Most of us are cautious eaters.
If you're munching grapes, you'll probably skip one that's all shriveled and discolored in favor of the round, healthy ones. Likewise, when you choose a can of food at the supermarket, you'll likely choose one that's exactly can-shaped, not one that's misshapen and swollen.
Reasons for swelling
A can of food can swell for two separate reasons. The first is what food scientists call "hydrogen swelling," and it only happens to cans of acidic food such as tomatoes or citrus fruit.
The acids in the food begin to attack and dissolve the metal lining of the can, and one of the by-products of this chemical reaction is hydrogen gas. Because the can is sealed, the gas builds up, causing the can to swell.
Poorly processed foods
The second reason a can swells is the result of poorly processed food. Before it's canned, food needs to be sterilized to remove all the potentially dangerous microorganisms- especially those that can live inside a can without oxygen.
If some of these organisms are left behind, they can thrive in the can, spoiling the food and giving off carbon dioxide gas. In this case, carbon dioxide causes the can to swell.
Tell your grocer
Standing in the supermarket aisle, there's no way you can tell whether a can is swollen because of hydrogen or carbon dioxide gas, but that shouldn't really matter. Assume that the food has spoiled, inform the store manager, and choose a healthy, can-shaped can instead.