Introduction

This is the first of two advisory notes on prepacked chilled fish. It summarizes the advantages of presenting prepacked fish to the shopper, discusses the important properties of the packaging materials used, and briefly describes the types of equipment available for handling and distributing prepacked fish. Note 52 will contain advice on making prepacks, including recommendations for selection of the raw material and for the preparation and packing of wet, breaded and smoked fish products.

Why prepack fish?

The retail food market in Britain is changing rapidly; the number of shops handling a narrow range of foods is decreasing, while supermarkets and large self service stores are increasing in number and size. There is considerable scope for expanding the market for fish through these modern outlets, provided a good quality product is attractively presented for sale in a suitable package.

Some large stores are selling wet fish from a traditional slab, with a fishmonger in attendance, but this method has a number of disadvantages; a large space is needed, the slab has to be manned continuously, the products are inconvenient to handle and there may be some smell and possibly contamination. In contrast, properly packaged fish products can be easily handled by nonspecialist staff, examined by the shopper for type, quantity and price, purchased and carried home in the shopping basket with other foods. A well printed, eye catching package can encourage impulse buying and quickly establish a brand reputation. Packets of frozen fish have been sold for many years from frozen food cabinets containing other foods; prepacked wet and smoked fish can be sold in much the same way from the chill cabinet in competition with meats and other perishable foods.

Does prepacking increase shelf life?

Prepacking is mainly a method of presentation, not of preservation; the shelf life of a wrapped wet fish product is virtually the same as that of an unwrapped one. There is sometimes a small increase in shelf life, but not enough to justify keeping prepacked fish longer in the shop. The reputation of a branded pack depends on careful stock control.

Where should the prepacking be done?

Only good quality fish should be packed; therefore prepacking should be done in factories at or near the port where the quality of the fish can be carefully inspected and controlled. Poor quality fish disguised in a package will bring that brand, and possibly all prepacked fish, into disrepute.

Packaging materials for chilled fish

The most useful materials for making small packages of chilled fish are the thin flexible films produced mainly from plastics. Plastics are mainly synthetic or naturally occurring polymers or resins. Polymers are large molecules built up by the repeated addition of small chemical units; when more than one type of unit is added the resulting molecules are called copolymers.

Plastics raw materials, which are mainly products of the petrochemical industry, are made into flexible films by manufacturers known as converters. Different films may be joined to form compound films called laminates, whose properties are a combination of their component materials. Normally the laminates are made either by extruding two single films together or by sticking the films together with an adhesive. Sometimes chemicals known as plasticisers are added to make the films easier to work with.

The arrangement of the molecules in some films can be altered to improve their properties. For example, some films can be made more flexible by heating and stretching simultaneously in two directions during manufacture and then cooling them; a film treated in this way is described as biaxially orientated. Other films are heated, stretched and then kept stretched while they cool; these, when subsequently wrapped round a product will, when heated to a particular temperature, shrink to give the familiar shrink wrap.

Other packaging materials for wet fish are used in conjunction with flexible films; for example small trays of cardboard, pulpboard or expanded polystyrene are often used to support the product and improve presentation.

The following properties of flexible films for wrapping fish are of importance:

Thickness or substance

The thickness of single plastics films is measured in micrometres, µm, but cellulose film, and flexible laminates and their components, are described by substance, that is the weight of a given area, in g/m². Other properties are dependent on thickness, for example strength and opacity.

Working temperature

This is the range of temperature, in °C, in which the film can be worked without becoming too brittle or too limp.

Heat sealing

When a film is heated to a certain temperature which depends on the type of material, the molecules move out of position and start to slip; this property is used for sealing bags and pouches. A film that has good protective properties but forms a poor heat seal can often be improved by coating it or laminating it with a film that seals well. The heat sealing temperature of a film is the lowest temperature that will give an acceptable seal.

Water vapour transmission

Fish requires the protection of a reasonably good barrier to water vapour to prevent it drying. Water vapour can pass through a film in two ways; the film may be porous so that vapour passes through holes in the material, or the film may be permeable, that is water vapour diffuses through it by dissolving in the material. Thin films are often porous, but porosity can be overcome by using a thicker film; a permeable film cannot be made impermeable in this way. Permeability is dependent on temperature, pressure and humidity; thus the water vapour transmission rate for a given material should be qualified by stating the temperature, pressure and humidity at which the rate applies.

