6. TREATMENT OF FISH AFTER FREEZING

6.1 Glazing
6.2 Packaging of frozen fish

As soon as fish are removed from a freezer, they should be glazed or wrapped (unless they have been packaged before freezing) and immediately transferred to a low temperature store. When it is known that storage will be for a short period only, glazing or wrapping may not be necessary or practical. Blocks of whole cod, frozen at sea, are usually transferred to the ship's cold store without a protective wrapper or glaze but this may be added later, prior to long term storage on shore. During relatively short terms of storage, fish without a protective wrapper or glaze can be severely dehydrated in a poorly designed or operated store.

6.1 Glazing

The application of a layer of ice to the surface of a frozen product by spraying, brushing on water or by dipping, is widely used to protect the product from the effects of dehydration and oxidation during cold storage. The ice layer sublimes rather than the fish below and it also excludes air from the surface of the fish and thereby reduces the rate of oxidation. Heat added by the glazing process is often considerable and the fish may require to be recooled in a freezer before being transferred to the cold store.

In order to form a complete and uniform glaze on the surface of the fish, the glazing process requires to be closely controlled. The amount of glaze applied depends on the following factors:

• Glazing time

• Fish temperature

• Water temperature

• Product size

• Product shape

Glazing by dipping in a container of water is not recommended. The initial temperature of the water may be relatively high; this is reduced as glazing proceeds and the thickness of glaze will therefore vary. The glaze on IQF fillets has been shown to vary between 2 and 20 percent using this method, even when the immersion time was kept constant. In practice, the time will not be constant and this will give rise to even greater inconsistency. The water will also become contaminated after some time; therefore, this method is not recommended. If a dipping method is used to apply a glaze, the container should be continuously supplied with chilled water and fitted with an overflow.

Spray glazing methods are suitable, but again it is difficult to obtain a completely uniform glaze and it may be necessary to invert the fish to ensure that all surfaces are treated.

The glazer shown in Figure 26 has a number of features which enable a complete uniform glaze to be applied.

1. A constant speed belt will ensure a fixed time in the glazing zone;

2. The overhead and underside spray provide a constant supply of chilled make-up water and glazes both the upper and lower surface of the product;

3. The double belt arrangement forces fish to invert providing an even glaze.

4. The adjustable baffle may be used to rearrange overlapping fish on the belt so that each fish is totally exposed;

Glazing when the fish surface temperature is at -70°C or lower, e.g. immediately after cryogenic freezing, results in a glaze which is fractured and broken due to thermal stress during the formation of the ice. This glaze is easily dislodged during subsequent handling. If the fish are immersed in the glaze water for too long, a thick glaze is formed but the equilibrium temperature of fish and ice is high and only slightly below 0°C. This glaze will be soft and easily dislodged during subsequent handling.

Good glazing practice can be beneficial, particularly when other aspects of storage and transport are far from ideal, but poor glazing involving partial thawing of the fish and slow refreezing in cold storage may do more harm than good.

6.2 Packaging of frozen fish

6.2.1 Need for Packaging

After fish has been frozen, it can be subjected to many forms of deterioration between production and eventual consumption. Contamination from humans, animals, insect and atmospheric sources is possible. Physical damage can be caused by bad handling during stacking, transit and storage or display in freezer cabinets. The sensory properties can be adversely affected by tainting and textural and flavour changes can be caused by dehydration and chemical changes which can take place under poor cold storage conditions. To prevent or reduce losses in product quality, it is essential that the frozen product is packaged in such a way as to provide an effective barrier with sufficient impact and compression strength to prevent mechanical damage. The packaging material must have adequate barrier properties to reduce losses due to dehydration and pick-up of taints. Further considerations are the printability of the material, so that a well designed attractive illustrated package can be produced. The package should give the consumer information on the nutritional properties of the product and instructions on how the product should be prepared, stored and give the 'use by' or 'best before' date.

A final consideration which is becoming increasingly important is the environmental issue. Considerations should include the impact of the packaging material on the environment, whether or not the package is re-usable or recyclable, whether the package is made from renewable resources and if the package produces pollution when it is being destroyed.

