How Long to Cook Tomatoes When Canning
Canning can be a tricky process, with the amount of time it takes to can tomatoes depending on the type of canner you use and the altitude at which you live.
Generally, if you're using a boiling-water canner, pints of tomatoes will take at least 40 minutes to process and quarts will take at least 45 minutes; however, it could take up to an hour if you live at an altitude above 6,000 feet.
In a pressure canner, your tomatoes will take between 15 and 40 minutes, depending on what altitude you live at and what pressure you use for your canner. For instance, it takes 25 minutes to can tomatoes under 10 pounds of pressure at sea level, but at high altitude, you'll need to use a higher pressure setting or allow more time for the jars to process. You should also allow the jars to sit in the canner for at least 10 minutes after you've released the pressure before you move them.
No matter which method you use, it's important to let the processed jars sit undisturbed until they're completely cool, at least overnight if not longer. Then, check the seals to make sure that all of the jars have been sealed properly. If any of the lids pop back up when you press on them, use those jars right away; they are not safe to store.