Page 357 - Biosystems Engineering

334 Cha pte r Ele v e n

with uniform wall thickness and higher strength. The industrial pro-
duction rate of glass packages is slower compared to other food pack-
aging containers. It takes about 10 to 12 s from the time that the gob
falls in the blank mold to the time the formed package exits. 2
Postprocessing Treatments The temperature of the glass container is
lowered quickly in the second mold, which produces internal stresses
in the containers, making them very fragile. The glass containers are
subjected to annealing to remove internal stresses. This is achieved
by raising the temperature of the containers to about 540°C (about
softening temperature), holding them at this temperature for few
minutes, and then cooling the containers slowly to well below soften-
ing temperature. Glass containers are also subjected to surface treat-
ments to improve their mechanical properties and chemical resis-
tance. The inner surface is treated with SO or fluorocarbon to form
2
sodium sulfate or sodium fluoride, which increases the container’s
resistance to chemical attack. An outer surface is treated with a vapor
containing tin or titanium prior to annealing. This treatment forms a metal
oxide layer, which improves adhesion properties of postannealing
coating. Before the annealing process ends, the outer surface of the
container is sprayed with an aqueous solution of waxe, stearates, sili-
cones, oleic acid, or polyethylene to improve its lubricity. 1

11.2.3 Metal Packaging
Aluminum and steel are the most commonly used metals for packag-
ing foods. Aluminum is used in the form of an alloy containing mag-
nesium and manganese, whereas steel is coated by tin or chromium
oxides. Lead and copper are used for soldering or welding tinplate
and chrome-coated steel containers. Aluminum is purified from
aluminum oxide (Al O ) using an electrolytic process. Aluminum oxide
2 3
is obtained from ore, which is mined as bauxite. The energy require-
ments to produced purified aluminum are extremely high, seven- to
ten-fold the energy required to produce the same mass of tinplate or
steel. Aluminum is typically added with alloying agents to increase
its mechanical properties and corrosion resistance and to improve
formability. Steel is produced from pig iron and may contain manga-
nese, chromium, nickel, molybdenum, cooper, tungsten, cobalt, and
silicon, depending on the desired properties. Based on its chemical
composition, steel is classified into three categories: carbon steels,
alloy steels, and stainless steels. Carbon steel is most important for
food packaging applications. Tinplate is produced by coating the
steel with tin using an electrolytic process. The method of electroplat-
1,2
ing is very intricate. Because tin is an expensive metal, the industry
has developed tin-free steel as an alternative to tinplate. Tin-free steel,
also known as electrolytic chromium-coated steel, is produced by
coating low-carbon steel with chromium or chromium oxide.