Temperatures matter, from the beginning to the end of the canning process.
Home canning involves more than just heating jars of food in a canner. There are many steps in the process, from food preparation to checking that jars sealed after processing – all of which contribute to a product that’s safe to eat. Two of these steps include the time the water in a boiling water canner comes up to a full rolling boil (or a pressure canner comes up to pressure), known as the come up time, and the time when when jars sit at room temperature after being removed from the canner, known as the cool down period. These periods in the canning process are as important as processing time itself. Be sure to include the steps listed below when canning. And of course, always follow a recipe from a reputable source.
- Fill the canner with clean water and heat to 140°F for raw pack food or 180°F for hot pack food. The water should not be fully boiling when the filled jars are placed in the canner. In large part, the reason for the below-boiling starting temperature is to prevent thermal shock and resultant jar breakage, but the amount of time it takes from starting temperature to a full boil or pressure (the come-up time) is included in the overall calculations for processing time.
- Keep jars hot until ready to fill and fill them quickly to ensure that the food or hot liquid doesn’t cool down before the jars are put into the canner. The initial temperature of the food is also part of the process calculations.
- When all jars are in the canner, turn up the heat, and start timing when the canner has come to a full rolling boil (or correct pressure).
- At the end of processing, remove the jars and place them on a towel or rack, away from drafts, with at least 1” of space between jars. Allow the jars to cool naturally, uncovered, in still, ambient air for 12-24 hours. This cool down period is actually when most of the bacteria are killed off, so it’s considered a critical step in the process. Don’t try to rush it.
For further information on canning, visit the National Center for Home Food Preservation (NCHFP) or contact your local Cooperative Extension office.
Brought to you by the UC Master Food Preservers of El Dorado County
Last updated July 2025