and deveined shrimp US market
(count/kg)

Codex recommendations a/
for all shrimp styles except whole(count/kg)

08 to 12

up to 15

<21

13 to 15

16 to 20

22 to 33

16 to 25

21 to 25

34 to 44

26 to 30

26 to 30

45 to 55

31 to 35

31 to 35

56 to 66

36 to 40

36 to 40

67 to 77

41 to 50

41 to 50

78 to 88

51 to 60

51 to 60

89 to 110

61 to 70

61 to 70

111 to 132

71 to 90

71 to 90

133 to 154

 

91 to 110

155 to 176

 

111 to 130

177 to 198

 

131 to 200

199 to 220

 

201 to 300

221 to 286

 

301 to 500

287 to 440

   

441 to 660

   

661 to 1 100

   

>1 101

a/Codex Alimentarius Commission, CAC/RS92-1976, Recommended International Standard for Quick-Frozen Shrimps and Prawns

Table 2 Size grading of frozen cephalopods

Whole squid
(count/kg)

Squid fillets
(count/kg)

Whole octopus
(count/kg)

Up to 16

   

16 to 20

up to 10

0.5 to 1.0

21 to 25

10 to 20

1.0 to 1.5

26 to 30

20 to 40

1.5 to 2.0

 

49 to 60

2.0 to 2.5

 

60 to 80

2.5 to 3.0

 

80 and over

3.0 to 4.0

If, for some reason. there are two or more distinguishable quality grades of a given product. They should be marketed separately. preferably under a different brand name. so that the sale of the high quality grade does not suffer from its association with the lesser quality grade. For instance delays before handling and freezing fish at sea can result in a quality grade which is less than the ideal achieved when the fish are frozen quickly after catching. In some countries it is the custom to label these fish so that they can be identified by the processor at the point of first sale and marketed either as a product where a lower quality is less critical or as a cheaper brand of the same product.

From the point of view of calculating the capacity requirement of the associated refrigeration plant, differences in the composition of fish seldom have any practical significance.

The refrigeration requirement depends to a great extent on the water content of the fish, there fore, fish with a high oil, shell or bone content and consequently lower water content would, in theory require a lower capacity refrigeration plant than that required for such products as lean fish fillets. Individual calculations can therefore result in significant differences between the refrigeration requirement for one product and another. For example. the heat to be removed in reducing cod fillets from 10C to -20C is 76 kcal/kg, whereas the requirement for oysters in their shell is 36 kcal/kg. This considerable difference is due to the high shell content of the oysters, but the differences between different species of white fish, for instance, would be very little and of no practical significance.

The question to be asked at this stage, however, is whether the freezer is likely to be required to freeze a variety of products either at present or in the future. If this is the case, then the freezer capacity should be based on the requirement for freezing fillets of lean fish. Such as cod, since this requirement is likely to be the most demanding. The need for versatility is more likely to prevail when the requirement is for an air blast freezer since this type of freezer can be adapted to freeze products of a variety of shapes and sizes.

Other differences in chemical composition have little or no effect on the freezing requirement other than to depress the temperature at which freezing commences. Higher salt contents may depress the freezing point or temperature of "thermal arrest" which will be between -0.5C and -1.0C for most products. This is more likely in processed fish. such as smoked products, where the salt content is increased by brining and concentrated by drying.

2.2 Fish Handling

Prefreezing treatment starts immediately after the fish is caught, whether the fish are frozen at sea or eventually on shore.

Freezing does not improve the fish quality. but good freezing and cold storage practice can maintain the fish at the quality pertaining at the time they were frozen. Good quality frozen products. therefore. can only be achieved with good prefreezing practice.

All fish should be chilled rapidly. immediately after catching. Stowage in ice is the most widely used method of chilling either in boxes. on shelves or in ponds in the fish-hold. When large quantities of fish are caught and are difficult to handle quickly with normal icing methods, chilled sea-water (csw) or refrigerated sea-water (rsw) systems are used.

Chilling practice for fish prior to freezing is identical to that recommended for fish that are chilled and sold unfrozen. Quick chilling is important even when the time between catching and freezing is short. particularly in tropical countries where only a few hours exposure at ambient temperatures may be equivalent to many days of chilled storage. This is illustrated in Figure 1 which compares the rate of bacterial growth at different temperatures.

