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< prev - next > Food processing Packaging and bottling KnO 100270_Packaging_materials_for_foods (Printable PDF)
Packaging materials for Foods
Practical Action
dry foods and, when lined with polyethylene, for cooking fats. They are lightweight, resist
compression and may be made water resistant for outside storage. Other products that are
handled in lined drums include fruit and vegetable products, peanut butter and sauces.
Corrugated board resists impact, abrasion and compression damage, and is therefore used for
shipping containers. Smaller more numerous corrugations give rigidity, whereas larger
corrugations or double- and triple-wall corrugated material provides cushioning and resists
impact damage. Corrugated cartons are used as shipping containers for bottled, canned or
plastic-packaged foods. Wet foods may be packed by lining the corrugated board with
polyethylene or a laminate of wax-coated greaseproof paper and polyethylene, and used for
chilled bulk meat, dairy products and frozen foods.
Flexible plastic films
In general, flexible plastic films have relatively low cost and good barrier properties against
moisture and gases; they are heat sealable to prevent leakage of contents; they add little weight
to the product and they fit closely to the shape of the food, thereby wasting little space during
storage and distribution; they have wet and dry strength, and they are easy to handle and
convenient for the manufacturer, retailer and consumer. The main disadvantages are that (except
cellulose) they are produced from non-renewable oil reserves and are not biodegradable. Concern
over the environmental effects of non-biodegradable oil-based plastic packaging materials has
increased research into the development of ‘bioplastics’ that are derived from renewable
sources, and are biodegradable. However, these materials are not yet available commercially in
developing countries.
There is a very wide choice of plastic films made from different types of plastic polymer. Each
can have ranges of mechanical, optical, thermal and moisture/gas barrier properties. These are
produced by variations in film thickness and the amount and type of additives that are used in
their production. Some films (e.g. polyester, polyethylene, polypropylene) can be ‘oriented’ by
stretching the material to align the molecules in either one direction (uniaxial orientation) or two
(biaxial orientation) to increase their strength, clarity, flexibility and moisture/gas barrier
properties. There are thus a very large number of plastic films and small-scale processors should
obtain professional advice when selecting a material to ensure that it is suitable for the intended
product and shelf life. Typically, the information required includes: type of plastic polymer(s)
required; thickness/strength; moisture and gas permeability; heat seal temperature; printability
on one or both sides; and suitability for use on the intended filling machinery (see also Technical
Brief: Filling and Sealing Packaged Foods).
A summary of the main different types of flexible plastic films is
as follows:
Plain cellulose is a glossy transparent film that is odourless,
tasteless and biodegradable (within approximately 100 days). It
is tough and puncture resistant, although it tears easily. It has
dead-folding properties that make it suitable for twist-wrapping
(e.g. sugar confectionery). However, it is not heat sealable and
the dimensions and permeability of the film vary with changes in
humidity. It is used for foods that do not require a complete
moisture or gas barrier, including fresh bread and some types of
sugar confectionery. Cellulose acetate is a clear, glossy
transparent, sparkling film that is permeable to water vapour,
odours and gases and is mainly used as a window material for
paperboard cartons.
Fig. 4. Milk packaged in
flexible film. (Photo:
Peter Fellows)
Polyethylene (or polythene)
Low-density polyethylene (LDPE) is heat sealable, inert, odour free and shrinks when heated. It is
a good moisture barrier but is relatively permeable to oxygen and is a poor odour barrier. It is less
expensive than most films and is therefore widely used for bags, for coating papers or boards and
as a component in laminates. LDPE is also used for shrink- or stretch-wrapping (see Technical
Brief: Filling and Sealing Packaged Foods). Stretch-wrapping uses thinner LDPE (25 - 38 m)