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Put food safety first in home canning


Don’t believe everything you hear when it comes to home canning. Two unproven methods — oven and dishwasher canning — are getting attention, but neither are safe, according to a University of Missouri Extension expert.

“The purpose of canning is to destroy decay and illness-causing bacteria,” said Janet Hackert, a nutrition specialist in Bethany, Mo.

“Canning information is frequently handed down from generation to generation,” she said, adding that the U.S. Department of Agriculture updated home-canning guidelines in 1989.

Canned foods should be processed using one of two methods: boiling water, also know as water-bath processing, for high-acid foods and pressure canning for low-acid foods. With both methods, heat forces air out of the jars. As they cool, a vacuum forms, sealing the jars.

Improperly canned foods provide an ideal environment for Clostridium botulinum bacteria spores to grow. As the bacteria multiply, they release a toxin that can cause serious illness, and even death.

“Foods must be heated to above boiling to destroy the bacteria. You most certainly can’t do this in an oven or a dishwasher or a microwave. Air is not a good conductor of heat, so food in jars placed in an oven will not heat evenly. The food in jars will not reach the required temperature, so the bacteria will survive,” said Hackert.

Another danger is the possibility of the jars exploding when the oven door is opened, causing serious burns or cuts, and destroying the oven.

Microwave oven or dishwasher processing are equally unsafe because the jars heat unevenly, and the food won’t reach the required temperature.

Steam and open-kettle canning — methods that still have followers — also produce a weak seal and are no longer recommended. Even heating is essential to forming a good seal between the jar and the lid.

More advice on safe canning:


  • Wash fresh produce.
  • Wash jars; check for chips along the rim.
  • Use flats only once.
  • Have dial gauge on pressure canner checked each season. Gauges can be tested at county Extension offices.
  • Process food according to current U.S. Department of Agriculture recommendations later than 1989.
  • Avoid jars with bails and caps made of glass or one-piece zinc, porcelain-lined.


For additional information on home canning, refer to MU Extension’s Quality for Keeps publications, such as “Before You Start to Can, Learn the Basics” (GH1451) or “Steps to Success in Home Canning” (GH1452). The many other publications in the Quality for Keeps series are listed in the left sidebar (under Related publications) on these pages. You can also contact your local county Extension center for more information.


Resource: Janet Hackert, (660) 425-6434,


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