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I've jarred soup with my pressure cooker. I know what a failed seal means, its gone bad. I have several jars which appear from the outside to have air displaced within a thickened body of soup and some off odor as though they have gone down the road to fermentation, however, the seals are completely sealed hard. Is it possible that they are not safe for consumption even though the seal is entirely intact?

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    The question title and body didn't match but instead asked the question in the opposite sense. I've edited you because it's clear that neither answer should be taken to mean "Yes, it's possible that a sealed jar is guaranteed to be safe" but based on the title it could be read that way.
    – Chris H
    Nov 16, 2023 at 15:15
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    It's funny: On the one hand, Clostridium botulinum and its toxins can be present without bad odor or taste; on the other hand, since this is soup, both will die when you cook it thoroughly (10 minutes boiling says the university of Florida). Therefore: (1) Trust your senses. If it looks, tastes or smells bad, don't eat it, and not (just) because of botulism. (2) Cook preserved, non-sour foods, even if they look, smell and taste great. Most botulism cases involve preserved food eaten cold, often polar native. Nov 17, 2023 at 0:13
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    Also, since Clostridium botulinum is anaerobic, if anything it's less likely to grow in imperfectly sealed containers, paradoxically. Nov 17, 2023 at 0:16
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    @Peter-ReinstateMonica (...) often polar native - what does this mean? (English is not my first language, sorry)
    – WoJ
    Nov 17, 2023 at 12:26
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    @Peter-ReinstateMonica but would anyone boil for 10 minutes when reheating canned soup? It would have to be at reheating because boiling doesn't destroy the spores, and I reckon most people would just get it to bubbling, stirring a couple of times
    – Chris H
    Nov 17, 2023 at 12:42

3 Answers 3

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It's completely possible. Sealing by itself doesn't preserve the food, effective processing does, then the seal keeps it from regaining any microbes which could cause spoilage.

If your food is sealed and gone bad then your processing is at fault - it didn't reach the necessary temperature for a long enough time.

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    I wonder how well most people really understand pressure cooking. I know someone who has a pressure cooker that lost its gasket decades ago. They don't seem to understand that it's just an awkward pan without it.
    – JimmyJames
    Nov 17, 2023 at 15:36
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    @JimmyJames based on the number of youtube videos where people show canning techniques that they "learned from grandma" and haven't updated based on modern understanding, I'd say it's far too many. I encourage anybody thinking about canning to get one of the well-known reference books and check out the USDA (or local equivalent) site to make sure they're using the most up to date information.
    – kettch
    Nov 17, 2023 at 16:51
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Yes, it's possible. If the soup is producing gas and smelling bad, then it's bad, even if it hasn't popped the seal yet.

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    If it is producing a detectable smell, don't that mean the seal is breached by definition? How else would the smell be detectable?
    – terdon
    Nov 19, 2023 at 15:05
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You are using a process known as "canning". (That you are using jars rather than cans is mostly immaterial; the purpose and principles of the process are the same.)

Canning preserves food by keeping microbes out; that's the purpose of hermetically sealing the cans (jars). However, in order for this to be effective, you must also eliminate any microbes already present. This is the purpose of cooking the food in the cans (jars). Such food is then effectively "sterile".

Cans (jars) go bad when microbes get into the food and start doing what microbes do, which includes producing gas. This is why you see "discard if button is raised" on canned goods; the 'button' is an indicator that the pressure inside has increased due to microbiotic activity producing gas.

Any sign that microbial activity is happening or has happened inside the can, or that microbes may have been introduced — bulging, odors, leaking, failed seals, fermentation — means the contents are no longer sterile, and consuming them is probably no different than if the food had sat, open, on your counter for however long since the food was canned. It might not kill you, but you'd be taking a pretty big risk. Thus, any canned (or jarred) food showing any such indications should be assumed "bad" and discarded accordingly.

As another answer noted, if the seals are good, you probably didn't process the jars properly, resulting in microbes still being present from when you filled the jars. It is absolutely possible for this to happen and for the food to be unsafe.

For comparison, consider homemade beer. Although those bottles need to be sealed in much the same manner, they are most definitely not sterile. In this case, great care is taken to ensure that only desirable microbes are present (resulting in fermentation and carbonation) inside, but it's a good illustration of how "properly sealed" doesn't equate to "sterile".

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