Packaging and storage of herbs and spices 95
5.7.5 Single films
The most important types of film for food packaging are described below.
Cellulose films
Plain cellulose is a glossy transparent film which is odourless, tasteless and biodegradable
within approximately 100 days. It is tough and puncture resistant, although it tears
easily. It has low-slip and dead folding properties and is unaffected by static build up,
which makes it suitable for twist wrapping. It is not heat sealable and the dimensions
and permeability of the film vary with changes in humidity. It is used for foods that
require a complete moisture or gas barrier, including fresh bread and some types of
confectionery.
Oriented polypropylene is a clear glossy film with good optical properties and a
high tensile strength and puncture resistance. It has a moderate permeability to moisture,
gases and odours, which is not affected by changes in humidity. Biaxially orientated
polypropylene has similar properties to orientated polypropylene but is stronger.
Polyethylene terephthalate (PET) is a very strong transparent glossy film which
has good moisture and gas properties. It is flexible at temperatures from – 70 ∞C to
135 ∞C and undergoes very little shrinkage with variations in temperature and humidity.
Low density polyethylene (LDPE) is used as a copolymer in some tubs and trays.
It is heat sealable, chemically inert, odour free and shrinks when heated. It is a good
moisture barrier but has relatively high gas permeability, sensitivity to oils and poor
odour resistance. Low slip properties can be introduced for safe stacking or, conversely,
high slip properties permit easy filling of packs into an outer container. It is the least
expensive of most films and is therefore widely used.
High density polyethylene (HDPE) is stronger, thicker, less flexible and more
brittle than low density polyethylene and has lower permeability to gases and moisture.
Sacks made from 0.03–0.15 mm HDPE have a high tear strength, tensile strength,
penetration resistance and seal strength. They are waterproof and chemically resistant
and are used instead of multi-wall paper sacks for sipping containers. Other types of
film structures include uncoated polyvinylidene chloride (PVdC), polystyrene and
ethylene vinyl acetate (EA).
Coated films
Films are coated with other polymers or aluminium to improve their barrier properties
or to impart heat sealability. A thin coating of aluminium produces a very good
barrier to oils, gases, moisture, odours and light. Metallized film is less expensive
and more flexible than foil laminates which have similar barrier properties. Metallized
polyester has higher barrier properties than metallized polypropylene, but polypropylene
is used more widely as it is less expensive.
Laminated films
Lamination of two or more films improves the appearance, barrier properties and/ or
mechanical strength of a package. Laminates typically include nylon-LDPE, nylon-
PVdC-LDPE and nylon-EVOH-LDPE for non-respiring products. The nylon provides
strength to the pack, EVOH or PVdC provides the correct gas and moisture barrier
properties and LDPE gives heat sealability. PVC and LDPE are also used for commonly
respiring MAP products.