Figure 1 The effect of temperature on bacteria growth

Table 3 gives the maximum prefreezing storage times that should be allowed at 0C after quick chilling to ensure a good quality frozen product. Little data exists. however. of corresponding times for higher temperatures, but reference to Figure 1 shows that they are likely to be extremely short and this situation should therefore be avoided.

Table 3 Prefreezing storage at 0C for a good quality product

Species

Prefreezing
storage time.
(days)

Cod

3

Haddock

2

Lemon sole

5

Shark/dogfish

1

Herring

3/4 to 1

Sardines

1

Tuna

1

Shrimp (Pandulus)

up to 2

Salmon

up to 3

Prefreezing handling and storage is extremely important and the following documents should be consulted for detailed information:

2.3 Preparation of Raw Material

Most operations in the processing of fish can now be done by cost effective machinery. The best conditions for machine processing pertain when labour costs are high and there is also a high degree of standardization in the product and processing requirement. Other factors, such as power availability and import restrictions on machines may, however, influence the choice between a manual or machine operation.

Most machine developments have been made initially to satisfy the conditions that pertain in temperate climates, although some machines have been specially developed, or modified, to meet a need for processing some species of tropical fish. Care, however, should be taken to ensure that a machine is suited to a specific requirement and that the yields and throughputs are measured values which apply to the species and quality of the fish to be processed. The following list gives information about machines used for the prefreeze processing of fish.

2.3.1 Gutting machines

Gutting machines are used on fishing vessels and on shore for marine fish, and also on fish farms for species such as trout. They open the belly and remove all parts of the entrails. The head is left on the fish. The liver, roe and milt are not collected.

Machines are designed for gutting cod, haddock, hake, trout, mackerels and similar fish of lengths from 25 cm to 75 cm, with a throughput of approximately 30 fish/min.

Floor area: 2.0 m
One operator  
Power: 3 kW 220/380 V 3-phase
Water consumption: 35 litres/min
Cost : US$ 22 500

2.3.2 Scaling machine

Type of fish : flounders, bass, perch, pike, pollock, whiting, haddock, hake, herring and many other types of fish
Capacity of machine: 2 500 kg to 4 000 kg/h, depending on size and type of fish being scaled
Floor area required: 3.0 m
Operators: one
Power: 1.5 kW
Water consumption: 15 litres/min
Cost: US$ 7 500

2.3.3 Heading machine

Designed for heading: cod, haddock, pollock, and similar fish of lengths from 25 cm to 100 cm
Throughput: 35 fish/min
Floor area required: 3.0 m
Operators: one
Power: 1.5 kW 220/380 V 3-phase
Water consumption: 12 litres/min
Cost: US$ 10 000

2.3.4 Filleting machines

A manufacturer's range of machines is given in Table 4.

Table 4 Filleting machines

Type of fish

Fish size (cm)

Capacity (per min)

Yield (%)

Floor area (m)

Operators

Power (kW)

Water consumption (l/min)

Cost (US$)

Codling, haddock, pollock, whiting

25-42

40

47
with skin and belly flaps

4.0

one

2.5

13

36 000

whiting and small haddock

25-40

40

47
with skin and dorsal fin

4.0

one

2.5

10

37 500

Cod, pollock, haddock and similar fish

30-70

24-40

50
skinless

12.0

one

10

30

105 000

Cod, pollock, haddock, hake and similar fish

30-55

25-40

45
skinless

4.0

one

1.5

15

37 500

Cod, haddock and pollock

35-70

24

47
with skin and belly flaps

3.5

one

2.5

25

39 000

Cod, pollock, haddock and hake

45-85

24-34

47
with skin and belly flaps

6.0

one

3

25

60 000

Herring

20-37

120-150

55

10.0

two

3

35

37 500

2.3.5 Skinning machines

Baader 51

This machine is capable of skinning fillets of any size cut from cod. pollock, merluza, haddock and redfish.

Operators: one or two
Throughput: 30-140 fillets/min
Power: 1.5 kW
Water consumption: 12 litres/min
Area: 2.0 m
Cost : US$ 15 000

Trio skinning machine (refrigerated drum)

Machine capable of skinning fillets of any size cut from herring. mackerel and white fish. The only machine suitable for skinning block fillets. The fillets are placed with skin -side up on a special designed conveyor. which passes under a rotating refrigerated drum. By touching the drum the skin of the fish is immediately frozen to the drum surface and subsequently the skin and the flesh are separated by means of a band knife.

Operators : one or two
Throughput : herring and mackerel -100-150 fillets/min white fish -30-60 fillets/min depending on size maximum width of fillets -330 mm
Power : 6.0 kW
Water consumption: 18 litres/min
Area: 3.5 m
Cost : US$ 27 000

2.3.6 Bone separator or mincing machines

This machine minces fish or separates the raw meat from bones, fins and skin. The fish pieces are fed into the machine through a filling hopper, then a conveyor belt of strong elastic material conveys the fish to a perforated drum. The meat is extruded through the perforations in the drum into the interior chamber while bones and other solid particles remain on the external drum shell, from which they are removed by a stripper.

When processing pin-bone V-cuts without skin, it is possible to obtain an edible fish yield of 90 percent.

Operators : one
Throughput: 400 kg raw material/h
Power : 1.5 kW
Area: 1.5 m
Cost : US$ 15 000

2.3.7 Battering and breading machines

Suitable for continuously battering and breading fish fillets, fish sticks, fish portions, etc.

The products to be breaded are put on a stainless steel conveyor belt of the batter machine either by hand or automatically. They then pass through a curtain of batter before moving to the breading machine where a layer of bread-crumbs, adjustable in thickness, is placed on the product. An air blower removes superfluous bread-crumbs.

Operators : one or two
Throughput: approximately 350 kg/h, depending on size of machine
Power : 2.5 kW
Floor area: approximately 4.0 m
Cost : US$ 22 500

2.3.8 Forming machine

Machines suitable for forming (moulding) a large variety of fish and other food products. Almost any combination of ingredients can be used which makes these machines highly flexible.

The shape of product can be formed by simply changing the forming set. The product is loaded into a self-feeding hopper then it is forced into the forming plates by means of a piston. When they have been filled, the plates are moved forward to the knock-out position and the contents are deposited onto the out feed conveyor.

Operators : automatic
Throughput: 2 000 kg/h (approximately)
Power : 7 kW
Area: 3.0 m
Cost : US$ 22 500

2.3.9 Automatic shrimp-peeling machine (comprising five machines)

(i) Peeler, (ii) cleaner, (iii) waste separator, (iv) deveiner, (v) grader.

These machines deliver completely peeled, deveined and graded raw shrimp meats for packaging or processing. The flow of shrimp is continuous and is accomplished by means of flumes or conveyors.

(i) Peeler

Peels approximately 400 kg of shell-on raw shrimp/h. Shrimp are bulk-loaded into an attached feeder-tank and are then carried by conveyor-belt into the machine where peeling is done automatically by a series of rollers which leave the shrimp meat on top.

Capacity : 300 kg/h
Power : 4 kW
Water : 300 litres min
Floor area: 10 m
Cost : US$ 45 000

(ii) Cleaner

Detaches gristle and other waste appendages in a continuous gravity flow.

Capacity : handles output of two peelers
Power : 0.6 kW
Water : water with peeled meats from discharge of peeler is sufficient
Floor area: 3.0 m
Cost : US$ 4 700

(iii) Separator

This machine separates the waste material detached from the edible shrimp meats as a continuous process.

Capacity : handles output of two peelers
Power : 0.6 kW
Water : 50 litres/min
Floor area: 2.5 m
Cost : US$ 5 400

(iv) Deveiner

The deveiner takes fully peeled shrimp which are flumed with water into the machine directly from the peeling, cleaning and separating operation.

Shrimp slide down the sloping blades which slit the backs of the shrimp to expose the veins.

Fingernail-like projections on the drum wall separate veins from shrimp.

Capacity : handles output of two peeling machines
Power : 0.5 kW
Water : 200 litres/min
Floor area: 12.0 m
Cost : US$ 18 000

(v) Grader

Accurately grades a sizes of shrimp, using spiral rods which can be easily adjusted to successfully pass shrimp of different sizes.

