By C. K. MURRAY & D. M. GIBSON

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.

DEPARTMENT OF TRADE AND INDUSTRY

TORRY RESEARCH STATION

TORRY ADVISORY NOTE No. 51

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