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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 all food
applications for absorption and migration.
Gases such as oxygen, carbon dioxide and nitrogen together with water
vapour 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
environmental perception.
Use of plastics in food packaging
Plastics are used as containers, container components and flexible packaging.
In usage, by weight, they are the second most widely used type of packaging
and first in terms of value. Examples are as follows: