Garlic and herbs in oil

Why

Spores of the bacterium Clostridium botulinum are common in soil and therefore on garlic, herbs and vegetables; they are harmless as such, but in a moist, low-acid food without air they germinate and the bacteria start producing botulinum toxin, one of the most potent poisons known. Oil covering raw garlic, fresh herbs or chilli creates exactly these conditions: air cannot reach the pieces and neither garlic nor basil is acidic enough. Heating garlic in oil does not destroy the spores, which survive even boiling, and the toxin cannot be detected by look, smell or taste; a bulging lid or an off smell need not appear. Homemade mixtures are therefore never stored at room temperature, the fridge only slows the bacteria down, so American university extension services give 4 days at most, and only freezing stops growth. Shop-bought garlic oils and pestos are treated by the manufacturer, for example by acidification or by preservatives and heat treatment, and keep only as long as the label says; a homemade mixture has no such protection.

How to do it

• Make pesto, chermoula, garlic or herb oil and similar mixtures only in amounts you will use within 4 days, and keep them covered in the fridge meanwhile.
• For longer storage freeze them, for example in portions in an ice cube tray; thaw them in the fridge and use them as soon as possible.
• Do not leave them standing at room temperature; food with pesto or chermoula should not stand warm for more than 2 hours (Cooling and storing leftovers card).
• Heating garlic in oil mellows the flavour but does not make it safe to keep; oil from roasted or fried garlic also goes in the fridge for the same time.
• Make garlic and herbs acidified for storage in oil outside the fridge only by a tested method; a few drops of lemon or vinegar added by eye do not guarantee safety.
• Store shop-bought garlic in oil, garlic oil and pesto as the label says, and in the fridge once opened.

If it goes wrong

Throw away homemade oil with garlic or herbs that has stood at room temperature, and any mixture that has been in the fridge for more than 4 days; do not taste or reboil it, as neither look nor smell gives anything away. Do not open a bulging or leaking jar or package; wrap it in a bag and throw it away. If blurred or double vision, drooping eyelids, or difficulty swallowing, speaking or breathing appear after eating, call emergency services at once: botulism needs antitoxin quickly.