Garlic and herbs in oil

A jar of garlic steeping in olive oil is one of the most useful things in a cook’s armoury, and on this site it is a particularly tempting one. Fat and aromatics are how shelf-stable food stops tasting like rations. An infused oil is cheap, keeps a long time by appearance, needs no refrigeration by appearance, and makes almost anything better.

It is also, made the obvious way and left on a shelf, one of the few genuinely dangerous things a home cook can produce.

Why this specific combination

Two ordinary properties that are harmless apart:

Oil excludes oxygen. Submerging something in oil creates an anaerobic environment — which is precisely the condition Clostridium botulinum requires and most spoilage organisms do not.

Garlic and herbs are low-acid. There is nothing in them to suppress bacterial growth.

From UGA Extension’s guidance on infused oils:

Oils provide an anaerobic environment, and when combined with low-acid food matrices such as garlic and herbs, this provides the perfect conditions for specific bacteria to thrive.

— Carla Luisa Schwan, How to Safely Make Infused Oils, UGA Extension C1334

The spores are already present. They are not a contaminant you can avoid by being clean — per the FDA’s Bad Bug Book, the organism and its spores “are widely distributed in nature… found in both cultivated and forest soils”. Garlic is a bulb that grows in soil. Washing does not remove spores, and the spores are heat-resistant.

The dangerous part is what happens next. Nothing about the jar changes:

Often, there’s no visible sign that a food is contaminated.

No smell, no cloudiness, no fizz. The usual sensory checks that keep people safe around other foods do not operate here at all.

What botulism actually is

Worth being plain about, because the risk is small but the outcome is not.

The symptoms, per the same source:

Initial symptoms may include double vision, blurred vision, drooping eyelids, slurred speech, difficulty swallowing, dry mouth, and muscle weakness. If the disease is not treated, symptoms may progress to paralysis of the arms, legs, trunk, and respiratory muscles.

And recovery, when it happens:

Patients with severe cases that involve paralysis of the respiratory muscles may need mechanical ventilation and intensive care for weeks or months.

The onset window is worth holding onto. Eighteen to thirty-six hours means the jar you suspect is not the meal you just ate — it is yesterday’s. If you are somewhere remote and the early signs are double vision and difficulty swallowing, you want to be moving toward help early rather than waiting to see whether it passes.

What to do instead

Refrigerate it, and use it within four days. UGA’s guidance for infusions made without acidification is that they must be kept refrigerated and discarded four days after being made, or frozen for longer storage.

Which, on this site, mostly means: don’t make it. Four refrigerated days is not a store-cupboard ingredient, and if you had reliable refrigeration you would not be reading a site about cooking from long-life ingredients. Infused oil is a thing you make at home, in a fridge, for the week.

There is a tested method for shelf-stable infused oil, and it is real: UGA describes acidifying the garlic or herbs to a pH of 4.2 or less within 24 hours using a citric acid solution, after which the oil can be kept at room temperature. Two points matter more than the rest of it:

Do not replace the citric acid solution with vinegar or lemon juice, as they have not been tested.

and the fact that it is a measured procedure with a pH target, not a ratio you can eyeball. We do not reproduce the method here — home preserving instructions are deliberately out of scope for this site, and a procedure like this should be followed from its source, not from a recipe blog’s summary of it. If you want to do it, read the original.

Commercially prepared garlic-in-oil is a different product. It is acidified and validated as part of manufacture. Buying it is the straightforward answer.

Getting the flavour another way

The point of garlic oil is garlic flavour carried in fat. There are routes to that which raise none of this:

• Cook the garlic into the dish. Fresh or dried garlic bloomed in hot oil at the start of cooking gives you the same thing, in the pan, eaten the same day. This is the answer almost every time.
• Dried garlic and garlic powder keep indefinitely, weigh nothing, and are a genuinely good store-cupboard ingredient rather than a compromise.
• Chilli oil made with dried chillies is a different proposition from garlic oil, because dried chillies are not a low-moisture-plus-low-acid problem in the same way — but the general caution about what you are submerging still applies, and anything fresh going under oil brings the risk back.

The rule that covers all of it: oil is a storage medium, not a preservative. Putting something under oil does not make it keep. It removes the oxygen, which for this particular organism is help rather than hindrance.