Explain the Process of Canning?

The basic principles of canning have not changed dramatically since Nicholas Appert and Peter Durand developed the process. Heat sufficient to destroy microorganisms is applied to foods packed into sealed, or "airtight" containers. The canned foods are then heated under steam pressure at temperatures of 240-250°F (116-121°C). The amount of time needed for processing is different for each food, depending on the food's acidity, density and capability to transfer heat. For instance, tomatoes require less time as compared to green beans, where corn and pumpkin need far more time. Canning uses metal or glass containers into which food is placed and sealed under reduced atmospheric pressure. Food does not completely fill the container; a headspace is required for expansion of food during heating. The headspace cannot be more than 10% of the total container volume. The containers are then processed in steam or hot water at temperatures above 115°C after which they are cooled. The cooking vessels required for these high temperature cooks must be capable of handling steam at pressures in excess of 1 atmosphere (14.7 psi). To have a steam environment at a temperature greater than 100°C elevated pressures are used.