Gas transmission

Gases like oxygen or carbon dioxide for example are transmitted through a film in much the same way as water vapour. For vacuum packs, the film must be a good enough gas barrier to maintain the vacuum during the life of the pack.

Odour and taste

All packaging materials in contact with fish must be odourless and should not affect the flavour of the fish. In addition, the wrapping should prevent the fish from tainting, or being tainted by, other foods.

Toxicity

Flexible films, and any adhesives or plasticisers in them, must be harmless and nonpoisonous in contact with food. The films should conform to the recommendations of the Toxicity Sub-Committee of the British Plastics Federation.

Types of flexible film

Some properties and uses of the more common flexible films used for packaging are briefly described. A single example of a proprietary name is given for each type, but this is in no way an endorsement of that particular make in preference to any other.

Cellulose: example Cellophane

Regenerated cellulose is made from wood pulp; it is not a plastics polymer. It absorbs water; therefore its properties are affected by the moisture content of the film and by the humidity of the surrounding air. The effect of moisture can be reduced by coating it, usually with nitrocellulose, or by converting it to cellulose acetate which, however, has somewhat different properties from cellulose but can be used for packaging fish. Cellulose can also be coated with a polymer, usually polyvinylidene chloride, PVdC, to reduce the gas transmission rate.

Suppliers use special letter codes to differentiate between types of cellulose film, and code numbers to indicate the substance, or weight of a unit area.

Suitably coated cellulose films are normally heat sealed at temperatures between 120 and 175°C; the advice of the manufacturer should be sought for any particular film.

Polyethylene: example Polythene

The ethylene polymer is available in three main grades, low, medium and high density film. Polyethylene, PE, film is slightly opaque; the opacity increases with density but can be reduced by radiation treatment. PE film is waxy to the touch, and is a good moisture barrier. Low and medium density PE films are flexible even when cold. Low density PE film is a poor gas barrier, but resistance to gas transmission increases with density. Low and medium density PE films are not resistant to oils and fats, and high density film is only slightly resistant. PE is readily heat sealed. High density PE film is suitable for boil-in-the-bag packs, whereas low and medium density films are not.

PE is frequently laminated with other, often more expensive, films so that its good moisture barrier and heat sealing properties can be combined with other desired properties. Working and heat sealing temperatures for PE film are

	working temperature °C	heat sealing temperature °C
low density PE	-50 to 50	120 to 135
medium density PE	-50 to 60	125 to 140
high density PE	-60 to 70	135 to 155

Polypropylene: example Propathene

Polypropylene, PP, is available either as an extruded film or as an orientated film, often designated polypropylene 0. The orientated film is usually stronger and more resistant to transmission of water vapour and gas.

PP resembles polyethylene but is stronger, lighter and more rigid. It has a higher melting point than PE, but becomes brittle at about -10°C. It can be heat sealed, and the orientated film has slightly lower water vapour and gas transmission rates than a medium density PE. It is resistant to fats, acids and alkalis. PP is used as a single film, coated or uncoated, and in laminates.

	working temperature °C	heat sealing temperature °C	shrink temperature °C
extruded PP	-10 to 70	170
orientated PP	-10 to 60	170	150

Polyvinylidene chloride; example Saran

Polyvinylidene chloride, PVdC, is inert in contact with food and can be used either as a film or as a coating on other films. It is often linked chemically with polyvinyl chloride, PVC, to produce a range of copolymers.

It is an excellent barrier to water vapour and oxygen, and is therefore useful in preventing fat in fish from going rancid. It is resistant to fats and oils and to many organic solvents. It can be sealed by using impulse or high frequency heat sealing equipment, but is easily decomposed when overheated. PVdC and its copolymers are most frequently used as thin coatings on other cheaper films.

	working temperature °C	heat sealing temperature °C	shrink temperature °C
PVdC	-20 to 60	130	-
shrinkable PVdC	-30 to 45	130	85

Polyvinyl chloride: example Vitafilm

There are several food grade films made from polyvinyl chloride, PVC, but plasticisers used in ordinary grades of PVC film often make this material unsuitable for packing fish. Some of the plasticisers are poisonous and others have distinctive odours that can be transferred to the product.

