Home canning and botulism

Why

Spores of the bacterium Clostridium botulinum survive boiling, and in a moist, low-acid food without air, which is exactly what a sealed jar is, they can germinate and produce botulinum toxin; only acidity at pH 4.6 or below reliably holds them back. Acid foods such as most fruit, jams, sauerkraut and cucumbers in a sufficiently acidic brine therefore only need heat up to 100 °C, whereas low-acid foods, meaning vegetables including mushrooms, meat, fish and milk, must reach 116–121 °C in the jar, which only a pressure canner can do. In a boiling water bath the same destruction of spores would take 7–11 hours, so neither boiling the jars repeatedly in a pot nor turning them upside down nor heating them in the oven or dishwasher makes a low-acid food safe. Tomatoes are borderline in pH and are therefore acidified with lemon juice or citric acid for canning, as the recipe states. Time, temperature and the amount of acid are measured for a specific recipe and jar size, and because the toxin cannot be detected by look, smell or taste, canning follows a tested method without changes.

How to do it

• Can only from a tested recipe for that food and jar size, and do not change the amount of vinegar, sugar or vegetables; brines for the fridge are covered by the Quick vinegar pickles card.
• Process fruit, jams and acidic brines in a water bath at least 3 cm above the lids and start timing when the water reaches the recipe temperature; above 300 m, extend the time as the recipe says.
• Cucumber pickles can be pasteurised for 30 minutes at 82–85 °C instead of boiled, but only from a recipe that says so; watch the temperature with a thermometer, it must not fall below 82 °C.
• Can vegetables, mushrooms, meat and fish only in a pressure canner with a gauge, following the recipe; a small kitchen pressure cooker is not designed for this, so without a canner freeze them.
• Let processed jars cool undisturbed, lid up, for 12–24 hours, then check that the lid curves inwards, and store them in a cool, dark place.
• Put a jar whose lid has not sealed in the fridge and use it within a few days, or reprocess it within 24 hours with a new lid.

If it goes wrong

Do not taste from a jar with a bulging or loose lid, or one that leaks, foams, shows mould or smells unnatural: wrap a sealed one in a strong bag and throw it away, wipe up spilled liquid wearing gloves and disinfect the surface with a chlorine product, leaving it on for 30 minutes. Low-acid vegetables, mushrooms or meat canned without a pressure canner are better thrown away, even if they look fine. If blurred or double vision, drooping eyelids, or difficulty swallowing or breathing appear after eating, call emergency services at once.