Canning uses heat to destroy molds, yeasts, bacteria, and other organisms that cause spoilage. How much heat is needed and consequently what kind of canner to use depends on whether produce is acidic or not. In general, boiling-water canners are used for acidic produce and are much faster while pressure canners are used for nonacidic produce and require longer processing times.
If you’re using a boiling-water canner, fill the pot about halfway with water—enough to reach an inch or two above the jars after they’ve been loaded into the canner. Preheat the water to about 180 degrees for hot-packed produce or 140 degrees for raw-packed produce to prevent the jars from cracking. Insert the canning rack and capped jars. Cover the canner and bring water to a steady, gentle boil, which must be maintained throughout the required processing time. If you’re using a pressure canner, follow the manufacturer’s directions, which will vary depending on the type of canner and the altitude. Generally, put two inches of hot water in the pressure canner, add the jars, put the lid on the pot, and increase the pressure before you start tracking processing time. When processing is finished, let the pressure drop before opening the canner and removing the jars.