If the can does not pop when you open it, that should be the first indicator. The pop is caused by pressure between the can and the food, and if that pressure is not there then it leaked out of the can from some where.
The so-called POP is air rushing INTO the can, not OUT OF the can. Food in cans is vacuum packed, not pressurized.
Although I do try to rotate stock, I keep canned goods a long time. As long as the seal hasn't been broken, they're just fine...to a point. I've got a couple cans of pumpkin that are at least 5 years old...maybe more. I think it's time for them to go.
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