MSU Extension Food & Health

Open kettle canning, what is it?

Your grandma may have done it, your neighbor may still do it, and they “haven’t gotten sick yet.” When preserving food by canning, some have gotten lucky with what is referred to as “open kettle canning.” This is the practice whereby food, tomatoes for example, is cooked to boiling in a pot, or kettle, then added to a canning jar and left to seal without further processing. So, if people get away canning this way without getting sick, why should we bother doing it a different way?

The canning process

First let us look at how canning actually preserves food. Food in its fresh state can spoil because of a few different factors such as high water content, which allows pathogens to grow, active enzymes that continue to ripen the food or because of a reaction with oxygen. The canning process counteracts these factors. It removes oxygen, destroys enzymes and prevents growth of undesirable bacteria, yeast and molds. A critical part of the caning process is the tight vacuum seal that is formed on each jar. This seal forms when the heated jars are removed from the canner, during the cooling process, and stops any liquid from leaking out and prevents any air or pathogens from getting in.

One pathogen of concern when canning, is Clostridium botulinum, which can lead to a type of food poisoning referred to as botulism. Botulinum is a bacterium that is present either as a spore or in a vegetative state. This spore can be present in soil for years without causing harm because the conditions do not support its growth. However, once the conditions are ideal for this spore to multiply, it can become deadly within just three to four days. Botulinum spores only multiply in the absence of oxygen, so they are harmless on fresh foods – but if canned food is not processed correctly, it can become deadly. 

The last step in any canning process involves processing the jars in either a water bath canner or pressure canner. This last step is a critical step and must not be overlooked. In a high-acid environment (which includes most fruit or pickled products), there is enough acid to block the growth of botulinum or destroy it when heated to a proper temperature, as in the boiling water method. However, in a low-acid environment (vegetables and meats), botulinum spores require a temperature of 240 degrees Fahrenheit for destruction, and this can only be reached with a pressure canner.

Risky foods

Tomatoes are a common food that was, and still is by some, processed in an open kettle method. Tomatoes in particular are a concern because often their acidity level falls on the borderline of being a low-acid food. When processing tomatoes, it is always recommended to add acid – in the form of bottled lemon juice or citric acid to ensure a proper acidity before they are canned in the boiling water method. 

Unfavorable consequences

Michigan State University Extension recommends following proper canning procedures. When water bath or pressure canning is missed, the bacteria, yeast and molds are not killed and can cause further spoilage. Jars that may have sealed in the open kettle method, may later become unsealed because of the growth of these microorganisms and spoiled food. After all of that hard work, why would you want to risk losing jars to spoilage or even worse – causing a foodborne illness that could have deadly consequences?

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