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I recently moved, and during the process I stored some things (canned soup for one) in a non temperature-controlled storage unit. As you know this winter has been science fiction cold, and the cans froze.

A while after the move, I decided to have some soup. It had been more than enough time for it to thaw, and the can wasn't cold. However when I dumped it out, the consistency was was ... well ... wrong. It was sort of like refried beans - but imagine that the refried beans had been left out for awhile and they had dry patches throughout, and then imagine somebody mixed in cottage cheese and sour milk and then blended it with maggots. It was kind of like that.

There was no danger of me eating it, and I didn't connect to the freezing immediately, but it has happened again.

Can anyone explain, or bare witness to, this horrifying soup behavior.

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  • Freezing expands soup. Where the cans still intact?
    – Mien
    Mar 1, 2014 at 14:27
  • As much as I could tell.
    – dgo
    Mar 2, 2014 at 6:58

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Your soup (and almost every other canned food) is mostly water. Water expands when it freezes.

This exerts a fairly large amount of force on the can, which will at minimum cause it to bulge (while frozen). It also potentially causes the can to rupture, possibly only a tiny amount at one of the seals.

If you were to transfer the cans to a freezer at this point, they'd remain safe, as freezing would prevent spoilage. Of course, you have no way to know for sure that the bulging is from freezing only, and not from microbes.

When it thaws, the water returns to its original size. This creates a vacuum inside the can again, at least if it didn't burst. That can also un-bulge the can. But any microbes that got in remain, and can begin spoiling (or worse) the contents.

But at that point, there may already be microorganisms allowed in when one of the seals gave temporarily under the pressure.

It may be possible to carefully inspect the seals to see if they've been damaged. It's also probably not worth the risk, especially for low-acid foods.

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  • The cans weren't (at least noticeably) bulging. That could be explained by what you said about the suction during thaw?
    – dgo
    Mar 2, 2014 at 7:02
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    @user1167442 yes, it could be. Or maybe those particular cans don't bulge much (which could mean the seals are failing quickly, and the excess pressure is being vented—allowing microbes in) or that there is a fair bit of headspace in the cans. If you really want to find out, you could get a new can (of the same product) from the store, put it in a ziploc bag (in case it bursts) and throw it in your freezer. Wait a few days and find out. If you want to eat it after that, thaw in the fridge or under cold running water and consume immediately. Quality will be reduced, but safety shouldn't be.
    – derobert
    Mar 2, 2014 at 7:18
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I would say that putting the cans in a NON-temperature controlled storage unit that the ambient temperature changes probably did in the soup. A side question, did the cans puff up or the tops and bottoms become able to make a popping sound when pushed. This is bad, maybe even botulism.

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  • Nothing obvious. I would've guessed that, but is it that visibly obvious with botulism?
    – dgo
    Mar 1, 2014 at 6:49
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    @user1167442 some bacteria release gasses as they grow, thus if they grow they'll create pressure in the can, causing it to bulge. But not all of the harmful ones do, and even the ones that do may get to harmful quantities before the pressure rises enough to bulge the can. But if you see a can bulging, you shouldn't eat it. Or even open it...
    – derobert
    Mar 1, 2014 at 15:55
  • @derobert why not even open it? Get on my hands?
    – dgo
    Mar 2, 2014 at 7:00
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    @user1167442 On your hands, around the kitchen, etc. You may want to review e.g., fsis.usda.gov/wps/portal/fsis/topics/food-safety-education/… (you'll not that it also has an answer to your original question—do not use the cans that have frozen and thawed)
    – derobert
    Mar 2, 2014 at 7:07

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