CAC/RCP 60-2005 Code of Practice for the Prevention and Reduction of Tin Contamination in Canned Foods
INTRODUCTION
History of use of tin
1. Tin is a soft, white, lustrous metal with an atomic weight of 118.7 and the chemical symbol Sn after its Latin name, Stannum. It has a relatively low melting point (231.9 °C) and is highly resistant to corrosion, which makes it an ideal element for the protective coating of metals. Over 50% of the world’s tin production is used for plating steel or other metals.
2. Today some 15 million tonnes of tinplate are produced each year using rapid and highly sophisticated production methods. These methods are able to control steel thicknesses and tin coating masses to within the extremely fine tolerances required for modern can making processes such as high speed welding.
Tin as packaging for canned food
3. Tin is used to protect the steel base from corrosion both externally (aerobic conditions) and internally when in contact with foods (anaerobic). Under the anaerobic conditions expected inside an internally plain processed food can, tin will normally behave as the sacrificial anode, dissolving very slowly whilst protecting the steel base from corrosion and creating a reducing environment in the can. It is this mechanism that has enabled the plain tinplate can to maintain its long history and proven track record of providing wholesome food on a year round basis and safe storage for long periods of time.
4. The later development of can linings (lacquers) enabled different types of food products to be satisfactorily packed. For example, some highly pigmented foods (beetroot, berry fruits) have their colours bleached by tin dissolution and are best protected from contact with tin by use of linings. A small number of food products (e.g. sauerkraut) have a different corrosion mechanism, in which the tin does not behave sacrificially and direct corrosion of the steel base can occur. These products should also have the additional protection of an internal lacquer system.