PVC has better gas barrier properties than polyethylene or polypropylene, and is resistant to oils and fats. It can be heat sealed. PVC is often used as a shrink or stretch wrapping for food products.

	working temperature °C	heat sealing temperature °C	shrink temperature °C
lightly plasticised PVC	5 to 60	180	-
orientated PVC	-5 to 40	-	90
normal plasticised PVC	5 to 35	150	-
heavily plasticised PVC	0 to 30	130	-
nontoxic PVC	5 to 35	150	-

Rubber hydrochloride: example Pliofilm

Natural rubber is dissolved and treated with a solution of hydrochloric acid to produce this material. It normally contains plasticisers, waxes, antioxidants and other additives; therefore it is essential to use a food grade film. It can be heat sealed easily and is generally used either as a stretch film or as a component of a laminate. Rubber hydrochloride is a good barrier to water vapour and oxygen, but its properties vary with the amount of plasticisers present.

	working temperature °C	heat sealing temperature °C
very heavily plasticised	-10 to 25	105
heavily plasticised	5 to 25	105
normal plasticised	10 to 30	120
lightly plasticised	15 to 35	125

Polyamide: example Nylon

The two polyamides most used for packaging are known commercially as Nylon 6 and Nylon 11.

Nylon 6 is slightly opaque, a good gas barrier but a poor water vapour barrier. It can withstand high temperature, can be sterilized, and can be used for boil-in-the-bag packs. It can be heat sealed, but only at a rather high temperature. It is resistant to oils and fats. It is often used with a PVdC coating or is laminated with polyethylene.

Nylon 11 is more transparent than Nylon 6, and a better gas barrier; otherwise their properties are much the same.

	working temperature °C	heat sealing temperature °C
Nylon 6	- 40 to 150	220
Nylon 11	-140 to 140	180

Polyester: example Melinex

Polyester is a moderately good water vapour barrier and a good gas barrier. It requires a high temperature for heat sealing, and cannot be handled very easily on conventional equipment. It is transparent. It can be used for shrink wrapping, but is relatively expensive and is used mainly in laminates, particularly when vacuum has to be maintained in packs stored at above 0°C. Polyester copolymers have a lower melting point but some of the other desirable properties of polyester are reduced.

	working temperature °C	heat sealing temperature °C	shrink temperature °C
polyester	- 80 to 200	260	85
copolyester	-100 to 120	-	85

Aluminium foil

Aluminium foil when free from pinholes is a complete barrier to water vapour and gases; foil more than 30 µm thick is generally regarded as being free from pinholes. Since foil is easily damaged, it is frequently used in conjunction with a supporting film. A coating of polyethylene or a heat sealing lacquer is usually necessary to provide a sealing facility. Aluminium is resistant to fats and oils, and can be used at any temperature likely to be encountered in packaging. It is opaque.

Laminates

Single films rarely fulfil all the requirements of a specific packaging application, but it is possible by laminating two or more materials to obtain the right combination of properties to suit almost any application.

The following tables compares the principal properties of some of the single films and laminates that have been used for prepacking chilled fish.

Guide to films for prepacking chilled fish

Material		barrier to			odour transfer	use with wet fish
		water vapour	oxygen	carbon dioxide
single films
PE ld		good	poor	poor	nil	overpouch
	md	good	poor	moderate	nil	overpouch
	hd	very good	moderate	good	nil	overpouch (opaque)
PP		very good	moderate	moderate	nil	overwrap
polyamide		very good	very good	very good	nil	vacuum pouch
cellulose		poor	poor	poor	nil	overwrap
coated single films
cellulose/PVdC on one side		good	good	good	nil	overwrap
both sides		very good	very good	very good	nil	overwrap
PP/PVdC on one side		very good	very good	very good	nil	overwrap
laminates
PE Id/PP		very good	good	good	slight	overpouch
PE md/PP		very good	good	good	slight	overpouch
PE Id/PP with PVdC coat		very good	excellent	excellent	strong	vacuum pouch
PE md/PP with PVdC coat		very good	excellent	excellent	strong	vacuum pouch
PE md/polyester		very good	very good	very good	nil	vacuum pouch
PE md/cellulose		very good	very good	very good	nil	overpouch
PE md/cellulose with PVdC coat one side		very good	excellent	excellent	slight	vacuum pouch

Notes on the table:

1. Abbreviations:	PE	polyethylene	PP	polypropylene	PVdC	polyvinylidene chloride
	ld	low density	md	medium density	hd	high density
2 Film thickness:	PE 38 or 50 µm, PP 12·5 or 19 µm, polyester 12·5 µm. The equivalent thicknesses in the old imperial guage are given below.

metric thickness	imperial guage
µm
12·5	50
19	75
25	100
30	120
38	150
50	200
65	250

3. The types of pack referred to in the last column are described in advisory note 52 ‘Prepacked chilled fish: production’.

