How do I tell if food has gone off?

Judging whether food is still safe — what look, smell and dates actually tell you, and where judgement is not enough.

Difficulty: beginner
Time: 15 min
Read: 2 min
Safety: caution

Short answer

Use dates for the food types where they matter and your senses for the rest. Use-by dates on meat, fish and dairy are safety dates and should be respected. Best-before is about quality. But some dangerous spoilage is undetectable, so when the storage history is unknown, throw it away.

Two different questions get confused here: is this still nice, and is this still safe. Your senses answer the first reliably and the second only sometimes, because several dangerous bacteria and toxins produce no smell, taste or visible change at all.

Safety

Some pathogens and toxins are undetectable by smell, appearance or taste. Food left at room temperature for hours, food past its use-by date, and anything with an unknown storage history should be discarded regardless of how it seems. Never taste food to test whether it is safe. If in doubt, throw it out — the cost of the food is trivial against food poisoning.

Step by step

Check whether the date is a use-by or a best-before.Use-by is a safety date on perishable food and should not be exceeded. Best-before is about quality and can often be passed.
For meat and fish, trust smell and texture as well as dates.Sour or ammonia smells, sliminess, or a dull grey-green colour all mean discard. But fresh-smelling meat past its use-by date is still not safe.
For dairy, smell it.Sour milk is obvious. Yoghurt with a lot of separated liquid is usually fine; mould or fizzing is not.
For hard cheese, cut mould off generously with a clean knife.At least a centimetre around and below. For soft cheese, and any mouldy soft food, discard the whole thing — the growth extends far beyond what is visible.
Discard any bread with mould rather than cutting it off.Bread is porous and the growth extends well beyond the visible patch.
For eggs, use the float test and then smell.Fresh sinks flat, older stands upright, off floats. Crack any doubtful egg into a separate bowl — a bad egg is unmistakable.
For anything cooked, respect the two-day rule and reheat only once.Cooled quickly, refrigerated, eaten within two days, reheated until piping hot throughout.
Discard anything left out overnight, regardless of appearance.Some bacterial toxins are heat-stable and produce no detectable change.

Tips

Cooked rice is the strictest case: refrigerate within an hour, keep one day, reheat once. The bacteria involved produce toxins that reheating does not destroy.
Bulging or leaking tins should be discarded without opening — that indicates gas production inside.
Freezer burn is a quality problem rather than a safety one. The food is safe; it will just taste dry and dull.

Common mistakes

Tasting food to check whether it is safe — Some toxins are dangerous in tiny quantities and produce no warning taste.
Treating use-by as a suggestion — It is a safety date on perishable food, based on how quickly harmful bacteria can grow.
Cutting mould off soft foods and bread — The visible growth is a fraction of what is present in a porous or soft food.

Questions people ask

What is the difference between use-by and best-before?

Use-by is a safety date on perishable foods and should be respected. Best-before relates to quality — food is often perfectly good afterwards, judged by appearance and smell.

Can I cut mould off cheese?

On hard cheese, yes — cut at least a centimetre around and below with a clean knife. On soft cheese, discard the whole thing, as the growth extends throughout.