A can with domed ends or a jar whose lid has pushed upward is not a “check it and see” situation. A bulging container can mean gas has formed inside because microorganisms grew, the seal failed, or the contents were exposed to damaging conditions. USDA guidance is unequivocal: do not use swollen cans and never taste food from containers showing botulism warning signs. The safe action is to keep the container closed, handle it carefully, and discard it without sampling the food.
Why pressure inside a sealed container matters
Commercial canning is designed to destroy organisms capable of growing in a sealed product and to leave the package hermetically closed. Under normal conditions, the ends of a metal can stay flat or slightly concave rather than ballooning outward. If microbial activity occurs inside, gases can accumulate and increase internal pressure. Swelling can also result from freezing, overheating, or chemical reactions, but you cannot identify the harmless explanation reliably just by looking at the outside.
This is why the decision is based on the package condition rather than on a home diagnosis. USDA FSIS lists bulging, leaking, badly dented, and severely rusted cans among warning signs and specifically says not to taste suspect food. Botulinum toxin is especially important because a very small amount can be deadly and the food does not have to smell obviously rotten.
Do not open it to “release the pressure”
Opening a swollen can in the kitchen can spray liquid or contaminated material onto hands, counters, utensils, and nearby food. A loud hiss or forceful spurt is itself another warning sign. Do not puncture the can, press the lid flat, transfer the contents to another container, or boil the food as an improvised rescue step. Those actions add exposure without giving you a reliable safety test.
If a suspicious container is already leaking, avoid direct contact with the contents and clean any contaminated surface carefully. For ordinary household disposal, follow local waste guidance; USDA recommends securely bagging suspect canned goods so children, animals, or anyone handling trash are less likely to contact them. Home-canned low-acid foods deserve particular caution because incorrect pressure canning is a known botulism risk.
Swelling from freezing is still not something to guess about
A can can swell if the food inside freezes and expands. USDA notes that frozen cans can also be swollen because of contamination, so a swollen can should not be used simply because the garage or car was cold. If a can froze but did not swell and the seams remain intact, there is separate USDA guidance for thawing it safely in the refrigerator. That narrow exception is not a reason to keep a visibly bulging container.
Heat can also damage shelf-stable foods and accelerate spoilage. Cans should be stored in a cool, dry location, not beside a stove, under a sink, or in a hot vehicle. Package damage and temperature history therefore belong in the same assessment: a can that has changed shape is telling you its normal sealed condition is no longer trustworthy.
What a normal can should look like
Before putting canned food in the cart or pantry, check for flat ends, intact seams, no leakage, no deep dents, and no heavy rust. A tiny cosmetic dent away from a seam is a different issue and is covered by separate guidance, but swelling is not a cosmetic defect. The same principle applies to jars with loose or bulging lids and flexible shelf-stable packages that are unexpectedly puffed up.
The practical rule is intentionally conservative because the consequence of guessing wrong can be severe. A bulging canned-food lid is not a freshness clue that you confirm with a sniff. It is a container-integrity warning. Keep it closed, do not taste the contents, and discard or return the product according to the retailer’s and local waste instructions.
Sources
- USDA FSIS — Shelf-Stable Food Safety — Explains bulging-can warnings, botulism risk and disposal guidance.
- USDA FSIS — Meat and Poultry Packaging Materials — Warns never to use or taste food from leaking, bulging or badly damaged containers.
- FDA — Are You Storing Food Safely? — Lists swelling, leakage and severe damage as reasons to reject canned goods.