Proper Canning Processing Methods
The most popular food preservation method today is canning. Home canning isn't complicated and is safe as long as you follow proper processing instructions. Canning is simply the process of applying heat to food that is sealed in a jar in order to destroy any microorganisms that can cause food spoilage and to properly seal the jars.
All foods contain microorganisms, but proper canning techniques heat the food for a specific period of time killing them to prevent spoilage. Also, during the canning process air is forced from the jar forming a vacuum and sealing the jar as it cools. This process prevents microorganisms from entering and recontaminating the food. The processing method and processing times must be followed exactly as stated in recipes and they are never interchangeable.
There are many canning methods used in recipes but only two are approved by the United States Department of Agriculture (USDA): water bath canning (boiling water canning) and pressure canning (steam pressure canning).
Periodically, old methods of preserving food are revived. These methods are not reliable and not recommended. Advancements in technology bring new methods and equipment to the forefront. Unless proven to be safe and reliable by the USDA they must be avoided.
The pH level of the food you are canning determines the processing method. See Identifying Acidity Levels in Food to Determine Canning Method for more information.
Water bath canning (boiling water method or hot water canning)
High acid foods are processed in a water bath (boiling water) canner. Heat is transferred to the food by the boiling water which completely surrounds the jar and two-piece cap. A temperature of 212°F (boiling) is reached and must be maintained (rolling boil) for the time specified by the recipe. This method is adequate to destroy molds, yeasts and some bacteria, as well as to inactivate enzymes. This method is used foods such as fruit, items made from fruit, pickles, pickled food, and tomatoes.
A water bath (boiling water) canner must not be used for processing low acid foods. The water bath (boiling water) method never reaches the high temperatures needed to destroy certain bacterial spores and their toxins, which can produce botulism.
The processing times for water bath (boiling water) methods in recipes are for processing at or below 1,000 feet above sea level. If you are located at an elevation greater than 1,000 feet, you must increase the processing time to compensate for the lower boiling point of water. See Canning Altitude Adjustments for more information.
Pressure (steam pressure) canning
Low acid foods must be processed in a pressure (steam pressure) canner. In order to destroy all bacteria, their spores and the toxins they produce, low acid foods must be heated to a temperature of 240°F and held there for the time specified in the recipe. Since the steam inside the canner is pressurized, its temperature exceeds the boiling point of water. At 10 pounds pressure, using a weighted-gauge canner, the temperature will reach 240°F (at or below 1,000 feet above sea level), which is hot enough to destroy the bacterial spores that emit toxins. This method is used for foods such as vegetables, meat, poultry, and fish.
The pounds of pressure given for low acid recipes are based on the use of a weighted-gauge canner and for processing at or below 1,000 feet above sea level, for the time indicated in the recipe. The weighted-gauge or sometimes referred to as a dial gauge is located on the lid of the canner. When canning above 1,000 feet sea level, you must increase the processing temperature by adjusting the pounds of pressure used: the processing time remains the same. See Canning Altitude Adjustments for more information.
A pressure canner should not be confused with a pressure cooker, which is used to cook food quickly. A pressure cooker does not have enough room for both the canning jars and the water needed to create the right amount of pressure to preserve foods.
The only way to reliably produce a safe canned product is to use the correct canning method for your type of food, following the recipe instructions to the letter, and complete each processing step.