How to Can Tomatoes
Although the basic process of canning has been relatively unchanged since it was developed during the Napoleonic Wars (no kidding!), over time, guidelines and recommendations have and will change as our understanding of microorganisms and food spoilage expand. Tomatoes are one food where the recommendations have recently changed. Although your grandma may have canned her tomatoes in a water-bath canner straight from the garden, this is no longer recommended. Water-bath canning requires food to be high-acid (a pH of 4.6 or lower) and tomatoes fall too near the line to guarantee safety. Small variations such as degree of ripeness, a break in the skin that allowed a little mold growth or the use of an extra-sweet variety could raise the pH past that threshold. Fortunately, all’s not lost! See below for instructions on how to lower the pH to safely water-bath can tomatoes, as well as how to pressure can them.
To water-bath can tomatoes: Use ripe, firm, well-washed tomatoes. Remove stems, blanch (dip in boiling water 30-60 seconds, until skins split), peel and prepare as desired (whole, sliced, diced or crushed). Place tomatoes into freshly washed canning jars and fill with liquid (typically water or tomato juice) reserving ½” headspace (the distance between the top of the jar and the top of the liquid) for contents’ expansion. Acidify by adding commercially bottled lemon juice (2 Tablespoons per quart or 1 Tablespoon per pint) or powdered citric acid (1 teaspoon per quart or ½ teaspoon per pint). You may also add salt or a little sugar to adjust the flavor at this time. Adjust lids. Place jars into canner when water boils. Water should cover jar tops by at least one inch. Quarts are processed for 45 minutes, pints for 35. Timing begins after water returns to a full boil.
To pressure can tomatoes: Prepare as directed above, omitting the lemon juice or citric acid. Arrange jars, vent canner and process as directed in your pressure canner’s instructions. Tomatoes are pressure canned for 15 minutes at 10 lbs pressure at sea level.
For more detailed instructions, altitude adjustments, etc, see: http://anrcatalog.ucdavis.edu/pdf/8116.pdf
Further instructions and a number of acidity-adjusted tomato recipes: http://www.ext.colostate.edu/Pubs/foodnut/09341.html