When Does the Processing Time Start in a Pressure Canner, and How Should the Canner Cool?

The first step in pressure canning is to vent the air, and this step cannot be skipped. Add the amount of water specified in the recipe to the canner, arrange the jars on the rack, and close the lid. When the burner is turned on, leave the steam valve or vent opening open. Once steam starts escaping in a steady, continuous column, wait 10 minutes. This allows the steam to replace the air remaining inside the canner, and the time must not be shortened. Measure the amount of water to add to the canner according to the appliance’s own manual, not by guesswork.

Venting the air is not a minor detail; it determines the temperature. A steam-and-air mixture is cooler than pure steam at the same pressure; that means that even if the pressure gauge shows the correct reading, if air remains inside, the actual temperature is below the target. The center of the jar then does not receive the full processing time, and the process is incomplete. This failure, which gives no visible sign from the outside, can occur when the 10-minute wait is skipped. That is why the venting time is measured with a timer, not judged by eye.

When the 10 minutes are up, attach the weight or close the vent, and the pressure will begin to rise. Start the timer when the pressure specified in the recipe is reached—not when the lid is closed and not when steam starts escaping. On a weighted-gauge canner, this is when the weight begins rocking at regular intervals. On a dial-gauge canner, it is when the needle reaches the target reading. If the pressure falls below the target during processing, increase the heat and start the time over from the beginning; do not continue from where it left off. Writing down the time when the timer starts prevents kitchen distractions from changing the result.

Keep the burner steady throughout the processing time, and do not leave the canner unattended. Repeated pressure fluctuations can draw liquid out of the jars; the headspace in a jar that loses liquid is disrupted, and the vacuum seal may be faulty. Adjust the burner in small steps; do not suddenly turn it down or up. Do not add water to the canner at this stage, and never open the lid. The time spent heating and cooling is not included in the processing time; the minutes in the chart are only the time spent under pressure.

When the time is up, turn off the burner and leave the canner where it is. Let the pressure drop to zero on its own; do not run cold water over the canner, open the vent to release pressure, or remove the weight early. A sudden pressure drop can make liquid spurt out of the jars, lift the lids, and crack the glass. After the gauge reaches zero, wait another 10 minutes, then remove the weight and open the lid with it facing away from your face. Arrange the jars on a towel without handling them, and let them cool on their own.