6.2.2 Types of Packaging Material for Frozen Fish

The range of packaging material for frozen fish is very wide and is dependant on the form of the product being packed. Whole fish frozen in a vertical plate freezer, for example, may require little packaging, other than an ice glaze, to prevent dehydration. Small pelagic fish frozen in vertical plate freezers may benefit from being frozen in water blocks where the freezer plate is lined with a robust plastic paper lined bag. This can be filled with water after the fish have been placed between the freezer plates. A processed fish product, for example fish sticks, may be wrapped in a primary package which is in direct contact with the frozen food and then stored in an outer carton. A number of primary packages may be collected together and packaged in a master carton (secondary package) for delivery or display in a freezer cabinet. The secondary packages can be bought together in a tertiary package, for example a wrapped pallet, and used for bulk transportation.

6.2.3 Primary Packaging Material

6.2.3.1 Plastics

The primary package in contact with the frozen product is generally a plastic derived from a natural hydrocarbon source. The choice of which plastic wrapper is dependant on the type of barrier required, and if the product is to be cooked or heated in the container. Migration of the plasticisers from the wrapping is a potential health hazard and the type of wrapping which can be in contact with food is covered by national legislation. The non-biodegradability of plastic wrapping material is an environmental issue and toxic compounds eg dioxins can be produced when, for example polyvinylidene chloride (PVDC) or polyvinyl chloride (PVC) plastics are incinerated at low temperatures.

Many different types of packaging materials are used for fish, and it would be unrealistic to try to list them all. Tables 10 and 11 list various films and laminates, their barrier properties and Table 12 lists various packaging methods and their applications . Table 13 summarises the properties of the basic material in comparison with nylon and (Polyacetol, PA) and polythene (Polyethylene, PE), and Table 14 summarises the properties of laminate with polyethylene and nylon. Plastics with good moisture vapour barriers and stability at low temperatures are required for the storage of frozen fish, and a good vapour barrier is required to prevent oxidation of fats in frozen fatty fish. The group of plastics called polyolefins, which include polyethylene, polypropylene and its co-polymers are most widely used. Generally, the higher the density of the polyethylene the better the barrier properties, with polypropylene being the best. Polypropylene is able to withstand temperatures up to around 100°C and is therefore suitable for boil in the bag products. To package ovenable products, a modified polyethylene CPET (Crystaline Polyethylene Tetraphthalate) is used, which maintains low temperature flexibility.

Polypropylene or polyethylene laminated with polyamide or polyester are frequently used for boil in the bag type products, and laminated plastic aluminium foil may be used when good vapour and moisture barriers are required, particularly with fatty fish to prevent fatty acid oxidation.

Plastic and paper based packaging is transparent to microwaves (passive packaging) and therefore can be used to contain foods which are to be microwaved. Thin polyester films can be metalised with aluminium and then laminated to a supporting board. When microwaved the aluminium absorbs a certain amount of radiation, generating heat and cooking the product.

Other considerations concerning primary packaging materials are the sealability of the material by heat and also its printability properties.

6.2.3.2 Cartons

Cartons can also be regarded as primary packages when used as a protective sleeve to the product. The boards for the cartons can be made of :

1. Kraft boards. These are frequently used for packaging frozen foods and are usually made from fully bleached materials. They are strong, of good appearance and are suitable for direct contact with food.

2. Folding box boards. These usually have one fully bleached side which is suitable for direct contact with food.

3. Recycled fibre boards. These are usually used for secondary and tertiary packaging.

6.2.4 Secondary Packaging

Secondary packaging is usually a carton which holds a number of primary packages. The secondary package is usually made from boards but can be bands of paper or plastic.

6.2.5 Tertiary Packaging

Tertiary packaging is used to hold a number of secondary packages. Tertiary packaging may be palletised for easy handling and wrapped with shrink, stretch wrap or corrugated outers or may be packed in re-usable containers. Wooden pallets are in regular use, but can become a source of contamination. Plastic pallets, which can be colour coded, are more easily cleaned but will support the growth of mould in frozen fish factories.

6.2.6 Packaging Machinery

Packaging equipment may vary between a simple hand operated tool to an extremely complex machine and examples are listed in Table 15. For primary packaging of frozen fish two types of carbon, top load and end loads are generally used.