Capacity : handles output of two peeling machines
Power : 0.5 kW
Water : 20 litres/min
Floor area: 1.5 m
Cost : US$ 3 200

Total floor area required for all five machines, when assembled in their correct positions, 80 m.

2.3.10 Weighing and grading machines

Machines specially developed for use in wet environments and suitable for weighing and grading wet or frozen fish.

Machines can grade up to 31 different weight classifications. all with user definable weight intervals, with speeds up to 200 weighing operations/min and can provide totalized and analysed production data on request.

Although machines can be purpose-designed or modified for individual products, existing models are mainly for the more common requirements ranging from small fillets to large cod.

A conveyor belt carries the product over the weighing machine where it is weighed with extreme precision. The product is then transferred to the conveyor belt on the grading unit where it is delivered into the appropriate bin by a pneumatic-controlled sorting arm.

Cost : machine with weight-range 0-2 500 g, US$ 33 000
Floor area: 15 m

2.4 Ingredients (Additives)

2.4.1 General

Most countries permit only very limited addition of non-nutrient ingredients (usually called additives) to unprocessed foods. and fish is no exception. The additives listed below include (i) additives that have been accepted by the Codex Alimentarius Commission for use in fish and included in Codex Standards. (ii) additives proposed in Draft Codex Standards. and (iii) additives known to be permitted in a number of countries. All the named additives have been approved for food use by the FAO/WHO Joint Expert Committee on Food Additives.

Food regulations of individual countries vary widely. and acceptance of Codex Standards by governments is, so far, limited. It cannot, therefore, be assumed that any of the additives listed will be widely permitted, and it is imperative that the regulatory authorities of the country of destination of the frozen product be consulted on:

  1. whether a particular additive is permitted in that product.
  2. whether the amount that may be added is restricted. and
  3. any specific labelling requirements.

The lists below include additives to fish fillets. to minced fish. to mixtures of fillets and mince, and to the fish component of composite products such as fish sticks. Additives appropriate only to non-fish components of such composite products are excluded. Also excluded are ingredients not normally subject to legislative control. such as salt. sugars. vinegar. herbs and spices.

Specifications of purity for many of the additives mentioned have been prepared by the Joint Expert Committee on Food Additives, but are not widely used. All additives should be of "food grade" quality and should meet any additional requirements of the country concerned.

It is difficult to estimate the cost of treatment with any of the additives listed. since the conditions of use vary so widely. As a broad generalization, the cost of materials for any of the treatments listed is unlikely to exceed 10 cents (US)/kg fish; this estimate does not include cost of labour and equipment employed.

2.4.2 Additives

2.4.2.1 Drip reduction agents:

Soluble salts of phosphoric acids are the only commonly approved agents for minimizing drip loss on thawing. These additives also act as lubricants and binding agents in the preparation of blocks of fillets, of minced fish, or of mixtures of fillets and mince. Many processors now use proprietary mixtures of phosphates, prepared for particular purposes, rather than the individual substances listed below.

Table 5 Phosphates

Tetrasodium or tetrapotassium diphosphate
(also called sodium or potassium pyrophosphate)
Pentasodium or pentapotassium triphosphate
(also called sodium or potassium tripolyphosphate)
Other linear sodium or potassium polyphosphates

These phosphates are added to fish most commonly as a 5-10% solution, either by dipping the fish in a large volume of the solution or by adding a small measured amount of solution to the fish and mixing.

Codex Standards and some national standards set limits on the amounts of added phosphate; in Codex Standards the maximum permitted amount is
5 g/kg of fish, calculated as phosphorus pentoxide (P2O5).

2.4.2.2 Binding agents:

Phosphates, described in the previous section, act as binding agents by forming a sticky layer of swollen protein on the surface of a fillet. Many other substances, including natural and synthetic gums, have been proposed as binders. The most used are salts of alginic acid, especially the sodium, potassium and propylene glycol salts. As in the case of phosphates, they are usually applied as solutions in water. Codex Standards place a limit of 5 g/kg on the amount of alginates permitted.

A closely related use of alginates is as a component of glaze on frozen fish to improve the adhesion and flexibility of the glaze layer.

