When it goes wrong · Can
A bulging lid, spurting contents, or any off smell on opening.
Can you save it?
Reviewed for this symptom
Discard it unopened where animals cannot reach it, and do not taste it, smell it closely or open it to check. Wash anything it touched with hot soapy water. This is the one kitchen failure that can be fatal.
Safety boundary: Never taste to check. Suspected botulism means discarding the jar unopened and cleaning surfaces; symptoms after eating home-canned food need immediate medical attention.
How to confirm the diagnosis
Observable cues, not inferred user state
- A domed or bulging lid
- Liquid spurts when opened
- Any off smell or foaming
Not this?
- A lid that flexes or pops when pressed.
- Liquid siphoned out during processing.
- Cloudy liquid in a vegetable jar.
What actually happened
Only this symptom
Spoilage, potentially botulism. Discard without tasting, and dispose of it so animals cannot reach it. This is the one kitchen failure that can genuinely be fatal.
The full mechanism
Shared by every can failure
Canning is the one kitchen technique where the safety margin is not a matter of taste, and the reason is Clostridium botulinum. Its spores survive boiling water indefinitely and germinate in exactly the conditions a sealed jar provides — no oxygen, moisture, moderate temperature. What determines the method is acidity. Below pH 4.6 the spores cannot germinate, so high-acid foods — most fruit, properly acidified tomatoes, anything in a vinegar brine — are safe processed in a boiling water bath at 100°C. Above pH 4.6, which covers all vegetables, meats, and unacidified tomatoes, boiling water is not sufficient at any duration, and a pressure canner reaching about 116°C is required. The seal itself is formed by physics rather than by the lid: heating drives air out, and as the contents cool and contract they pull the lid down.
The full Can lesson — how to tell, where to practise it, what it carries to.
Next time
Prevent this specific failure
Follow tested recipes exactly — acidity, headspace, time and pressure — and pressure-can every low-acid food.
What right looks like
Shared by every can failure
- The lid becomes concave and does not flex when pressed — that is the vacuum, and it is the only seal indicator that matters.
- A distinct ping as each jar seals during cooling.
- Headspace as specified, usually a centimetre or two — too little and the contents force the seal, too much and air remains.
- Rings are removed after cooling, so that a failed seal is obvious rather than held in place by the band.