Open Kettle Canning
Since the late 1980's we have been teaching that open kettle canning is no longer safe. Open kettle canning involves heating the food to boiling, pouring it into the jars, applying lids, and allowing the heat of 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 is not heated adequately to destroy the spoilage organisms, molds and yeasts that can enter the jar while you are filling the jar, and it does not produce a strong seal on the jar. This method is not safe! Processing jars in a boiling water bath or in a pressure canner drives air out of the jar and produces a strong vacuum seal.
Open kettle canning is not safe! It is especially dangerous when used for 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) that should be pressure canned.
Just because a lid "pops," it doesn't mean the contents inside the jar are safe. The time saved with open kettle canning is not worth the risk of food spoilage or illness.
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. The Jarden Company that manufacturers most canning jars in this country states 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 not comparable to the moist heat of a boiling water bath. Processing in an oven will not heat the contents in the coldest part of the jar in the same way as boiling water.
Thirdly, oven heat will not increase the temperature inside the jar above boiling to be adequate to destroy botulism spores in low acid foods. Only in the enclosed conditions of a sealed pressure canner will you be able to increase the internal temperature to 240°F. Oven canning is not recommended!