Food which is kept too long decays because it is attacked by yeasts, moulds and bacteria. The canning process, however, seals the product in a container so that no infection can reach it, and then it is sterilized by heat. Heat sterilization destroys all infections present in food inside the can. No chemical preservatives are necessary, and properly canned food does not deteriorate during storage.
The principle was discovered in 1809 by a Frenchman called Nicolas Appert. He corked food lightly in wicked-necked glass bottles and immersed them in a bath of hot water to drive out the air, then he hammered the corks down to seal the jars hermetically. Appert's discovery was rewarded by the French government because better preserved food supplies were needed for Napoleon's troops on distant campaigns.
By 1814 and English manufacturer had replaced Appert's glass jars with metal containers and was supplying tinned vegetable soup and meat to the British navy. The next scientific improvement, in 1860, was the result of Louis Pasteur's work on sterilization through the application of scientifically controlled heat.
Today vegetables, fish, fruit, meat and beer are canned in enormous quantities. Within three generations the eating habits of millions have been revolutionized. Foods that were previously seasonal may now be eaten at any time, and strange foods are available far from the countries where they are grown. The profitable crops many farmers now produce often depend on the proximity of a canning factory.
The first stage in the canning process is the preparation of the raw food. Diseased and waste portions are thrown away; meat and fish are cleaned and trimmed; fruit and vegetables washed and graded for size. The jobs are principally done by machine.
The next stage, for vegetables only, is blanching. This is immersion in very hot or boiling water for a short time to remove air and soften the vegetable. This makes it easier to pack into cans for sterilization. Some packing machines fill up to 400 cans a minute. Fruit, fish and meat are packed raw and cold into cans, and then all the air is removed. When the cans are sealed, the pressure inside each can is only about half the pressure of the outside air. This is ‘vacuum' packing.
During the sterilization process which follows, the cans are subjected to steam or boiling water, with the temperature and duration varying according to the type of food. Cans of fruit, for example, take only 5-10 minutes in boiling water, while meat and fish are cooked at higher temperatures for longer periods. After sterilization, the cans are cooled quickly to 32℃.to prevent the contents from becoming to soft.
The final stage before despatch to the wholesale or retail grocer is labelling, and packing the tins into boxes. Nowadays, however, labelling is often printed on in advance by the can-maker and no paper labels are then required.