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I grew up eating green beans which were pressure-canned at home. When my mother heated them for a meal, she brought them to a boil then removed the lid and let them boil for at least a minute with the lid off. She said this was necessary to be safe. Is it necessary or was that just something passed down from before pressure canning was available?
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Thanks for your great question! If you are canning green beans today and you pressure can them correctly (for Utah County altitude, you need to process them at 13 lbs pressure if using a dial-gauge canner or 15 lbs pressure if using a weighted gauge canner), any microorganisms that may have existed in the jar will have been destroyed by the heat of the processing. With a low-acid product like green-beans, the microorganism we are most concerned with is the C. botulinum spore. If this spore (which is around us, in the air, in the soil, etc.) is placed in a low-acid, moist environment with no oxygen (which is exactly what we do when we can green beans), it can develop a toxin that is deadly. In order to destroy to botulism spore, we need to heat the contents of the jar to an internal temperature of 240 degrees F. The only way to do that is with a Pressure Canner, which uses the pressure to increase temperature beyond boiling point (212 degrees F) of water. As long as the contents of the jar have been processed correctly - at 13lbs or 15lbs pressure for 20 minutes in pints or 25 minutes in quarts, then you should be able to open up that jar of beans and eat them straight out of the jar without any negative effect.
In other words, the contents of the jar do not need to be boiled prior to consuming. If you don't know how long it was processed, or are concerned it might not have been long enough, then you could go ahead and boil the contents (full rolling boil) for about 10-15 minutes prior to consuming. This doesn't help in every situation, but can render the botulism spore inactive for a period of time and help you feel sure that it is safe to eat.
One other motto to consider: When in doubt, throw it out!
Please let me know if you have further questions related to canning or food safety! I'd be happy to answer them for you!
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