Water Bath Canning Step-by-Step - The Washington Post |

Photo by Cathy Barrow/for The Washington Post

For shelf stability, the USDA recommends water bath canning to process and seal jars of low-pH (high acid) foods like jams and pickles (as well as some tomato products). The filled jar is closed with a gasket-edged lid and placed in the canner. A threaded ring holds the lid in place during the processing. (While the lids cannot be reused, the rings can be.)

While in the water bath, the jars boil for the time indicated in the recipe, and the food inside the jar heats to the same temperature as the boiling water (212 degrees), equalizing the pressure in the jar. When the jar is pulled from the water bath, the gasket on the lid seals, making a pinging sound. The dimple should be flat and the lid should be concave, when sealing is complete. Some jars ping while still in the water bath. Some ping an hour later. Be patient.

Any lidded pot can be used as a canner as long as the jars (atop a rack) can be submerged in the boiling water. The rack can be a cake rack or even a circle of canning rings tied together. Anything that will ensure the jars will not be knocking against the bottom of the pot when the water is vigorously boiling will work. Even a folded towel is a suitable cushion, but annoyingly spectre-like as it pillows up in the boiling water. It’s helpful to have a kettle of boiling water at the ready to make adjustments to the water height once the jars are in place.

Set up a baking sheet lined with a towel to hold the jars both before and after processing. The hot jars should not come in contact with a cold surface or they could crack; the towel provides a cushion.

TROUBLESHOOTING: Seals will fail when there is food residue between the flat lid and the jar; clean the jar rims and threads thoroughly. Seals will fail if the jar is not held upright going in and coming out of the water bath and if the processed jars are moved before they are entirely cool. Seals will fail if the jar is filled with cold food, so process as soon as possible after cooking.

Place a rack on the bottom of the canner and fill with water so that it will cover the jars by 1 inch. If you have hard water, add 2 tablespoons white vinegar for sparkling jars. Set the canner over high heat, cover and bring to a boil. Reduce the heat so the water is at a simmer until it is time to can.

Line a rimmed baking sheet with a folded tea towel. In hot, soapy water, wash the number of jars needed (indicated in every canning recipe) plus one extra, just in case. Invert the clean jars on the towel and let them dry. Have at hand the lids and rings, a ramekin or small bowl for any extra, a teaspoon to adjust the headspace, a few folded paper towels, a ladle and a chopstick.

Make the recipe as instructed.

Place a jar funnel inside a jar. Ladle the hot food into the jar to the recommended headspace. Headspace is measured from the top of the jar to the food within. The threads on the rim of every canning jar indicate 1/4-inch, 1/2-inch and 1-inch headspace. Headspace is one of the factors used to determine how long food must process in the waterbath to reach a safe temperature of 212 degrees.

Repeat with the remaining jars. Use the teaspoon to adjust the headspace, as needed. Run the chopstick around each jar’s contents to remove any air bubbles.

Dampen a paper towel and carefully clean the inner and outer jar edge and threads of the jar. Once the jar is wiped clean, place the flat lid, gasket side down, on top. Add the outer ring and finger tighten.

Increase the heat under the canner to return the water to a boil. Using a jar lifter, transfer the upright jars into the canner and atop the rack or towel. When all the jars are in place, cover the pot and wait for the water to return to a vigorous boil. Process for as long as the recipe instructs, starting the timer only when the water is boiling again.

Remove the jars one at a time, returning them to the towel-lined baking sheet. Keep the jars upright (resist the urge to pour off any water pooled on the top of the jar.) Listen for the ping that indicates the jar has sealed and look for the dimple on the top of the lid to be flat and the lid concave. Let them cool in place for several hours.

When cooled, remove the ring on each jar and lift it by the flat lid. If the lid releases, the seal is unsuccessful. Jars that do not seal should be refrigerated and consumed within 1 month. If all the jars in a batch fail to seal, review your process. If the seal failure is clear immediately, empty the contents of the jars into a pan, reheat to an active boil, and reprocess. The flavor and texture of the foods may suffer from being heated and processed twice.

Wash and dry the jars, be sure to label with both the contents and date, and place in a dark, cool space to store.

The contents of most water bath-processed jars will stay delicious for up to 18 months.

NOTE: High-pH (low acidity) foods such as non-pickled vegetables, any proteins, dried beans and stock must be pressure canned, an entirely different process. When preserving and processing, use recipes from known sources and follow the directions precisely with regard to jar size and time to process. Adding extra ingredients, especially proteins, mushrooms, peppers, garlic and onions, can change the pH of a recipe and increase the likelihood of botulism or molds.

Correction: An earlier version of this method included an incorrect temperature that the food inside jars reaches to equalize the pressure. It also included an incorrect description for the lid of the jars once they are sealed properly. It has been corrected.

From food writer Cathy Barrow.

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