Page 349 - Biosystems Engineering

326 Cha pte r Ele v e n

foodstuff wrappings including their processing and manufacturing,
physical and barrier properties of packaging materials, and recent
technological advances.

11.2 Packaging Materials
Packaging materials are used to protect a food during storage and
distribution. They may provide protection from microbiological,
chemical, and physical contamination. The most commonly used
materials for packaging of foods are polymer, glass, metal, paper, and
paper-based materials. Once dominated by metal, glass, and paper,
polymeric materials are now increasingly being used for packaging.
Composite materials are also being used in a growing number for
packaging of foods due to their desirable characteristics. Such materials
are formed by combining two or more materials that have quite different
properties. Together, these properties give composite unique prop-
erties. The following section describes commonly used packaging
materials and package-manufacturing process using these materials.

11.2.1 Packaging Polymers
Polymers include a wide range of natural and synthetic materials;
however, only a small portion is used for packaging of foods. Syn-
thetic polymers, commonly known as plastics, are compounds hav-
ing a very high molecular weight. They are constructed of many
repeating units or building blocks, known as monomers, combined
together by a chemical reaction. These monomers are gases or liquids
at room temperature and pressure, whereas polymers are normally
solids under these conditions. Other components in plastic are small
amounts of additives (typically <1 percent) such as heat and light
stabilizers, antioxidants, plasticizers, and ultraviolet absorbers as
well as lubricants, slip agents, and antistatic agents to improve their
manufacturability. 1,2,6
Packaging polymers may be classified into thermoplastic and
thermosets depending on their cross-linked structures. Most food-
wrapping polymers are thermoplastics that are not cross linked and
have either amorphous or semicrystalline structures. Thermosets are
hard and stiff materials usually formed at high temperature by cross-
linking polymer chains with covalent bonds into a three-dimensional
network. The applications of thermosets in food packaging are lim-
ited mostly to coatings, adhesives, caps, and trays. 2,6
The properties of plastics are determined by the chemical and
physical nature of the polymers used in their manufacture. The prop-
erties of polymers are determined by their molecular structure,
molecular weight, degree of crystallinity, and chemical composition.
These factors in turn affect the physical properties of the polymers.
In terms of chemical composition, polymers can be classified as