Pressure Canners and Preserving
The basic principles of canning have not changed dramatically since Nicholas Appert developed the process in the 18th century. 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 240-250F (116-121C). The amount of time needed for processing is different for each food, depending on the food's acidity, density and ability to transfer heat. For example, tomatoes require less time than green beans, while corn and pumpkin require far more time.
Non acidic foods must be processed at higher heat than acidic foods. This is because acidity helps destroy harmful bacteria. Non acidic food requires heat to kill the bacteria, and heat at a temperature considerably above boiling temperature. This is achieved through the use of specialised pressure cookers known as pressure canners. Pressure canners only work on stove tops and have the added advantage that they can be used as a water bath as well as a pressure canner and so therefore can be used to preserve both acidic and non acidic foods.