6.2.6.1 Top Load Cartons

Top load cartons (Fig 27) which are formed immediately prior to packaging, are supplied flat and unglued. The cartons are erected by a machine at speeds of between 20 and 320 cartons per minute and glued with hot melt adhesives which can tolerate cold store conditions. Top load cartons are used for some forms of fish sticks, the product being loaded in through the widest opening. A top load processing line normally consists of three machines for carton erecting, filling and closing respectively. Closing is usually effected by heat seal or adhesive to make the container tamper proof.

6.2.6.2 End Load Machines

End load cartons (Fig 28) are used for products which are sufficiently robust to allow the product to be pushed in at the end of the carton. These are frequently used as a primary package for frozen fish portions or fish sticks. The process is slightly more expensive than top load processing, but has the advantage that the process is carried out by a single machine.

6.2.6.3 Auto Loading

Many machines have intelligent product - transfer units (IPTU's) which will automatically monitor and load the frozen product into the container and can be set to accept different tolerances. This technology is widely used in packaging frozen fish products to ensure that the weight of the product within the packet is with the designated tolerance.

6.2.6.4 Bags

Frozen fillets can be packed directly into bags made from materials with good gas vapour and moisture barrier properties. The level of sophistication can range from manual weighing and loading to a highly sophisticated form-fill-seal technology where a specified weight, volume or count of product is filled into a formed bag which is heat sealed. Such equipment can be used for packing IQF cooked, peeled shrimp.

6.2.6.5 In-Line Pouch Forming Equipment

In-line pouch forming equipment was developed in the mid 1960s to reduce the labour required to product the package, fill and seal the container. Such machines (Figure 29) are widely used in the frozen fish industry, particularly in situations where sauces are added to the product. The material for the package is formed from a roll of packaging laminate (bottom web) which is heat formed in a dye, either by compressed air or vacuum to form the pouch. The product is then loaded, either by machine or hand into the pouch and any added liquid will be added by an automatic depositor. The package then has the top sealing web brought into contact with the bottom web, evacuated and then heat sealed. Some products may have an inert gas injected into the package as an alternative to evacuation, to prevent the packaging coming into contact with the product. The packages are then separated by cutters which cut both along and across the webs which join the packages together.

Typical films for this type of packages are:

Bottom web - nylon 75 microns + polyethylene 50 microns = 125 microns

Top web - nylon 20 microns + polyethylene 50 microns = 75 microns

6.2.7 Machinery For Tertiary Wrapping

Tertiary packaging is the final stage of the packaging procedures and machinery is available which will collate, package and palletise the secondary packages. Tertiary packaging normally takes place after the product has been frozen. A typical operation would involve the orientation and collation of a number of secondary packages which would be automatically loosely wrapped in shrink wrap. The shrink wrap would then be heat sealed before the wrap packs are conveyed through a heated tunnel, where the wrapping shrinks to assume the geometry of the pack. Alternatively, the secondary packages can be packaged in cases, which may be formed round the product (wrap around) or the case may be preformed and glued before the packages are loaded.

The tertiary packs are then frequently palletised for storage or distribution. The pallet and contents themselves may be wrapped in stretch wrap or similar materials for further protection. This may be done by hand, but is increasingly carried out by mechanical pallet wrappers.

Table 10 Characteristics of some film used for the packaging of solid foods

LDPE, low-density polyethylene; HDPE, high density polyethylene; PP, polypropylene; OPP, oriented polypropylene; PVC, polyvinyl chloride; PVDC, polyvinylidene chloride; PS. polystyrene; SAN, styreneacrylonitrile; PET, polyester; PA, polyamide; OPA, oriented polyamide; EVAL, ethylene-vinyl alcohol; MXXT, a PVDC coating.

Table 11 Characteristics of some laminates used for the packaging of solid foods

M, Metallized; Kraft, paper; DM. one side nitrocellulose coated; A, aluminium foil; TPP, this porous paper; XS. cellulosic film coated with PVDC

Table 12 Packaging methods and applications

Table 13 Properties of basic materials

Notes: 1. Tensile, tear and water vapour (WV) qualities are relative to PE which has unit value

2. Gas permeability is related to PA which has unit value

Table 14 Typical laminates compared with PE (Polyethylene) and PA (Nylon)

Table 15 Packaging machinery

Note: Data for typical machines. Local prices and availability may vary.