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Unsafe Canning Methods

Date: May 1989 (Reviewed June 1996)

Source: University of Wisconsin

There are no safe short cuts in canning. And there is no substitute for adequate heat treatment. Some people have had "good luck" for years, but each season is different and you never know if this is the year everything spoils. This is because heat-resistant bacteria are not always present. When these organisms are absent, the under-processed foods might keep. If they are present, the food spoils. So you must always assume they are present.

The open-kettle method involves placing hot food in jars and sealing with no further heat treatment. This method is NOT recommended for home canning of any food because it seldom sterilizes food. Without sufficient heat to destroy bacteria and sterilize the food, the product may spoil. Foods ferment when open-kettle canning does not destroy yeasts, or permits them to enter the jar before it is sealed. Simply getting lids to seal on jars will not prevent food from spoiling.

Oven-canning is extremely hazardous. When you can food it is important to know and control temperature. With oven-canning the temperature will vary according to accuracy of the oven regulator and how heat circulates. The dry heat penetrates food jars very slowly. Oven-canning can be dangerous regardless of brand of oven, jar, cap or lid you use. Jars may explode, wrecking the oven and seriously cutting or burning someone. Most important, the temperature of the food in the jars during oven-canning is not high enough to destroy dangerous bacteria.

Do not use any chemicals or preserving powders as a substitute for heat treating home-canned food. These will not prevent food from spoiling, or give you a good product. If you do not use a heat treatment, there will not be a vacuum in containers and this will generally accelerate food deterioration. The only safe procedures are boiling water bath process for acid foods and pressure canning for low-acid foods.

Some new kitchen appliances will create a vacuum in the jar without heat treatment. These are dangerous because there is no heat to destroy spoilage organisms and botulism bacteria could grow under the vacuum seal.

Potentially unsafe devices are now sold for canning food in a microwave oven. These have not been shown to destroy all bacteria present in the food or heat uniformly. Problems may also occur with siphoning of the food out of the jars and lid failures. Do not use these devices to can food at home.

If you require further information, please contact your county office of the NDSU Extension Service.

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