Rescues · TO CAN
A bulging lid, spurting contents, or any off smell on opening.
Why it happens
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.
Recovering it
A jar that failed to seal within twenty-four hours can be reprocessed with a fresh lid, or simply refrigerated and used. Anything suspected of spoilage is discarded unopened and untasted — the usual kitchen instinct to check by smelling or tasting is exactly wrong here. Follow tested times and pressures rather than adapting a recipe, since the processing schedule is calculated for a particular density and acidity and cannot be reasoned about by feel.
The mechanism
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 TO CAN lesson — how to tell, where to practise it, what it carries to.