Lesson- 9 Metal
Two basic types of alloyed metals are used in food packaging i.e. steel and aluminum. Steel is used primarily to make rigid cans, whereas aluminum is used to make cans as well as thin aluminum foils and coatings. Nearly all steel used for cans was coated with a thin layer of tin to inhibit corrosion, and called as “tin can”. The reason for using tin was to protect the metal can from corrosion by the food. Tin is not completely resistant to corrosion, but its rate of reaction with many food materials is considerably slower than that of steel.
The strength of the steel plate is another important consideration especially in larger cans that must withstand the pressure stresses of retorting, vacuum canning and other processes. Can strength is determined by the temper given the steel, the thickness of the plate, the size and the geometry of the can, and certain construction features such as horizontal ribbing to increase rigidity. This ribbing is known as beading. The user of cans will find it necessary to consult frequently with the manufacturer on specific applications, since metal containers like all other materials of packaging are undergoing constant change.
Aluminum is light weight, resistant to atmospheric corrosion, and can be shaped or formed easily. However, aluminum has considerably less structural strength than steel at the same gauge thickness. This means that aluminum has limited use in cans such as those used with retorted foods. Aluminum works well in very thin beverages cans that contain internal pressure such as soda or beer. This internal pressure from CO2 gives rigidity to the can. Aluminum in contact with air forms an aluminum oxide film which is which is resistant to atmospheric corrosion. However, if the oxygen concentration is low, as it is within most foods containing cans, this aluminum oxide film gradually becomes depleted and the underlying aluminum metal is then no longer highly resistant to corrosion. (potter)
9.1 Metals used in packaging
The metal materials used in food packaging are aluminum, tinplate and electrolytic chromium-coated steel (ECCS). Aluminum is used in the form of foil or rigid metal.
9.1.1. Aluminum Foil
Aluminum foil is produced from aluminum ingots by a series of rolling operations down to a thickness in the range 0.15–0.008 mm. Most foil used in packaging contains not less than 99.0% aluminum, with traces of silicon, iron, copper and in some cases, chromium and zinc. Foil used in semi rigid containers also contains up to 1.5% manganese. After rolling, foil is annealed in an oven to control its ductility. This enables foils of different tempers to be produced from fully annealed (dead folding) to hard, rigid material. Foil is a bright, attractive material, tasteless, odorless and inert with respect to most food materials. For contact with acid or salty products, it is coated with nitrocellulose or some polymer material. It is mechanically weak, easily punctured, torn or abraded. Foil is used as a component in laminates, together with polymer materials and, in some cases, paper. These laminates are formed into sachets or pillow packs on FFS equipment (see Section 9.3.6). Examples of foods packaged in this way include dried soups, sauce mixes, salad dressings and jams. Foil is included in laminates used for retortable pouches and rigid plastic containers for ready meals. It is also a component in cartons for UHT milk and fruit juices.
Tinplate is the most common metal material used for food cans. It consists of a low-carbon, mild steel sheet or strip, 0.50–0.15 mm thick, coated on both sides with a layer of tin. This coating seldom exceeds 1% of the total thickness of the tinplate. The mechanical strength and fabrication characteristics of tinplate depend
on the type of steel and its thickness. The minor constituents of steel are carbon, manganese, phosphorous, silicon, sulphur and copper. At least four types of steel, with different levels of these constituents, are used for food cans. The corrosion resistance and appearance of tinplate depend on the tin coating.
126.96.36.199. Tin coating
The role of tin coating is an essential component of the can construction and plays an active role in determining shelf life. The most significant aspect of the role of the tin coating is that it protects the steel base-plate which is the structural component of the can. Without a coating of tin, the exposed iron would be attacked by the product and this would cause serious discoloration and off-flavors in the product and swelling of the cans; in extreme cases the iron could be perforated and the cans would lose their integrity. The second role of tin is that it provides a chemically reducing environment, any oxygen in the can at the time of sealing being rapidly consumed by the dissolution of tin. This minimizes product oxidation and prevents colour loss and flavor loss in certain products.
188.8.131.52 Tin toxicity
High concentrations of tin in food irritate the gastrointestinal tract and may cause stomach upsets in some individuals, with symptoms which include nausea, vomiting, diarrhoea, abdominal cramps, abdominal bloating, fever and headache. Tin corrosion occurs throughout the shelf life of the product. It is therefore imperative to take steps to reduce the rate of corrosion. Accelerating factors include heat, oxygen, nitrate, some chemical preservatives and dyes, and certain particularly aggressive food types (e.g. celery, rhubarb). A high vacuum level is one effective method of reducing the rate of tin pick-up in cans with un-lacquered components.
9.2. Electrolytic Chromium-Coated Steel (ECCS)
Electrolytic chromium-coated steel (ECCS), sometimes described as tin free steel, is finding increasing use for food cans. It consists of low-carbon, mild CR or DR steel coated on both sides with a layer of metallic chromium and chromium sesqueoxide, applied electrolytically. ECCS is less resistant to corrosion than tinplate and is normally lacquered on both sides. It is more resistant to weak acids and sulphur staining than tinplate.
Figure1: Structure of ECCS plate
9.3. Aluminum Alloy
Hard-temper aluminum alloy, containing 1.5–5.0% magnesium, is used in food can manufacture. It is lighter but mechanically weaker than tinplate. It is manufactured in a similar manner to aluminum foil. It is less resistant to corrosion than tinplate and needs to be lacquered for most applications. A range of lacquers suitable for aluminum alloy is available, but the surface of the metal needs to be treated to improve lacquer adhesion.
Lead was a problem with older, soldered cans but levels are now very low. However, some tinplate is contaminated with minimal amounts of lead. The manufacture of lead soldered cans may still be found in the developing world.
The presence of lacquer or enamel very effectively limits dissolution of tin into the product, and so the use of lacquers is becoming increasingly common, even with those products which were previously packed in plain tinplate cans. There are several different types of lacquer in common use today. By far the most common type is the Epoxy Phenolic group, which are suitable for packing meat, fish, vegetable and fruit products. These have largely replaced the oleoresinous group, which had a similar wide range of application. Some canners use cans lacquered with vinyl resins, which have the important quality of being free from any taste and odor, and are therefore particularly suitable for dry packs such as biscuits and powders, but also some drinks. White vinyl lacquers have been used where staining of the underlying metal caused by reaction with the product is a problem. Also, white vinyl lacquers have been used for marketing reasons in order to present a hygienic/clinical appearance and not the aesthetically undesirable corrosion patterns on tinplate.