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Oven canning is a method of sealing jars filled with hot, high-acid foods in a hot oven, but it is not recommended for low-acid foods. While it can process a large number of jars at once, it is also susceptible to user error and does not guarantee a constant minimum temperature to kill bacteria.
Oven canning consists of placing jars filled with hot, freshly prepared, high-acid foods such as fruits, jams, jellies, or marmalades in a hot oven to heat and seal them. This method has several advantages, the most significant of which is the ability to process a large number of jars simultaneously. It also has a number of disadvantages, including the fact that many food safety experts have deemed a canning method unsafe.
Unlike stovetop preserves, oven preserves don’t require a water bath. It doesn’t require carefully placing jars in boiling water with a pair of tongs and listening for the sound of the lid sealing properly. When done correctly, the lids will seal after being screwed onto pre-heated jars, then heated for another 15 minutes and cooled on a broiler or in the oven. The jar also moves all the jars at once because they are on a tray.
Proponents of oven canning point out that the method provides a quick and relatively hands-free canning method for high-acid foods such as apples and pears. High acid foods are more resistant to bacteria than low acid foods such as vegetables or meat. Low-acid foods require longer heating at a higher temperature for proper preservation, so oven canning is not recommended for them.
The kilns give off dry heat rather than steam, so the vessels are heated slowly and may shatter in the heating process. The dry heat of the oven could also compromise the rubber seal of a container lid if the jar is left in the oven for too long. When properly sealed, the lid creates a vacuum, keeping air and bacteria out of the jar. Dry heat also changes the temperature of food more slowly than steam or a water bath, giving bacteria more time to reach the food, especially if it doesn’t reach a temperature of 212 degrees Fahrenheit (100 degrees Celsius).
This method also does not guarantee that the food is canned at a constant minimum temperature needed to kill bacteria. Most oven canning instructions include heating the canning jars without the lids on, at least for the latter part of the process. As the can removes the lidless jars from the oven to install the lids, the food, the jar, and the surrounding area will cool, increasing the risk of contaminating the contents. Ovens can also vary in temperature from what the knob or digital thermostat reads, so the oven may not heat up to the required temperature. The temperature inside the oven can also vary, depending on where the jars are placed – the center, the sides or the top, for example – so the jars may not reach uniform temperatures.
The oven canning method is also highly susceptible to user error. Jar contents may leak and spill, even when the container adheres to the required 0.25-0.5 inch (about 0.6-1.3 cm) margin in each jar. A bin also runs the risk of burns or broken glass when removing glass jars from the oven.
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