Avoid Open Kettle or Oven Canning

Open Kettle Canning

Open kettle canning involves heating the food to boiling, pouring it into jars, applying lids, and allowing the heat within the jar to cause the lid to seal. Many years ago, it was commonly used for pickles, jams and jellies, and sometimes used for tomatoes and applesauce.

The reason open kettle canning is no longer recommended is that the food may not be heated adequately to destroy the spoilage organisms, molds and yeasts that can enter the jar while you are filling the jar, and it often not produce a strong seal. Processing jars in a boiling water bath or in a pressure canner is preferred because it drives air out of the jar leaving a strong vacuum seal.

Open kettle canning is especially risky canning tomatoes or tomato products where the acid level may be low enough to allow bacterial growth. Never open kettle can low acid foods (meats, vegetables, soups). These must be pressure canned.

Oven Canning

Occasionally people ask about processing jars in the oven. They claim a friend or neighbor promotes it as a simple method of canning. What they fail to understand is that oven heat is not the same as heat from a boiling water bath or from steam in a pressure canner.

First of all, placing jars in the dry heat of the oven may cause the glass to crack and shatter causing injury to you. Jar  manufacturers state emphatically that it is not safe to heat glass jars in the dry heat of an oven. Jars are not designed to withstand oven temperatures and can break or even explode causing injury from broken glass.

Secondly, dry heat is a less efficient way to transfer heat into the product and under processing is likely to occur.

Thirdly, in low acid foods, oven heat will not increase the temperature inside the jar to above boiling and temperatures inside the jar will not be adequate to destroy botulism spores. The only safe way to process low-acid foods is use a pressure canner that will heat the product internal temperature to 240°F.