Corrosion and RancidityExtra Information
You might have noticed that old iron gates turn reddish brown in colour?
The turning of iron materials into reddish brown colour is rusting of iron. Some other materials also tarnish the same way. The black coating on silver and the green coating on copper are other examples of corrosion. Corrosion is the process by which metals are eaten up gradually by the action of air, moisture or a chemical on their surface.
If a metal is reactive, its surface may be attacked slowly by the action of oxygen and water (moisture) in the atmosphere. The metal reacts with the oxygen of air and water vapour of air forming compounds on its surface. The formation of these compounds tarnishes the metal, that is, it makes the surface of the metal appear dull. The compounds formed on the surface of metal are usually porous and gradually fall off from the surface of metal, and then the metal underneath is attacked by air and water. This process goes on and on. In this way, the action of air and water gradually eats up the whole metal.
When we keep food materials containing fat or oil for a long time we get an unpleasant smell and the taste also changes. This is because the food materials get rancid.
Rancidification is the decomposition of fats and oils by hydrolysis or by oxidation. Antioxidants are often added to fat-containing foods in order to retard the development of rancidity due to oxidation.
Rancidification can be prevented to some extent by keeping food in refrigerators, by storing food away from sunlight and in air tight containers. Rancidity can be prevented when the food packaging material is filled with nitrogen along with the food materials. The nitrogen gas prevents the food material from being oxidised and thus prevent rancid.