Hi Ann, 

The question of when you can reprocess home canned foods that have been processed incorrectly has become a hot topic of conversation in some online canning forums, and there is some confusion and disagreement about when you have a 24 hour window to reprocess and when you only have two hours. Here is what our Extension resources say on the topic.

First, the "24 hour rule." The USDA Complete Guide to Home Canning explains that you may safely reprocess home canned foods within 24 hours of the original processing if the jars did not seal. You must use a new lid, however, and reprocess for the entire length of time listed in the recipe. It's worth noting that within that 24 hour window, you also have the option to refrigerate or freeze your unsealed jars of food. If you miss that window and more than 24 hours passes from the time of the original processing, however, reprocessing, freezing, or refrigerating would no longer be safe. The food in that case should be disposed of. 

The National Center for Home Food Preservation further clarifies when this rule applies in one of their newsflashes. They state that for safe reprocessing, "you must have followed an up-to-date research-tested recipe on your first attempt, and you must reprocess within 24 hours." Older Extension resources have sometimes interpreted the 24-hour rule more broadly, permitting reprocessing in cases where an incorrect time or elevation adjustment was made during the original processing, for example. This more recent statement from the NCHFP has a higher standard of safety, however, and it only allows for reprocessing in cases where the only fault in the original attempt was a failed seal. If you made other risky modifications to a recipe during the first attempt at canning, we cannot vouch for the safety of the product sitting on the counter for a day.

This brings us finally to the 2-hour time frame that you'll find in more recent resources. Nevada Extension, Iowa State Extension, and the Ball company all agree that if a food was not processed using a research-based recipe and time, you only have 2 hours from the original processing to open those jars and refrigerate, freeze, or reprocess following research-based instructions. This 2-hour time frame is in keeping with the more general food safety standard that perishable foods should only be kept at room temperature for 2 hours before they become unacceptably risky to eat. 

So, in short, if you did everything safely on the first go-round of canning but a jar failed to seal, you have 24 hours to decide how to rescue that food by refrigerating, freezing, or reprocessing.  If you did not follow a safe and research-based recipe on the first attempt, though, you only have 2 hours to rescue that food. With an unresearched recipe, we just cannot vouch for the safety of the food sitting in the jar at room temperature for more than a couple of hours. 

Thanks for using Ask Extension!

Jared