Page 360 - Biosystems Engineering

Food Package Engineering 337

environment (internal and external corrosion). A metal such as alumi-
num is also used extensively as foils for packaging, caps, lids, and
laminates. Aluminum foil is also used in multilayer retort pouches.
Other metal containers are trays, collapsible tubes, and aerosol con-
tainers. Most metal packaging intended for food packaging is coated
on the inside and outside surfaces. The coating on the container
surfaces is called lacquer or enamel. An internal coating prevents
food contamination by metallic ions. Many types of internal coatings
are available for food containers including oleoresins, vinyl, acrylic,
phenolic, and epoxy-phenolic. External surfaces of a container are
coated to improve corrosion resistance and provide a barrier layer
against external abrasion. The coating also improves a container’s
appearance and its marketability.

11.2.4 Paper and Paper-Based Packaging
Paper and paper-based packaging include bags, wrappings, paper-
board, corrugated boards and boxes, folding cartons, composite cans,
and fiber drum. The packaging is used in primary, secondary, and
tertiary packaging. Pulp, which is obtained from plant fiber, is the
raw material used to produce paper-based packaging. Paper and
paper-based packaging is based on a renewable resource and is bio-
degradable. The main source of fiber for making pulp are trees,
including spruce, pine, birch, eucalyptus, fir, poplar, and jasmine.
Plant fibers consist of cellulose, hemicelluloses, and lignin, and their
proportions in fiber may vary depending on the type of tree and dif-
ferent parts of the tree. The characteristics of paper and choice of
paper-making technique are determined by the amount of cellulose,
hemicelluloses, and lignin present in the fiber.

Manufacturing Paper
The first step in paper making is to prepare a pulp from fibers. Pulp
can be produced from wood chips using two processes: chemical and
mechanical. In the chemical process, diluted (5 percent) sodium
hydroxide at high temperature (170°C) is used to prepare the pulp. In
addition, sodium sulfite and a small amount of sodium sulfate solu-
tion are used to improve the yield and mechanical resistance of fibers.
Mechanical pulp is produced by forcing wood chips against a rotat-
ing grindstone. This mechanical process is less expensive due to the
high yield obtained.
Mechanical pulp contains 70 to 80 percent fiber bundles, broken
fibers, and fines in addition to individual fibers. Most often, the pulp-
making process is improved by combining mechanical and chemical
processes. In addition, thermal treatment is used to enhance the pulp
yield.
The next step is bleaching the pulp to improve fiber brightness.
Mechanical pulp is bleached using hydrogen peroxide followed by