The warning to discard swollen cans is widespread but the reason usually goes unexplained. Once the reason is known the rule gets taken far more seriously.
Why does swelling happen?
The pressure in a sealed can should be constant.
After sterilisation there is a slight vacuum inside; the lid is drawn inward.
A lid domed outward means gas has been produced inside.
That gas has four possible sources.
The first is microbial growth. Bacteria produce gas as they metabolise the product.
That is the commonest and most dangerous cause.
The second is chemical reaction. Acidic products can react with the metal and produce hydrogen.
That appears in acidic cans that have stood a long time.
The third is freezing. When a can freezes the contents expand and push the lid.
The fourth is overfilling, a production fault.
There is no way to distinguish those four from the outside.
A swollen can should therefore be discarded in every case.
What is the botulism risk?
That is the most serious cause of swelling.
Clostridium botulinum is a bacterium that develops without oxygen.
A can provides exactly that environment.
When sterilisation is done properly the spores are killed.
But where the process fell short, or the can was damaged later, spores can survive.
Under suitable conditions they multiply and produce gas.
They also produce a toxin.
That toxin is extremely potent and affects the nervous system.
Its most dangerous property is leaving no sensory trace.
No smell, no taste, no change in appearance.
The "let me smell it" approach is useless here.
The only external sign is the swelling of the can.
When is a dented can risky?
That distinction is finer.
Not every dent is dangerous.
What decides it is where the dent sits.
Soft, non-sharp dents on the body are generally not a problem.
The metal deforms while the seal survives.
Dents at the seam are critical.
The folded join at the top and bottom rim is what provides the seal.
Damage there can create a microscopic opening.
Air and microorganisms enter through that opening.
The leak may be invisible.
Sharp, deep dents are risky too; the metal can crack.
If the can moves under finger pressure at the dent, the seam has been damaged.
| Defect | Risk | Decision |
|---|---|---|
| Lid domed outward | Very high | Discard without question |
| Dent at the seam | High | Discard |
| Deep, sharp dent | High | Discard |
| Soft dent on the body | Low | Usable |
| Surface rust | Low | Clean and use |
| Perforating rust | Very high | Discard |
When does rust become a problem?
Rust on the outside comes from humid storage.
Surface rust is generally not a problem and can be cleaned with a dry cloth.
But deep rust thins the metal.
If the metal flexes or collapses under pressure at the rusted area, the can is about to perforate.
In a perforated can the contents have met air.
Rust on the inside is more serious; that shows only after opening.
Where the internal lining is degraded the product should not be used.
Storage conditions should be managed to prevent rust.
Humidity should stay below 60 percent and cans kept off the floor.
They should not sit anywhere water might drip onto them.
What should be checked after opening?
Even where a can looks sound, checking continues once it is opened.
Five signs are looked for.
The first is sound. A properly sealed can pulls air inward with a distinct hiss when opened.
The absence of that sound means the vacuum is gone.
The second is liquid spurting. Liquid escaping on opening means there was pressure inside.
The third is smell. A sour, yeasty or off smell is suspect.
The fourth is appearance. Foam, bubbles or cloudiness indicates microbial growth.
The fifth is colour. Marked darkening or an unusual colour is suspect.
Where even one of those signs is present, the product should not be used.
Tasting should never be used as a check.
How is a suspect can disposed of?
This is an overlooked subject.
Care is needed when discarding a swollen can.
It should not be opened; the contents can spray during opening.
It should go into a bag, the bag tied, and then discarded.
The contents should not be poured down a sink.
Hands should be washed with soap after touching the can.
The can should not be left in a fridge or on a kitchen bench.
In a business the suspect product should be set aside and labelled.
A "do not use" label should go on it and the incident recorded.
The supplier should be informed too; other cans in the same batch may be affected.
The batch number should be noted.
What should be checked when buying?
Inspection begins at the shelf.
Four things get checked when a can is picked up.
The first is the lid: drawn in, flat or domed outward.
The second is the rims and seams: any dents or deformation.
The third is the surface: rust, scratches or signs of leakage.
The fourth is the label; a torn or damp label suggests the can was damaged.
Cans on lower shelves take more knocks; those deserve careful inspection.
Dented cans in a discount section may be cheap but carry risk.
A soft body dent is acceptable; a seam dent never is.
Care is needed when stacking cans at home too; stacking too high deforms the ones underneath.
How is a suspect can safely discarded?
That is an important step for safety.
A swollen or leaking can should not be opened.
The contents can spray during opening and contaminate the surroundings.
The can goes into a bag and the bag gets tied tightly.
It then goes into a second bag before being discarded.
That provides a double barrier.
The contents should not be poured directly into a sink or a bin.
Surfaces that touched the can get cleaned with soapy water.
Hands get washed thoroughly.
Where other cans from the same batch exist they get checked too.
Where the problem appears in more than one can, reporting it to the retailer and where necessary to the authorities is the right approach.
Keeping the batch number before disposal is worthwhile, since that number is the first thing asked for in any report.
