Cooking science

Spoilage

Spoilage is what makes food look, smell and taste wrong. Food safety is about whether food can make you ill. They are different questions with different answers, and the most important thing on this page is that neither one predicts the other.

Two different populations

Spoilage organisms are the ones that break food down and announce themselves: they produce off smells, slime, sourness, mould and discolouration. They are why milk smells sour and why meat goes tacky.

Pathogens are the ones that can make you ill. Many of them produce no smell, no visible growth and no change in taste at all. Food carrying a significant population of them can look and smell entirely normal.

So the two facts that follow are both true and both uncomfortable. Food can be badly spoiled and not dangerous — a jar of yoghurt gone sour, an over-fermented kraut, a stale loaf. And food can be dangerous and show nothing at all.

What spoilage looks like

Useful for judging quality. Not a safety test.

Sour smell in dairy

Lactic acid bacteria. Usually a quality change rather than a hazard.

Slime on meat or vegetables

Bacterial growth on the surface. A clear quality signal.

Mould

Visible fungal growth. Some moulds produce toxins; this is a food-safety question rather than a quality one.

Yeasty or alcoholic smell

Fermentation, wanted or unwanted depending on the food.

Rancid smell

Fat oxidation, not microbial at all. Old crayons, putty, wet cardboard.

Discolouration

May be oxidation, may be microbial, may be neither. Browning on an apple is not spoilage.

What this means in practice

• Do not use the smell test to decide whether food is safe. It tells you about quality. It does not detect the organisms that matter most.
• Follow the dates and the storage guidance instead. Use-by dates are a safety instruction; best-before dates are about quality.
• Do not scrape mould off and eat the rest, except where a food is designed to carry it — a blue cheese, a dry-cured salami.
• Sour is not automatically spoiled. Yoghurt, kefir, sauerkraut and sourdough are all sour on purpose.
• Rancid is not microbial. It is oxidised fat, and it is unpleasant rather than a microbial finding.
• When the two questions conflict, the safety one wins. Follow Food Standards Agency guidance rather than your nose.
• When in doubt, throw it out. The cost of being wrong is asymmetric.

Where this changes what a cook does

1. Judge the quality of an ingredient by smell, look and taste — that is what those senses are good for.
2. Judge the safety of an ingredient by how it has been stored and by the guidance, not by how it seems.
3. Treat a ferment that smells sour and clean as working, and one that smells putrid or shows fuzzy coloured growth as finished.
4. Treat spoilage as the sign that quality has gone, and act on it for that reason.
5. Do not reassure yourself that something is fine because it smells fine.
6. Do not throw away perfectly good sour or fermented food because it smells strong.
7. Keep the two judgements separate in your head. That separation is the whole point of this page.

Common questions