First, ready the jars, lids and screw bands. Jars, whether new or previously used, should be free of chips and scratches. New lids must be used for each batch, though screw bands, if in good condition, can be reused. Wash the jars, lids and bands well in hot soapy water, either by hand or in a dishwasher. Place the lids in a small saucepan with water to cover, bring to a simmer (180°F/82°C) and maintain the simmer until you are ready to use them. Avoid boiling the lids, or you may compromise the seal.
If the recipe processing time is more than 10 minutes, the jars need only be washed before use, since sterilization will occur during processing. If the recipe processing time is 10 minutes or less, however, the jars must be sterilized in the boiling-water canner before they are filled. To sterilize them, fill the canner pot two-thirds full of hot water. Fill the jars with hot water and, using a rubber-coated jar lifter, lower the jars, one at a time, onto the rack in the water-filled pot, making sure they are covered by at least 1 inch of water. Bring the water to a boil over high heat and boil for 10 minutes. Turn off the heat and leave the jars in the hot water. Remove them with the jar lifter and dry them as needed.
Always keep the jars warm until you are ready to fill them to ensure that they don't break when a hot mixture is added and that they seal properly. If you have cleaned the jars in a dishwasher and they don't need sterilization, leave them in the dishwasher with the door closed, removing them, one at a time, as needed. If you have sterilized them in the canner pot, leave them in the hot water until needed. You can also keep just-washed jars warm by immersing them in a large pan filled with boiling water and then turning off the heat, or by slipping them into a low-temperature oven.
If you have sterilized the jars in the water canner, the rack will already be in place. If you have not, before filling the jars, insert the rack into the canner. Fill the canner about two-thirds full with water and bring to a boil over high heat. At the same time, bring a teakettle full of water to a boil, and then adjust the heat to maintain a simmer, in case you need additional boiling water to cover the filled jars once they are in the canner.
Working with one warm, dry jar at a time, place a funnel over the opening. Depending on the recipe, use a ladle, slotted spoon or other utensil to fill the jars, leaving the amount of headspace called for in the recipe. To determine the headspace, measure the space between the top of the jar and the top of the food or liquid in the jar. Run a thin nonmetallic spatula or a chopstick around the inside edge of the jar to release any air bubbles trapped inside, and then adjust the headspace if necessary. Wipe the rim with a clean, damp cloth to remove any errant droplets that could prevent a proper seal. Use nonmetallic tongs or a magnetic wand to remove a hot lid from the simmering water, and dry with a clean kitchen towel. Top the jar with a dry, warm lid. Then screw a band over the lid just until it is secure. Do not turn the band too tightly, as the seal must allow air to escape from the jar during processing.
Immediately arrange the jars in the canner, using the jar lifter to lower them onto the rack. Do not let the jars cool before exposing them to boiling water, or they may crack. Make sure the jars are covered by at least 2 inches of water. Cover the pot with the lid and begin timing the processing after the water has returned to a rapid boil.
Once the time is up, use the lifter to remove the jars from the boiling water. Place the jars on a kitchen towel or rack, spacing them well apart to allow air to circulate, and let cool completely. You may hear the "ping" of the jar lids being sucked into a vacuum seal within minutes of removing them from the water, or it may take hours for the seal to occur.
When the jars have cooled completely, test the seal by gently pressing on the top of each lid. It should be taut and rigid to the touch and slightly indented. If the lid bounces back and makes a clicking noise when you press it, the seal is not good. To test it further, unscrew the band and gently lift the lid with your fingertips. If you are able to pick up the entire jar by holding the edges of the lid, the seal is good. If the lid slips easily from the jar rim, the seal is insufficient. Store any jar that does not have a good seal in the refrigerator for the time specified in individual recipes.
Most jars with proper seals can be stored for up to 1 year. Label the jars with their contents and the date on which they were sealed, and then store in a cool, dark place, as excessive heat or light can discolor the contents. Each time you open a new jar, check the contents for signs of spoilage. Be wary if the aroma is especially sour or musty. Also, dispose of the contents of any jar with visible mold or discoloration at the top or around any air pockets, or with tiny bubbles, a sign the contents have fermented. Finally, discard the contents of any jar that does not appear to have maintained a tight seal throughout its storage.
Most canning recipes have been formulated for canning at sea level. High altitudes require a longer processing time, because the higher the elevation, the lower the temperature at which water boils. In general, you should add 1 minute to the processing time for every 1,000 feet in altitude, or 2 minutes per 1,000 feet if the original processing time is more than 20 minutes. For altitudes higher than 5,000 feet, consider using a steam-pressure canner, which will allow for accurate processing time without overcooking the food.