2.4.2.3 Preservatives:

The use of preservatives in unprocessed fish is very limited. Codex , Standards and some national regulations permit the use of sulphur dioxide in raw crustacean shellfish to minimize certain specific deteriorative reactions such as development of "black-spot" on shrimps and prawns. The permitted limits laid down in Codex Standards are 100 mg/kg in the raw product and 30 mg/kg in the cooked product, calculated as sulphur dioxide. Sulphur dioxide is a gas and is usually applied in the form of a solution of a salt of sulphurous acid; sodium metabisulphite (Na2S2O5) is the most commonly used salt.

2.4.2.4 Antioxidants:

The only antioxidant widely permitted in fish is ascorbic acid (Vitamin C), used either as the acid or its sodium or potassium salt. The maximum amount permitted in Codex Standards is 1 g/kg, calculated as ascorbic acid. It is usually added as a solution.

2.4.2.5 Dyes:

The use of dyes on unprocessed fish is not widely permitted. In Codex Standards, a very limited range of dyes is allowed on frozen cooked shrimps and prawns, but on no other frozen fish. Frozen processed fish may contain dyes; smoked fish, for example, is often dyed to give a more pleasing and uniform colour. The dyes permitted for such purposes vary so widely from country to country that it is not possible to give any useful list of dyes. Not all dyes have been approved by the Joint Expert Committee on Food Additives mentioned earlier.

2.5 Products

2.5.1 General

Although fish are frozen in a variety of ways to give a wide range of fish products, most of them can be included in the following main categories:

2.5.2 Small pelagic

Small pelagic fish may be frozen on board in either:

  1. vertical plate freezers, preferably in polythene or coated paper bags inserted between the plates with or without water added;
  2. horizontal plate freezers in cartons;
  3. air-blast freezers in cartons.

Vertical plate freezers are the most widely used when the fish are to be frozen in bulk for later processing on shore, and Table 6 gives information on some of the more typical blocks produced in this way.

Table 6 Whole pelagic fish frozen in vertical plate freezers

Weight
(kg)

Dimensions
(mm)

Density
t/m

No of blocks/pallet
(t)

Stowage rate incl. Pallet
m/t

22.5

1 060 x 520 x 50

8

44

1.6

22.5

530 x 520 x 100

8

44

1.6

45

1 060 x 520 x 100

8

22

1.6

If small pelagic fish, such as herring, are frozen in the form of fillets, it is more acceptable to freeze them as smaller blocks in a horizontal plate freezer. Blocks of 3.5 and 7 kg frozen in cartons with polythene interleaving between the layers of fillets are typical products.

Whole fish, packed as shown in Figure 2, may be frozen in an air-blast freezer to give blocks of 3.5-7.5 kg.

Figure 2 Tray for air-blast freezing small, whole fish

2.5.3 Whole demersal

Air-blast frozen fish can be individually quick-frozen by laying them out on trays which are then loaded into the freezer on trolleys or pallets. Some larger fish, or fish such as salmon which are often required to be frozen so that they retain their rounded shape in the frozen state, can be frozen by hanging them from suitable racking in the freezer or on a trolley. Whole fish may also be frozen in trays with the fish packed head-to-tail, as shown in Figure 2.

Fish, such as halibut and shark, may be too large to freeze whole in the main freezer and, if space and requirement justifies the need, a small freezer room may be constructed where they can be frozen slowly in the vertical hanging position. This freezer room can also be used as an additional cold store for frozen product.

Freezing by immersion or spraying with sodium chloride brine has also been used in the past for whole fish, but fish frozen by this method would have a restricted market due to salt uptake, and the method is now little used.

2.5.4 Tuna

Depending on the species and form of the end product, tuna are frozen using one of the following methods:

  1. by immersion in sodium chloride brine,
  2. by spraying with sodium chloride brine,
  3. by blast-freezing -in a batch or continuous freezer,
  4. by "sharp" or semi-air-blast freezing,
  5. by plate freezer,
  6. by immersion in calcium chloride brine.

Freezing with sodium chloride brine is used where the end product is to be canned and mainly on smaller fishing vessels only. Disadvantages with this type of freezing are that the temperature of the fish cannot be reduced much below -15C, and care has to be taken to limit the salt uptake. After the fish has reached this temperature, the brine is drained off and the tuna may be further reduced in temperature during a period of "dry" cold storage. The fish require to be at least partially thawed to facilitate discharge at the cannery. More than 50% of almost 2 million tons of world catches of "true" tuna (i.e., yellowfin, bluefin, albacore, bigeye and skipjack) end up in a can.

