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I opened a store bought can ("tin") of diced tomatoes with a "pull top" lid, and unusually as I did I heard a noticeable popping noise. The lid appears to have bent inward slightly, like there was a negative pressure differential (pressure inside the can less than outside).

There is no unusual smell (although a couple of the pieces are a bit off color yellow which isn't necessarily unusual) and I know the normal issue is when there is a build-up of pressure inside something canned. At the same time this is a rather odd occurrence, and I'm wondering if it's safe to eat and if so what harmless thing could have caused this.

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Negative pressure is fine.

The process of canning hot contents will do that if there’s any air space (as the hot air will shrink as it cools)

It’s the opposite that you want to be wary of—- a pressurized can means that there has been microbial activity going on that generated gasses

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I have a differing opinion on this than Joe's answer...

If the top popped "in" when you opened it, that means that something was pushing it out prior to opening (this is not negative pressure, this is positive). Opening it equalized the higher internal pressure with the lower external pressure causing the button to "un pop". Having a button popped out with the jar sealed, then popping in as you open it, is a sign to discard the food.

What happens normally in the canning process is the hot product is placed in the jar, then the lid on top. As the product cools, it creates a negative pressure pulling the "pop top" into the jar and sealing the lid to the jar lip.

If the lid is popped out, the seal is still OK, but something inside is wrong. The button on the lid should be depressed prior to the first opening, not popped out. If the button moves at all when sealed this is an indication that the product should be discarded.

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  • This wasn't a can with a separate lid.. this was a "tin" can, but with one of those new pull tops so you don't need a can opener to open it. I didn't notice any bulging on the top of the can before I opened it, but I wasn't specifically looking for it either. I'll let this question sit a bit longer in case there are any other answers, but at the moment I'm leaning towards erring on the side of caution.
    – Michael
    Mar 19, 2022 at 17:54
  • Oh… I didn’t even think of a bulging can. I was thinking the can sucked in air as it was opened. If the can is bulging, definately don’t consume it.
    – Joe
    Mar 20, 2022 at 0:26

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