A pressure canner is the essential piece of equipment for safely canning low acid foods such as soup stocks and un-pickled vegetables. But did you know that it can double as a boiling water batch for fruits, pickles, jams and jellies?
The instructions for using a pressure canner as a boiling water bath are slightly different from those for either pressure canning or standard boiling water bath processing. Here's how to do it:
Place the pressure canner on the stove. Put a rack in the bottom of the pressure canner (most pressure canners come with one). Place your jars of food on the rack, leaving space between the jars. You need to be able to cover the tops jars with an inch of water - in my canner, this means I can only use this method with pint and half pint sized jars.
Pour in hot water until it covers the jars with an inch or more of water.
Screw or clamp on the lid of the pressure canner and leave the vent, or petcock, open. Turn on the stove to high heat. Start timing your boiling water bath according to the recipe instructions only once steam starts coming out of the vent vigorously.
If you were pressure canning, you would leave the vent open for 10 minutes, then close it to start building up pressure in the canner. When you use a pressure canner as a boiling water bath, leave the vent open for the entire processing time.
Remember that the processing time doesn't start when you turn on the heat, but rather when steam starts blasting through the vent. So for example, if a canning recipe ends with the instruction to "process in a boiling water bath for 20 minutes," you don't start timing that 20 minutes until you see and hear steam escaping.
Wait until the hissing sound of escaping steam ceases before taking off the pressure canner lid and removing the jars.
Very Important! -
Remember that even though you are using a pressure canner, with this method you are processing the food in a boiling water bath. This cannot be used in lieu of pressure canning. The same safety rules apply as with regular boiling water bath canning, i.e. you can only use this method for acidic foods including fruits, sweet preserves, tomatoes with added acid and pickles (including vinegar-containing chutneys). All un-pickled vegetables and animal products must be processed using your pressure canner as, well, a pressure canner.
Note that some older canning instructions used to say that it was not necessary to cover the jars completely with water when using a pressure canner as a boiling water bath. However, that method is no longer recommended by pressure canner manufacturers.