Oven canning consists of placing jars filled with hot, freshly prepared high-acid foods such as fruits, jams, jellies or marmalades into a hot oven for heating and sealing. This method has several advantages, with the most significant being the ability to process a large number of jars at one time. It also has a variety of disadvantages, including that many food safety experts have deemed to be an unsafe canning method.
Unlike stove-top canning, oven canning requires no water bath. It requires no gingerly placing of jars into boiling water with a pair of tongs and listening for the sound of the lid sealing properly. When done correctly, lids seal themselves after being screwed onto pre-heated canning jars, then being heated for an additional 15 minutes and cooled on a rack or in the oven. The canner also moves all the jars at the same time because they're on a tray.
Oven canning advocates stress the method provides a quick, 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 preserving, so oven canning isn't recommended for them.
Ovens give off dry heat instead of steam, so the jars are slowly heated and might shatter during the heating process. Dry oven heat might also compromise a canning lid's rubber seal 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 food's temperature more slowly than steam or a water bath, giving more time for bacteria to reach the food, especially if it doesn't reach a temperature of 212 degrees Fahrenheit (100 degrees Celsius).
This method also doesn't guarantee that the food is canned at a constant minimum temperature needed for killing bacteria. Most oven canning instructions include heating the canning jars without the lids on, at least for the latter part of the process. As soon as the canner removes the lidless jars from the oven to install the lids, the food, jar and surrounding area will cool, increasing the risk of tainting the contents. Ovens also can vary in temperature from what the knob or digital thermostat reads, so the oven might not be heating to the required temperature. The temperature inside the oven also can vary, depending on where the jars are placed — the middle, sides or top, for instance — so the jars might not reach uniform temperatures.
The oven canning method also is highly susceptible to user error. The jar's contents can bubble and spill, even when the canner adheres to the required 0.25-0.5 inches (about 0.6-1.3 cm) of headroom in each jar. A canner also runs the risk of burns or broken glass when removing the glass jars from the oven.