Few garden vegetables hold as much allure as tomatoes. Plucked warm and perfectly ripened from the vine, a good tomato is lush and juicy, with a perfect balance of sweetness and acidity. Unfortunately that perfect peak of ripeness only lasts a relatively brief moment before it's gone, and when your garden is at peak production it can be difficult to use them quickly enough. Canning the tomatoes for use later in the year is an obvious solution, and it's relatively easy to do.
Selecting and Blanching
Choose tomatoes that are ripe but still slightly firm for canning. Standard-sized canners hold either 9 pints or 7 quarts per load, which translates to 13 or 21 pounds of tomatoes respectively. The fruits should be free of visible bruising, mold or spoilage. Flawed tomatoes can be set aside for cooking or sauces. Before canning, blanch and skin the tomatoes. Cut an "X" in the bottom of each tomato with a paring knife and cut out the core. Drop the tomatoes in small batches into boiling water, for 30 to 60 seconds or until the skin appears loose where it's been cut. Transfer the tomatoes to a bowl of ice water, to stop them from cooking further. Their skins should then simply slip off.
Preparing the Tomatoes
Tomatoes can be packed hot or cold for canning purposes. To cold-pack, simply fill sterilized canning jars with whole or halved tomatoes and pour boiling water or tomato juice over them. For hot-packing, simmer the tomatoes for five minutes in water or tomato juice, and then fill your jars with the tomatoes and hot liquid. Hot-packed tomatoes will last longer and retain their color better. Whichever method you choose, you'll need to add a tablespoon of lemon juice or 1/4 teaspoon of citric acid -- available from bulk food stores -- to each pint to ensure it's acidic enough to maintain food safety. Double those measurements for quarts. Add a teaspoon of salt per pint as well, if you wish.
Preheat the water in your canner to 140 degrees Fahrenheit if you're cold-packing the tomatoes, or 180 F if they'll be hot-packed. Fill your jars to within ½ inch of the top, and use a sterilized knife or chopstick to remove any air bubbles. Wipe the rims clean with a paper towel. Screw clean, unused lids onto the jars until they're just finger tight, and lower your jars one at a time into the canner's rack. If it has the kind of rack with handles for easy lifting, fill the rack first and then lower the whole thing into the water. Add boiling water, if necessary, to submerge the jars to a depth of 2 inches. Bring the pot to a boil and begin timing. Boil pint jars for 35 minutes or quarts for 40 minutes at sea level. Add five minutes if you're above 1,000 feet of elevation, or 10 minutes if you're above 3,000 feet.
Tomatoes don't need pressure canning to be food-safe, but the shorter processing time yields a better-tasting product. You'll still need to acidify each jar with lemon juice or citric acid, even with pressure canning. Inspect the gasket and valves of your pressure canner before beginning, to make sure they're in good working order. Preheat water -- typically 2 to 3 inches, or as directed by the manufacturer -- to 140 or 180 F, as for water-bath canning. Lower the rack of jars into place, then position and lock the lid onto the canner. Open the canner's vent and bring the pot to a boil, allowing steam to vent for a full 10 minutes. Close the vent and bring the canner to 6 pounds' pressure for pints, and process for 15 minutes. For quarts, process at 11 pounds' pressure for 10 minutes. Remove your canner from the heat, and let it rest until the pressure releases naturally.
Testing and Storage
Open the canner, taking care to avoid any steam escaping from beneath the lid, and remove the jars. Take care to hold them upright, as you handle them. Rest the jars on a cooling rack or a towel, leaving at least an inch of space between them for air circulation. Let them cool for at least 12 hours. Check the seal on each jar. The lid should be slightly concave, indicating a good seal and a vacuum within the jar. If you tap the lid with a spoon, it should ring clearly. If instead you hear a dull thud, or if the lid is visibly not sealed, you can either re-process that jar or simply refrigerate it and use it up within a week or so. Well-sealed jars should be stored in a cool, dry place away from direct light, and used within a year for best quality.