Damaged tins: dents, bulges, rust — and never store food in an open one

A tin is not just a container, it is the preservation method. The food inside was sterilised in the sealed can, and it stays safe for years because nothing can get in. Everything on this page follows from that one fact: the risk is not the dent, it is whether the seal is still intact.

This matters more here than in a kitchen cupboard. Tins that live in a van, a rucksack or a crate under a bed get knocked about in a way that supermarket shelves do not, and a stack of tins that has spent a season sliding around a locker is worth a proper look.

Dents: shallow is fine, deep is not

The Food Standards Agency:

Food should remain fit to eat if a can is dented, providing the denting of the can is shallow and there are no other obvious signs that the can is damaged.

if the denting is deep, the can may have a hidden split, hole or break in the seal

— Food Standards Agency, Home food fact checker

So a dinged tin is not automatically a write-off — which is worth saying, because “throw away every dented tin” is both wasteful and not what the guidance says.

What you are actually looking for is damage to the seal: the rim seams at the top and bottom, and the side seam. That is where the can is joined, and it is the part that fails. A broad shallow dent in the middle of a wall is a cosmetic problem. A sharp crease running into a seam is a structural one, and the split it opens can be far too fine to see.

The practical test is what the damage sits on, not how bad it looks.

Rust changes the answer too:

If the dent has caused the can to rust, the food inside should not be distributed or consumed.

Surface rust on the outside of a tin from a damp locker is common. Rust at a dent or a seam is different, because rust is metal being eaten through, and the question is again whether the wall is still continuous.

Bulging tins: do not open them

A tin that has swollen is producing gas, and gas means something is alive in there. The FSA says to avoid

eating food from a visibly bulging can

and notes that

if the can spurts when it is opened, this may be a result of gas build up in the food

The CDC lists the signs to throw food out for:

The container is leaking, bulging, or swollen

The container looks damaged, cracked, or abnormal

The container spurts liquid or foam when opened

The food is discolored, moldy, or smells bad

A bulging tin should go in the bin unopened. There is nothing to be learned by opening it, and opening it aerosolises the contents over your hands and your worktop — in a van, over the surface you will make tomorrow’s breakfast on.

If you bought it bulging, the FSA’s position is that you return it to the retailer.

Why you cannot check by smelling or tasting

This is the part that overrides instinct. From the CDC:

You cannot see, smell, or taste the toxin that causes botulism.

Never taste food to see if it’s safe.

Most of us resolve food doubts by sniffing and then trying a bit. That works reasonably well for ordinary spoilage, which announces itself. It does not work here at all: a tin can contain a lethal dose of botulinum toxin and taste completely normal.

Note also that the CDC list includes “smells bad” as a reason to throw food out — a bad smell is evidence against, but a normal smell is not evidence for. The test only runs in one direction, which makes it useless as reassurance.

When in doubt, bin it. A tin of beans costs about a pound.

Once it is open

Separate issue, worth knowing, and routinely got wrong on the road:

You should not store food in an opened tin can. When a can has been opened and the food is exposed to the air, the tin from the can could transfer more quickly to the food inside.

The FSA notes this does not apply to cans designed to be resealed, such as cocoa or golden syrup.

So half a tin of tomatoes does not go back in the fridge in the tin, and it certainly does not sit on the side in the tin. Decant it. This is a metal migration point rather than a microbiological one — it is about what the food picks up, not about it going off — but the practical advice is the same, and on a trip the decanting also gets you a lid.

The microbiological side of an opened tin is simply that it is now ordinary cooked food with no preservation at all. Once the seal is broken, the tin’s years of shelf life are over and the contents are subject to the same rules as anything else you have cooked — which, without refrigeration, means eat it now. The same reasoning as cooked rice, for the same reason.

Best before is not use by

Tinned food carries a best before date, and the FSA is clear that how long canned food stays acceptable past it

depend[s] on the product, storage conditions and the brand

Best before is a quality date, not a safety date — food may legally be supplied after it. Use by is the safety date, and supplying food after a use-by date is an offence.

For a store cupboard built to sit for a year, that distinction is the difference between rotating stock sensibly and throwing away good food. It does not, however, do anything to the rules above: a tin that is well within its best before date and has a deep dent on the seam is still a tin with a deep dent on the seam.

The check, in order

Worth doing when you load up and again when a stack has been thrown around:

1. Bulging or swollen? Bin it, unopened.
2. Leaking, or sticky where it should not be? Bin it.
3. Deep dent on a seam, or a sharp crease? Bin it.
4. Rust at a dent or seam? Bin it.
5. Shallow dent, seams clean, no rust? Fine.
6. Spurts or foams when opened, smells wrong, looks wrong? Bin it — and do not taste it to decide.