Open kettle canning is a method of food preservation that involves putting boiling food into extremely hot jars and then sealing them. This method of canning is not recommended by numerous food safety organizations, including the United States Department of Agriculture. However, it can help to understand what the process is because this will make it easy to identify and avoid. As an alternative, cooks should consider canning in a pressure cooker or in a boiling water bath, as these two methods are much safer.
In open kettle canning, one pot is used to heat the food that is being canned, such as tomato sauce, while canning jars, rings, and lids are kept in a pot of boiling water. The cook removes one jar at a time, fills it with boiling food, and quickly seals it, setting it aside to cool. Once totally cooled, the canning ring can in theory be removed, although many people like to leave it on.
Some people use this as a shortcut method, because it is fast. If you plan on refrigerating or freezing the food afterwards, open kettle canning is an acceptable technique. However, for long term preservation and preservation at room temperature, it is not a safe canning method. Recipes which recommend this canning technique should be viewed with suspicion.
The primary problem with this canning method is that it does not raise the temperature of the food enough to eliminate many harmful organisms. In a pressure cooker or canning bath, the food is raised and held at a high temperature, ensuring even distribution of heat, and this kills the majority of organisms that could be potentially harmful. Open kettle canning also leaves a number of opportunities for contamination.
Organisms can be pulled from the counter or tools used to handle the food and jar, for example, and they can also be introduced into the pot used to boil the jars by sloppy cooks. Under normal circumstances, if a little bit of bacteria is introduced into a jar while the food is being packed, it is not a cause for concern, because the bacteria will be killed when the jar is boiled again. However, there are no second chances for getting rid of bacteria, mold, and fungi in open kettle canning, and as a result there is a high risk that the food will go bad, especially if it is held at room temperature.