Most on-board freezing is done by a so-called semi-air-blast method in an insulated room which has a capacity to hold about 2 t of fish. In the freezer, very cold air at a temperature from -55 to 60C is circulated by two fans of about 1.5-2.0 kW and it usually takes up to 36 h to freeze the fish to -45C at the deepest part of the fish. Larger fish, however, may take up to 72 h. After freezing, the fish are glazed before storage in an insulated hold.

Some of the larger tuna clippers are equipped with calcium chloride brine immersion freezers which have an advantage over the air-blast freezers in terms of reduced freezing times and lower energy consumption. Brine may be considered a problem due to residual calcium chloride, but since this only penetrates to a few millimetres below the surface, particularly if they are frozen pre-rigor, there is a wide acceptability by consumers.

Some operators pre-wrap the fish before immersion in calcium chloride brine and with this method it is possible to store the fish immersed in the brine until they are discharged.

Cold stores used with air-blast and calcium chloride brine freezers usually have floor, wall and roof cooling grids. This type of cold store cooling tends to be more expensive, but the method is used to ensure that product dehydration during storage is kept to a minimum.

Correct stacking of large tuna fish in a cold store is important in order to achieve load stability and also the maximum use of the storage space.

Tuna fish between 20 and 150 kg are usually frozen whole after they have been bled and washed, after the gills are removed (semi-dressed).

Fish of more than 150 kg are semi-dressed, then cut into two fillets. The fillets are then cut further into 1/2 or 1-kg portions which are then vacuum packed in polyethylene bags, sealed and frozen.

Flesh colour is a major quality consideration for fish used in the preparation of sushimi and sushi and the rate of change of the colour from red to an unacceptable brown during storage depends on the storage temperature. Storage temperatures of -40C and below are therefore recommended, and since there is a tendency for prices to be higher for fish stored at lower temperatures, processors are encouraged to operate their stores at -50C or even -60C.

It has been demonstrated that temperature is closely related to quality of the tuna and for this reason there has been a continual lowering of freezing and storage temperatures on board the vessels catching fish other than for canning. Temperatures of between -50C and -60C are now common for freezing rooms and fish-holds.

2.5.5 Frozen blocks (demersal)

Freezing in vertical plate freezers is the method mainly used for producing blocks of whole fish. Blocks of fillets can also be prepared, usually in the smaller blocks thicknesses only and by using partitions in the freezer to produce smallest blocks. Blocks can vary in thickness and other dimensions, depending on the size of fish to be frozen with, in general, thinner blocks being made with smaller fish and fillets. Optimum block dimensions will depend on the size range of the fish being frozen, but there may not be a requirement to freeze all sizes. For example, block dimensions of 1 060 x 520 x 50 mm were used for frozen-at-sea North Atlantic cod and this was suitable for about 98 percent of the catch; large fish were either headed to allow them to fit into the freezer or individually frozen in a freezer room.

A typical standard range of block dimensions and weights now available for freezing whole fish and fillets is given in Table 7.

Table 7 Vertical plate frozen blocks of demersal fish

Product

Block thickness
(mm)

Block dimensions
(mm)

Block weight
(kg)

Whole haddock

50

1 060 x 520

25 nominal

Whole haddock

75

1 060 x 520

37 nominal

Whole cod

100

1 060 x 520

50 nominal

Cod/haddock fillets

50

1 060 x 520

27.5 nominal

Cod/haddock fillets

50

530 x 520

13.7 nominal

Cod/haddock fillets

65

1 060 x 520

36 nominal

Cod/haddock fillets

65

530 x 520

18 nominal

Blocks of fish can also be frozen in air-blast freezers with the fish packed in metal containers and stacked on trolleys or pallets. The containers require to have well tapered sides in order to release the fish after freezing, usually by spraying water on the underside. Blocks frozen in this way are normally packed in cartons before storage and one reason for this is that they are not regular shaped, flat-sided blocks and, therefore, difficult to stack.

