The danger can begin before the jar is sealed
Botulism is caused by a nerve toxin made by Clostridium botulinum. The bacterium can leave behind spores—tough, dormant forms that are widespread in the environment and can survive ordinary cooking. If spores remain in food and later encounter conditions that let them grow, the bacteria can produce toxin. The toxin disrupts signals between nerves and muscles, potentially causing paralysis and making it hard to breathe.
A jar does not have to look spoiled to be dangerous. The toxin may be present without an unusual smell, taste, or appearance, so tasting a questionable preserve is not a safe way to check it.
Why vegetables need a different process
Acidity is one barrier to the bacteria’s growth. Many vegetables are low-acid, so they cannot reliably provide that protection on their own. Their spores must instead be controlled with enough heat throughout the food. Pressure canning reaches temperatures higher than boiling water can, making it suitable for properly processing low-acid foods.
A boiling-water bath can seal a jar, but a tight seal is not evidence that its contents received the heat treatment they needed. The seal only shows that the lid closed; it cannot undo spores that survived processing.
How one jar becomes the exception
Home-canned jars in the same batch can differ: one may have been filled or processed differently, or may not have received the required heat for its contents. That is why safety depends on following a tested recipe and the specified pressure-canning process—not on whether the lid “popped” or the jar looks normal. The critical protection happens inside the food, before the jar is stored.
