Page 355 - Biosystems Engineering

332 Cha pte r Ele v e n

longitudinal direction, whereas orientation in the transverse direc-
tion is produced as in normal extrusion blow molding by blowing air.
This biaxial orientation produces bottles with superior mechanical,
optical, and gas barrier properties. 1,2,6

Thermoforming In this process, a thermoplastic sheet, 75 to 200 μm
thick, is heated to its softening temperature with infrared radiation.
Mechanical or pneumatic (vacuum or pressurized air) forces are used
to press the soft sheet against the mold contours. The most common
thermoplastics used for thermoforming include PS, PVC, PP, PET,
LDPE, HDPE, cellulose acetate, and nylon. The sheets may be mono-
layer, multilayer, coated, or laminated structures. Typical food pack-
aging containers produced using thermoforming are trays, cups, and
tubs.
A number of processes as described above are used to process
thermoplastics into films, bottles, trays, foamed products, and other
packaging forms. Some polymer processes used are orientation, vac-
uum metallization, lamination, and coatings to improve the function
properties of final products. A detailed discussion of several of these
processes is found in the literature. 1,2,6,7 The use of polymer-based
packaging has increased over the years. The major advantage is their
broad spectrum of properties. It is relatively inexpensive, has low
density, is easily processed and shaped, and is easy to seal. The den-
sity of most plastics is similar to the density of paper but is less than
half of the density of glass or aluminum and about one-eighth the
density of steel. Plastics do not shatter like glass or buckle like metals. 9
The major disadvantages of polymer-based packages are their per-
meability to gases and vapors and their interacting properties with
food. 10

11.2.2 Glass Packaging
Molecular units in glass have a disordered arrangement but sufficient
cohesion to produce mechanical integrity. Glass is defined as an
amorphous, inorganic product of fusion that has been cooled to a
rigid condition without crystallizing. Glass is also indicated as a
supercooled liquid. Glass containers are considered to be a valuable
means of packaging liquids, semisolids, and liquid/solid mixture
products such as wines, liqueurs, soft drinks, sauces, dressings, and
pickles, among others. Using glass to store foods is reported to be
5000 years old. In terms of the chemical composition of common
glasses, silicon is their major component. Silicon does not occur freely
in nature, but it is found as silica of silicon dioxide or as silicate.
Glass is hard but fragile, easily susceptible to mechanical failures.
Glass is highly inert, impermeable to gases and vapors, and transpar-
ent to light. The glass transparency is modified by selecting appropri-
ate ingredients and glass can be given different desired colors. Glass