Why Do Cans Bulge?

If the two ends of a can bulge outward and do not spring back when pressed with a finger, the cause is usually gas produced by bacteria growing inside the can. If you see this, never open the can or taste its contents; throw it directly in the trash. Bulging usually means the can was not sterilized sufficiently during production or its lid was damaged in a way that lets air in, so microorganisms are growing inside; a chemical reaction between the food and the metal, or improper cooling, can also make a can swell. Since there is no way to tell from the outside whether toxins are present, treat every swollen can as dangerous.

What causes a can to bulge? During canning, after the food is placed in the can, it is heated at a high temperature to kill the microorganisms inside; the can is then sealed airtight. If this process is incomplete, surviving bacteria gradually produce gas, increasing the pressure inside the can and pushing the metal outward. This risk is higher in low-acid foods such as meat, vegetables, and fish; these canned foods require longer sterilization at a higher temperature. An impact to the lid or a seal around its edge that deteriorates over time can also let air in and cause the same result. This risk is higher with home-canned foods than with commercially produced foods because home settings often cannot reach the high pressure and temperature achieved by industrial autoclaves, which can allow some resistant bacteria to survive.

Why is a bulging can so dangerous? The microorganism of greatest concern in bulging cans is a type of bacterium that grows without oxygen and produces a potentially fatal toxin. The toxin is colorless, odorless, and often tasteless, so there may be no visible or olfactory sign of a problem when the can is opened. Trying to judge a swollen can by tasting it is extremely risky and can cause food poisoning. There is no reliable way to destroy this toxin at home, so the precaution is to discard the can without opening it.

Does every bulge mean the same thing? A can hit during transport may have a slight dent or irregularity in one area; this is generally limited to a single spot and differs from gas-related swelling. The main danger sign is the two ends of the can bulging outward symmetrically and resisting firmly when pressed with a finger. If the contents foam when the can is opened, gas escapes with a hissing sound, or a rotten smell is released, take that just as seriously and do not consume the contents. When selecting a can on a store shelf, it is normal for the center of the lid to be slightly concave. A lid that bulges outward or remains springy when pressed without making a clear “click” is the first sign of a problem that can be noticed while the can is still on the shelf.

What should you do if you find a swollen can? The safest approach is to put the can directly into a sealed bag and discard it without opening it. Wash your hands with soapy water after touching the can, and discard it where pets cannot reach it. If a swollen can was purchased from a store, take it back to the place of purchase with the receipt, if possible, and report it; this may help identify other swollen cans from the same production batch.

Brief summary
If the two ends of a can are swollen and stay that way, this is a sign that bacteria have grown inside and that a dangerous toxin may have formed. In that case, never open the can or taste its contents. A single dent caused during transport is a different situation, but if you are unsure, the safest choice is always to discard the can.