Canning is by far the most common method of preserving foods for long-term storage. Preservation occurs by heat, which destroys both microorganisms and enzymes that cause spoilage. The processing required depends on whether the food is high in acid or low in acid. Higher acid foods require less heat to destroy microorganisms. Most fruits (including tomatoes, usually) contain enough acid naturally to allow for processing in a hot water bath. Most common vegetables, meats, fish and poultry do not contain much acid; and, therefore require processing at much higher temperatures in a pressure canner. Refer to an up-to-date reference book for the current USDA processing recommendations for the food that you wish to can.