The most recent EU Directive relating to ‘plastic materials and articles intended to come into contact with foodstuffs’ (reference 2001/62/EC) defines plastics as being: ‘organic macromolecular compounds obtained by polymerisation, polycondensation, polyaddition or any similar process from molecules with a lower molecular weight or by chemical alteration of natural macromolecular compounds’.
Plastics are widely used for packaging materials and in the construction of food processing plant and equipment, because:
• they are flowable and mouldable under certain conditions, to make sheets,
shapes and structures
• they are generally chemically inert, though not necessarily impermeable
• they are cost effective in meeting market needs
• they are lightweight
• they provide choices in respect of transparency, colour, heat sealing, heat resistance and barrier.
Referring again to the Directive, molecules with a lower molecular weight are known as monomers and the macromolecular compounds are known as polymers – a word derived from Greek, meaning many parts. The first plastics were derived from natural raw materials and, subsequently, in the first half of the 20th century, from coal, oil and natural gas. The most widely used plastic today, polyethylene, was invented in 1933 – it was used in packaging from the late 1940s onwards in the form of squeeze bottles, crates for fish replacing wooden boxes and film and extrusion coatings on paperboard for milk cartons. In Europe, nearly 40% of all plastics is used in the packaging sector, andpackaging is the largest sector of plastics usage (Association of Plastics Manufacturers in Europe, APME). About 50% of Europe’s food is packed in plastic packaging (British Plastics Federation, BPF).
Plastics have properties of strength and toughness. For example, polyethylene terephthalate (PET) film has a mechanical strength similar to that of iron, but PLASTICS IN FOOD PACKAGING 175 under load the PET film will stretch considerably more than iron before breaking. Specific plastics can meet the needs of a wide temperature range, from deep frozen food processing (-40°C) and storage (-20°C) to the high temperatures of retort sterilization (121°C), and reheating of packaged food products by microwave (100°C) and radiant heat (200°C). Most packaging plastics are thermoplastic, which means that they can be repeatedly softened and melted when heated.
This feature has several important implications for the use and performance of plastics, as in the forming of containers, film manufacture and heat sealability. Thermosetting plastics are materials which can be moulded once by heat and pressure. They cannot be resoftened, as reheating causes the material to degrade. Thermosetting plastics such as phenol formaldehyde and urea formaldehyde are used for threaded closures in cosmetics, toiletries and pharmaceutical packaging but are not used to any great extent for food packaging.
Plastics are used in the packaging of food because they offer a wide range of appearance and performance properties which are derived from the inherent features of the individual plastic material and how it is processed and used. Plastics are resistant to many types of compound – they are not very reactive with inorganic chemicals, including acids, alkalis and organic solvents, thus making them suitable, i.e. inert, for food packaging. Plastics do not support the growth of microorganisms. Some plastics may absorb some food constituents, such as oils and fats, and
hence it is important that a thorough testing is conducted to check food applications for absorption and migration.
Gases such as oxygen, carbon dioxide and nitrogen together with watervapour and organic solvents permeate through plastics. The rate of permeation depends on:
• type of plastic
• thickness and surface area
• method of processing
• concentration or partial pressure of the permeant molecule
• storage temperature.
Plastics are chosen for specific technical applications taking the specific needs, in packing, distribution and storage, and use of the product into consideration, as well as for marketing reasons, which can include considerations of