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There are two methods of canning: in a boiling water bath or in a pressure canner. Knowing the difference between the two methods is absolutely essential if you want to preserve food safely in sealed jars at room temperature. Boiling water bath canning is the simpler of the two methods. It requires minimal equipment – basically just a large, deep pot and canning jars and lids. The most important thing to know about canning in a boiling water bath is that this method is only safe with high acid foods.
What are high acid foods? Fruits, anything pickled with a brine that is mostly vinegar (this includes chutneys); fruit-based sweet preserves such as jams and jellies; and tomatoes with a little added acidity. Jars filled with any of those foods can be safely processed in a boiling water bath because they are acidic (4.0 on the pH scale or lower). All
The takeaway here is that with a boiling water bath it is the acidity of each jar's contents, even more than the heat of the processing, that safely preserves the food.
Even though pressure canning also involves processing canning jars filled with food in hot water, it is a very different food preservation method from boiling water bath canning. Pressure canning enables you to can low acid foods that could be dangerous if they were canned in a boiling water bath (seriously dangerous–think botulism). It also requires a pressure canner, which is a very specialized piece of gear (and not the same as a pressure cooker). If you want to store more alkaline foods such as soup stocks, un-pickled vegetables, or meat in sealed jars at room temperature, you must process them in a pressure canner. Here's why: Although Clostridium botulinum and the toxin it produces is are killed at the temperature of boiling water, its spores can survive those temperatures. And guess what kind of environment they need in order to hatch? Someplace with moisture and without oxygen–exactly the environment they get within sealed canning jars. A pressure canner is capable of heating the food inside the jars to hotter than the temperature of boiling water, hot enough to kill off even the spores of botulism. That is why it is essential to use a pressure canner for low-acid foods.
Both boiling water bath canning and pressure canning create a vacuum seal that prevents molds from entering the jars. But as with store-bought canned foods, once the jars are opened they must be stored in the refrigerator.
Leda Meredith is the author of Preserving Everything: Can, Culture, Pickle, Freeze, Ferment, Dehydrate, Salt, Smoke...and More and Northeast Foraging: 120 Wild and Flavorful Edibles from Beach Plums to Wineberries. You can watch her foraging and food preservation videos, and follow her food adventures at Leda's Urban Homestead.