This time of year, most farmers' markets and home gardens are bursting with all kinds of tomatoes. Though, I'd like to think I could subsist on tomatoes alone, the reality is I can't possibly eat all the wonderful tomatoes put in front of me. The best way to preserve the summer harvest is to can the tomatoes.
Canning foods safely protects them from rot or off-flavors for 1 to 3 years. Canning used to be how many American families survived through winter before the advent of freezers and cheap (and more boringly flavored) commercial foods. It is still a superb technique to learn and use as part of the repertoire of accomplished cooks. The approach below use tomatoes as an example, but also works well with jellies, jams, and other vegetables packed in an acidic liquid.
The one thing to remember when canning tomatoes (or any other acidic foods) is that you need to boil everything. Boil the jars, boil the tomatoes and boil the tomatoes in the jar. The first two boils are necessary to sterilize the jars and the tomatoes, the third boil is to create a vacuum seal in the jar. This technique also works well for sauces and jams. For more tips on canning, refer to The Joy of Cooking by Rombauer and Becker.
So to be clear, the process goes like this:
Purchase canning jars. We prefer the wide mouth because they are easier to fill. Consider buying a variety of sizes. Even if you are only canning one kind of sauce, the variety will enable you to maximize your tomatoes – if a recipe calls for a small amount of tomato, you open a small jar, instead of opening a large jar that may not be completely used. Also, buy a pair of “canning tongs”. These tongs are specially designed to lift the jars out of the water.
Wash the jars. Put the lids and bands in one pot and the jars in another pot. The pot for the jars should be deep enough that the top of the jars can be covered by at least one inch of water.
Cover the jars completely with water and bring them to a boil. Continue boiling them for 10 minutes.
Cover the lids completely with water and put them on the stove. Bring to a boil, and turn off the heat. Let them sit in the water until you’re ready to use them.
Meanwhile, wash and coarsely chop tomatoes. Put them in a stainless steel (non-aluminum) pot. Bring the tomatoes to a boil, and continue cooking them for at least 10 minutes. Even if you smoked the tomatoes, or make your own ketchup, you still need to boil them.
Remove the jars from the water, draining the water out. Fill each jar with tomatoes, be sure to leave at least a ½-inch air-gap at the top. With a clean towel, wipe the lip of each jar clean.
Drain the water from the lids and cover each jar. Screw on the metal band, but not too tightly.
Return the jars to the boiling water and let boil for 10 minutes. Remove from the water and let stand for 20 minutes. Remove the band and test the seal of the lids – if it comes off easily, then the seal did not work and you must repeat the process. If the lid is tight, then you are all set! Otherwise, remove lid, and wipe rim of jar clean with a sterilized towel. Reseal jar with lid and band, and return to pot of boiling water for 5 minutes more.