Canning Basics: Make sure your canning jars start hot and stay hot
Canning jars should always be washed and rinsed well before being used. And while they don’t always need to be sterilized (unless the processing time is less than 10 minutes), the jars do need to be hot when filled with food, and kept hot until they’re placed in the canner. There are several reasons why, ranging from preventing underprocessing (food safety) to simply avoiding jar breakage.
Hot Food Should Stay Hot. Food prepared for the hot pack method is expected to be hot when it goes into the canner – processing times are based on starting with hot food (this goes for hot liquid used in the raw pack method as well). Putting hot food or liquid into a cold jar cools the food/liquid down, as does letting filled jars sit until they go in the canner. This is why the general recommendation is to fill only one jar at a time, seal it, and place it in the canner before going on to fill the next jar. So – keep those jars hot, and move them quickly into the canner.
Preventing Jar Breakage. Filling a jar with hot food or liquid when it’s cold, or placing a cold jar into a hot canner, could cause jar breakage.
Help Jars to Seal. Starting with hot jars can help the jars to seal during canning.
After washing and rinsing your jars, they can be kept hot in a dishwasher, in a pot of hot water, or in the canner itself.
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