Packaging equipment

Sealing

A pack is only as good as its seal. Pouches are normally sealed on three edges during manufacture, leaving one edge to be sealed after filling. Heat sealing is the normal method of closing wet fish prepacks, usually by means of an electrically heated bar or strip; the metal may be heated continuously or heated and cooled alternately by an impulse technique. Some heat sealers employ high frequency heating, whereby rapid reversal of the current generates heat within the film.

Some films, for example PVC and the polyamides, require the use of a special release coating on the jaws of the heat sealer to prevent sticking; polytetrafluoroethylene, PTFE, is often used for this purpose.

Automatic sealing machines usually employ a continuously heated bar which is clamped on the edge of the pouch for long enough to weld the films together. This type of machine is most suitable for PE and PP films. Some plastics, for example PVdC, cannot be sealed in this way, and here the impulse technique is used. The edge to be sealed is first heated and then allowed to cool while remaining clamped in the jaws of the sealer. Impulse sealing is slower, because of the cooling time required, and output is therefore reduced.

Small and comparatively cheap heat sealing machines can be bought or hired, so that a wet fish prepacking line can be started on a modest scale without investing large amounts of capital. Large machines handling thousands of packs an hour are expensive but they usually include a conveyor system.

Shrink wrapping

Filled trays overwrapped with a shrink film are first held on a heated plate to seal the film under the tray, and are then passed though a hot air tunnel set at the correct temperature to shrink the film. The film becomes taut over the product, thus making a more attractive pack. The hot plate and air tunnel are relatively cheap and can have a high capacity.

Pack design and printing

Attractive presentation is one of the keys to success in marketing prepacked fish, and this means more than just arranging the fish neatly in the tray or pouch. Clarity of film, style, layout and colour of the printing on the film, and easily read information on price, weight and kind of product are all very important selling factors.

Artwork and lettering on the film can be either surface printed or sandwich printed. Surface printing is usually cheaper, but the print may become blurred or damaged by scuffing. With sandwich printing, one of the component films of a laminate is printed before the laminate is made up, so that the print is protected within the sandwich.

The print is usually applied to the surface of a flexible film either by the flexographic or the photogravure process. Flexographic printing, in which the ink is applied from a raised moulded surface of rubber, is generally cheaper for small orders, but the range of inks that can be used is limited to those that do not affect the rubber mould.

In the photogravure process the ink is applied directly from an engraved metallic roller, giving better reproduction than in flexographic printing. Photogravure can be as cheap as the flexographic method for large orders. Film manufacturers should be consulted about printing requirements, because some surfaces may require special inks and colours, or may need treatment before printing.

Packaging for distribution

Fresh fish packed in flexible film requires to be kept cool in transit and protected against physical damage. The product will spoil if it is too warm, and undue pressure may break the wrapping or squeeze fluid from the fish to make the appearance of the pack unattractive.

Packs should be transported in a protective outer container of a size that can be readily handled by shop staff. Cardboard outers with spacers between individual packs can be used provided insulated, refrigerated transport is available. Alternatively expanded polystyrene containers can be used to give some degree of insulation; nonreturnable boxes of the kind used for carrying unwrapped fillets can be used, or specially designed boxes can be made in which built-in supports are provided for individual packs. In the plain nonreturnable polystyrene box, the packs stow best on edge and, if the packs are partially evacuated, the contents of the packs do not slide about within the pouch. The packs are kept cool in both types of polystyrene container by including ice in thick polyethylene bags; in the nonreturnable box the ice bags are laid on top of the rows of packs, and in the special containers they are put in the slots at intervals between the packs. The type of box selected and the amount of ice used will depend on the length of journey and the nature of the distribution chain. In a well designed polystyrene container with a few ice packs it is possible to keep the temperature of the packs close to 0°C for up to 16 hours in an uninsulated vehicle, with an outside temperature of 16°C.

Retail equipment

The retailer must have access to a chillroom capable of maintaining the packs within the temperature range 1-4°C, and must have a chilled display cabinet. Some existing chillrooms, designed to store bacon for example at about 7°C, may not be capable of running satisfactorily at a lower temperature. The main stock of packs should be held in the chillroom, and the small number on display in the cabinet replaced as they are sold. The shop lighting and any lighting in the display cabinet should be chosen to show off the packs at their best; some smoked fish packs for example may appear to have un unattractive colour when seen in certain types of fluorescent light.