The details of canning procedures vary with the nature of the food to be canned, but there are certain important operations common to canning of all foods.
(1) Cleaning (2) Blanching (3) Exhausting, (4) Sealing the container (5) Sterilizing the sealed container and
(6) Cooling the container
1. Cleaning :
The first step in canning, whether done in the home or on a large scale in factories, is the thorough cleaning of the raw food to be preserved. By this means most of spoilage organisms are removed.
On a large scale, cleaning is done with the help of various kinds of washers. The raw materials may be subjected to high pressure sprays or strong flowing streams of water, while passing along a moving belt.
Blanching consists of the immersion of raw food materials, especially vegetables and fruits, into hot water or exposure to live steam.
Blanching serves as an additional hot water wash. It softens fibrous plant tissues, inhibits the action of enzymes and fixes the natural colour of certain products making them more attractive in appearance.
Gases are expelled by passing the open can containing the food through an exhaust box in which hot water or steam is used to expand the food and expel air and other gases from the contents and the head space area of the can. After the gases are expelled, the can is immediately sealed, heat processed and cooled.
In the case of certain products, exhausting is done by mechanical means, rather than by the use of heat. There are special machines which withdraw the air from the cans and they seal them at the same time - 'vacuum packing'.
4. Sealing the container:
Each container must be sealed properly before it is subjected to the heat process, since re-contamination of the contents must be prevented.
5. Sterilizing the sealed container with its contents by heat processing
This is meant to bring about complete sterilisation to prevent spoilage of the food by microorganisms. This is usually done by the application of steam under pressure.
The temperature and time used for heat processing depend on the kind of food, on the pH of the medium and other factors. It should however be remembered that an excessive period of heating at higher temperatures than necessary will spoil the product.
A longer exposure to a relatively low temperature should be preferred to a short exposure at a higher temperature.
6. Cooling the container
The containers should be cooled rapidly to check the action of heat and prevent unnecessary softening of the food or change in colour of the contents. Cooling can be done by means of air or water.