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Ensuring safe canning for tomatoes

By Staff | Aug 28, 2009

It’s vital to preserve tomatoes properly to avoid serious food safety issues, including botulism poisoning.

Iowa’s sun-warmed, vine-ripened, juicy tomatoes may be a little late this year, but home cooks are busy tapping into their recipes for salsa, sauces and more to preserve the bounty of the season when it does come in.

While nothing compares to feasting on home-canned tomatoes in the middle of winter, it’s vital to preserve tomatoes properly to avoid serious safety issues, including botulism poisoning.

“We always get a lot of canning questions this time of year, and it’s especially important to understand safe canning methods for tomatoes and salsa,” said Paulelda Gilbert, a Webster County-based nutrition and health field specialist with Iowa State University Extension.

Gilbert is concerned that some home cooks are still using the “open-kettle” method, in which food is cooked and placed in hot canning jars that are sealed without further processing.

While previous generations often used this method, modern science has revealed the many food safety issues that can arise from open-kettle canning.

“The acid level in modern tomato varieties is not as high as it was in older varieties, and an acidic environment is what helps control the microorganisms that can lead to food spoilage,” said Gilbert, who noted that improper canning methods have caused some recent cases of botulism poisoning. Botulism, which is one of the most dangerous food-borne illnesses and affects the central nervous system, comes from dangerous toxins that are produced when Clostridium botulinum spores grow in low-acid foods.

Simple strategies ensure food safety

Fortunately, it’s simple to can tomatoes properly, from using commercially-prepared lemon juice to boiling-water-bath processing, Gilbert noted. ISU Extension offers the following advice:

  • Select the best tomatoes. Use only firm, ripe tomatoes that have no spoiled parts or mold. Do not can tomatoes harvested from frost-killed, dead vines. These tomatoes are low in acid and can be eaten fresh or frozen, but should not be canned.
  • Choose standard jars and lids. Check canning jars and lids for cracks, chips, dents and rust, since these defects can cause sealing failures. Commercial jars such as those used for mayonnaise are not recommended for home canning, because they are not designed for use with two-piece lids and because the glass is more likely to break during processing. Wash canning jars in hot, soapy water; rinse well. Prepare lids and process bands according to manufacturer’s directions.
  • Acidify tomatoes. Add bottled lemon juice to each jar before processing. Use 2 tablespoons of lemon juice per quart, or 1 tablespoon of bottled lemon juice per pint. Botulism spores do not survive a high acid environment, so tomatoes with added lemon juice can be safely canned with the water bath canning process. You can also salt the tomatoes, if desired.

Salt is not necessary for preservation in canned products but can be added for flavor. Use half a teaspoon per pint, or 1 teaspoon per quart. After adding lemon juice (and optional salt), fill the jars with the tomato mixture.

  • Process the tomatoes. To process in a boiling water canner, fill the canner halfway with preheated water. Load sealed jars into canner. Ensure that water can circulate freely around each jar.

Add boiling water to a level of one to two inches above the jars. Bring water in canner to a vigorous boil. Adjust heat to maintain a gentle boil and cover. Process for the time recommended.

Do not reduce the processing time. Keep water boiling during the entire processing period. If water evaporates, add boiling water to keep it at least one inch over the top of jars. Leave the lid on the canner. Remove jars when the processing time is up.

  • Remove and store jars. Take jars from canner and set upright on a rack or on a folded cloth away from drafts. Do not tighten lids. Allow jars to cool undisturbed for 12 to 24 hours, then check for sealing failures. To test jar, press center of lid. If lid is down and will not move, the jar is sealed. Remove screw bands carefully. Wash, dry, label and store jars in a cool, dark place.

If any jars have not sealed, the entire canning procedure can be repeated within 24 hours with clean jars and new lids, Gilbert said. If re-canning is not desired, the unsealed jars can be placed in refrigerator and used within a few days.

Contact Darcy Dougherty Maulsby at yettergirl@yahoo.com.

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