Water bath [[Canning]] preserves high-acid foods by processing filled jars in boiling water. The acid content of the food prevents the growth of Clostridium botulinum, so the lower temperature of boiling water (100 degrees Celsius or 212 degrees Fahrenheit) is sufficient for safe preservation. Foods suitable for water bath canning include fruits, tomatoes (with added acid), pickles, [[Jams and Jellies]], jellies, and fruit juices.
A water bath canner is a large pot with a lid and a rack that holds jars off the bottom of the pot. Any deep pot with a lid and a rack can be used as long as it is deep enough to cover the jars with 2.5 to 5 centimeters (1 to 2 inches) of water above the jar tops. The pot must have a flat bottom for stable heat distribution.
The canner is filled halfway with water and brought to a simmer. Jars are filled with prepared food, leaving the recommended headspace (typically 0.6 to 1.3 centimeters or 0.25 to 0.5 inches). Air bubbles are removed, and the jar rim is wiped clean. The lid is centered on the jar, and the screw band is tightened to fingertip-tight. The jars are lowered into the canner, and boiling water is added to cover them by 2.5 to 5 centimeters (1 to 2 inches). The canner is covered, brought to a rolling boil, and timed for the recommended processing duration.
Processing time is increased at higher altitudes because water boils at a lower temperature. At 300 to 900 meters (1,000 to 3,000 feet), add 5 minutes to the processing time. At 900 to 1,800 meters (3,000 to 6,000 feet), add 10 minutes. Above 1,800 meters (6,000 feet), add 15 minutes.