Frozen blocks can also be made by packing the smaller demersal fish in cartons and freezing in a horizontal plate freezer, and when freezer frames are used, block dimensions can be standardized.

2.5.6 Shrimp/prawn

2.5.6.1 Whole shrimp:

Whole shrimp can be frozen at sea by immersion freezing in a sodium chloride brine or sugar and salt solution, by plate freezing or by freezing in an air-blast freezer.

Freezing in sugar and salt solutions is claimed to give an improved glaze on the shrimp, makes them more attractive: and also ensures, that the shrimp remains separate to give an individually quick frozen product. This method of freezing, however, will result in flavour changes which may not always be acceptable.

Shrimp frozen in a vertical plate freezer are poured into polyethylene bags inserted between the plates, and water is added to strengthen the block improve heat transfer and protect the product during subsequent cold storage. The frozen blocks are usually packed in master cartons, made from plain or corrugated fibreboard, for storage.

Cooked whole shrimp can be satisfactorily frozen in a vertical plate freezer, but freezing by immersion is not normally recommended since this makes the shrimp difficult to peel and the meat texture is also adversely affected.

The above methods of freezing are equally applicable to freezing on shore, but in order to give a good quality product the shrimp should not be stored for more than two-three days at chill temperatures from the time of capture.

Another method of freezing, widely used in factory operations, is to freeze whole shrimp or unpeeled tails into 2-kg blocks in horizontal plate freezers. The shrimp are loaded into metal moulds and water added to fill the voids. After fixing on the lid the moulds are loaded into the freezer on metal trays and frozen to give solid blocks of shrimp with a net weight of about 2 kg. After freezing, the blocks are released from the moulds by spraying the under side with potable water and they are then packed into plastic bags or cartons. The packages are packed into master cartons for cold storage.

2.5.6.2 Meats only:

Peeled meats are frozen raw or cooked as individually quick-frozen products or in blocks or packages.

Small shrimp should always be individually frozen in a continuous freezer, either air-blast or cryogenic, since even short delays, which may be inevitable with a batch freezer, will result in a product temperature rise and even partial thawing.

Continuous belt freezers may be used for both cooked and uncooked produce but, for cooked meats, only, a freezer, known as a fluidized bed continuous freezer, is a150 satisfactory. With this freezer there will, however, be an additional weight loss due to the agitation of the product in the freezer, but the pulped meat or formed mush can be collected and utilized in some way.

A semi-fluidized bed freezer can also be used for cooked meats with a reduced agitation weigh loss. In this freezer, agitation is only applied at the start of freezing to achieve a separation of the shrimp and a belt is then used to convey the product through the rest of the freezer.

Meats are also frozen in small blocks in horizontal plate freezers for markets which may require them to be thawed and reprocessed before sale. Packaging in metal moulds, but within plastic bags or with a completely over lapped plastic liner, is one method which requires packaging after freezing. Inner plastic bags and outer cartons is another, which gives a finished product. Retail and catering packs can also be made in a similar fashion.

2.5.7 Fillets

The two main frozen fillet products are individually quick-frozen fillets (IQF) and fillet blocks.

IQF fillets may be frozen skin-on or skin-off and various prefreezing and post-freezing treatments, such as polyphosphating and enrobing in batter and bread-crumbs, may influence the choice of freezer.

Ideally, IQF fillets are better to be frozen in a continuous freezer, and air-blast, cryogenic and drum freezers have been used for this purpose.

Batch freezing of fillets is also viable in an air-blast freezer with the fillets laid out carefully on trays placed on shelves, trolleys or pallets. This method, however, is labour intensive and unless the fish are packaged and stored quickly after freezing, some thawing, particularly of thinner fillets, may result.

Fillets are also frozen as blocks in horizontal plate freezers with interleaving of a plastic film so that individual fillets may be removed by the consumer without thawing.

Skinless fillets are frozen into regular shaped, homogeneous blocks in horizontal plate freezers and this product i5 then cut up to form fish portions or fish fingers.

Fillet blocks are also frozen in horizontal and vertical plate and air-blast freezers for bulk storage and later processing. However, the advantage gained by freezing the fish flesh only, rather than whole fish, may be lost due to the greater susceptibility of fillets to adverse quality changes during subsequent cold storage.

