Oven canning is a controversial method of food preservation that uses low oven heat to process and seal jars of fruits, vegetables, and sometimes grains. It is typically not recommended by food safety experts for several reasons, including temperature accuracy and increased likelihood of contamination or spoilage. The process often involves a lot of guesswork or past experience. Reading up on the various methods and processes, as well as understanding the leading risks and expert recommendations, is essential before beginning.
“Dry” Oven Canning
When people talk about oven canning they are most often referring to “dry canning,” in which cans of prepared food are set in a warmed oven and allowed to process for a set amount of time. This method is quite controversial in the food community and is often harshly criticized as being dangerous and unsafe.
The theory behind dry canning is that cooks are able to kill any food-borne bacteria by slowly heating the food in glass jars. The oven is usually heated to around 200°F (about 93°C), and the jars placed on the oven’s racks and allowed to heat for 30 minutes to an hour. When the jars “pop” — that is, when their seals depress — they are believed to be closed off from contaminants, and they can theoretically be stored at room temperature for several years.
Food Safety Concerns with Dry Canning
Contamination is the main fear when it comes to dry canning. In order for food to be shelf-stable, it must be heated to a hot enough temperature that any latent bacteria in the food is killed off. The premise behind dry canning is usually sound, as a 200°F oven is generally hot enough to be considered sterile. Not all oven thermometers are accurate, however, and it can be tough for home cooks to know whether the external temperature is actually penetrating the jars.
There is no way for cooks to test the internal jar temperature without removing the lids and compromising the food. Any bacteria that remains in sealed jars can grow into toxins over time, which can cause serious food poisoning once the contents is eventually consumed. Sometimes spoiled food looks discolored or has an unpleasant taste, but not always.
Risk of Explosion
A more immediate danger of oven canning is explosion. Canning jars are not designed to be exposed to prolonged dry heat and have been known to crack or even shatter during processing. At best, this creates a huge mess; at worst, it can lead to severe burns, cuts, and infections.
Water-Based Oven Canning
Some of the downsides of dry canning can be avoided by using water. According to this process, jars are placed in a pan of water inside the oven rather than simply standing alone on the racks. This method is very similar to water bath canning, and while still controversial, tends to have fewer safety concerns.
The traditional so-called “water bath” canning method involves bringing a large pot of water on the stove to a boil, then submerging closed jars for a set amount of time. The boiling water creates both heat and pressure that sterilizes the food and forms a vacuum seal.
Setting jars in a similar water bath in the oven can achieve similar results, and often uses less energy. The jars do not always need to be submerged, either, as the heat of the oven combined with the steam from the water can produce a powerful seal in a shorter amount of time. Still, the practice is widely criticized as being far less precise than the standard water bath method, and home canners are advised to carefully weigh the risks against the perceived convenience when choosing one method over another.