University of Wisconsin - Extension

Vine-ripe tomatoes are among the most prized garden treats, commanding top dollar at farmers' markets during early and mid-summer. As the end of summer approaches, many people like to can their garden tomatoes and preserve other tomato products, such as catsup, salsa, tomato relish and chili sauce.

UW-Extension has developed a food preservation series that includes information on how to safely preserve tomatoes, tomato products, other vegetables for canning, and frozen fruits and vegetables. http://www1.uwex.edu/ces/pubs/pdf/BWSFPS_P.PDF">The "Wisconsin Safe Food Preservation Series" - PDF recently won a Gold Award for its editing and design from the national Agricultural Communicators in Education (ACE).

"It is important to follow some basic guidelines to avoid spoilage and the risk of food poisoning when canning tomato products at home," says Barbara Ingham, UW-Extension food science specialist.

Ingham says a variety of disease-resistant tomatoes in Wisconsin are well-suited to home canning. Even "low-acid" tomatoes contain enough acid for safe canning. However, tomatoes that are overripe or harvested late in the season may contain lower acid levels.

Because so many factors can affect tomato acidity, Ingham says the U.S. Department of Agriculture recommends that acid be added to home-canned tomato products. A half-teaspoon of citric acid or two tablespoons of bottled lemon juice per quart of tomato product is enough to increase the acid levels of recipes to safe levels.

Ingham says tomatoes can be canned using a pressure canner or boiling water-bath canner. Pressure canners are available in both dial-gauge and weighted-gauge models. Ingham recommends checking dial gauge canners each year to make sure they are working properly.

Your county UW-Extension office can check your dial pressure gauge to be sure it measures pressure accurately. This should be done at the beginning of each canning season. Canners with weighted pressure regulators don't require testing, but should be checked regularly.

Ingham says open-kettle canning of any tomato product is not safe. Because this method packs hot tomatoes in jars and seals them without any further heat processing, microorganisms may survive and the product can spoil or become unsafe. Microwave canning and oven canning also are unsafe.

For more information on safe food preservation techniques or to purchase the "Wisconsin Safe Food Preservation Series," contact your http://www1.uwex.edu/ces/cty">county UW-Extension office.