Tables 8 and 9 list a number of frozen fillet products for retail and catering sales.

Table 8 Individually quick-frozen fillets -polystyrene trays with stretch wrap

Product

Wt of pack
(kg)

No in pack

Type of pack

Dimensions of pack (mm)

IQF Fillets

.284

-

Poly tray with wrap

188 x 135 x 30

IQF Fillets

.227 - .298

-

Poly tray with wrap

220 x 130 x 30

IQF Fillets

.255 - .369

-

Poly tray with wrap

270 x 138 x 18

IQF Fillets

.173 - .355

-

Poly tray with wrap

215 x 130 x 30

IQF Fillets

.184 - .340

-

Poly tray with wrap

264 x 131 x 25

Note: The fillets in the pack might be above the level of the top of the tray; this must be allowed for when calculating density or stowage rate

Table 9 Individually quick-frozen fillets -polyethylene bags and cartons

Product

Wt of pack
(kg)

No. in pack

Type of pack

Dimensions of pack (mm)

IQF Fillets

.700

-

Poly Bag

 
IQF Fillets

.800

-

Poly Bag

 
IQF Fillets

.907

-

Poly Bag

356 x 258

IQF Fillets

2.7

-

Carton

374 x 235 x 100

IQF Fillets

3.2

24

Carton

379 x 238 x 115

IQF Fillets

3.6

24

Carton

379 x 238 x 115

IQF Fillets

4.1

-

Carton

333 x 250 x 150

IQF Fillets

4.5

-

Carton

375 x 284 x 100

IQF Fillets

6.4

-

Carton

410 x 270 x 185

IQF Fillets

9.1

-

Carton

435 x 278 x 250

2.5.8 Minced and/or formed frozen fish products

This category covers a wide range of products with fish fingers and fish portions being the most widely recognized and accepted products.

Fish fingers and fish portions are made from regular shaped blocks of fish which may be made wholly from fish fillets or fish mince or combination of the two raw materials. The blocks are sawn or cut, after tempering, into the size and shape required and often this type of product is enrobed with a batter and bread-crumb coating. The coating process is usually followed by "flash frying" in a cooking fat, which partially cooks the product and also helps to bind the coating. The heat added during this forming process must be removed before the frozen product is placed in cold storage; therefore, further freezing, in or out with the final retail pack, is necessary. Continuous freezers, either air-blast or cryogenic, may be used to cool and refreeze the individual portions or, if these are packaged in regular shaped cartons or other containers, this final freeze may be done in a horizontal plate freezer.

A variety of machines is now available for forming fish products from fish mince, cuttings and fillets. In addition, countries have their own specialist fish products, such as fish cakes in the UK, fish balls in the Scandinavian countries and surimi in Japan. All of these products may require special freezing facilities, such as arrangements to feed the freezers directly from the forming machines. This type of product is more likely to be processed with an inline production system, therefore, continuous air-blast or cryogenic belt freezers are the type likely to be used. The freezer conveyor belt may also require to be made from a flat sheet material rather than the link or mesh type of belt normally used, since the flat belt ensures a quick release without damage to the product.

Tables 10 and 11 list some of the frozen fish products made in the UK, and they show the different methods of packaging and range of packs sizes usually produced for products of this kind.

Table 10 Fish fingers in cartons and bags

Product

Wt of pack (kg)

No. in pack

Type of pack

Dimensions of pack (mm)

Fish Fingers

.150

6

Carton

110 x 110 x 30

Fish Fingers

.255

10

Carton

182 x 100 x 32

Fish Fingers

.580

24

Carton

200 x 100 x 55

Fish Fingers

2.890

120

Carton

280 x 196 x 112

Fish Fingers

.850

36

Bag Pack

115 x 95 x 135

Fish Fingers

1.450

60

Bag Pack

170 x 100 x 155

Table 11 Fish portions battered and flash-fried

Product

Wt of pack (kg)

No. in pack

Type of pack

Dimensions of pack (mm)

Cut cod Portions

.2

2

Carton

174 x 118 x 28

Cut cod Portions

.6

6

Carton

172 x 116 x 61

Machine formed Cod Portions

.2

2

Carton

182 x 117 x 28

Machine formed Cod Portions

.6

6

Carton

185 x 114 x 72