We must be careful to distinguish between garlic oil, the intrinsic essential oil of the garlic plant, Allium sativum, and garlic-infused oil, an edible vegetable oil (usually olive oil) that has been flavored with garlic.
Pure garlic oil is indeed nasty stuff that is never ingested per se.
One of its major ingredients is allyl trisulfide, a teaspoon of which in its pure form would kill half the people who swallowed it and burn the esophagus in the rest of them. But you could never eat enough garlic to come within miles of that amount, nor would anyone come within miles of you if you did.
When chemically pure garlic oil is required for non-food purposes (it is an effective antibacterial, antifungal, and insecticide), it is obtained, as are most plant essential oils, by steam distillation, in which the crushed plant material is boiled in water. The resulting mixture of steam and vaporized oil is condensed, whereupon the water and the oil settle out as separate layers.
All right, then. So is garlic-infused olive or other vegetable oil dangerous? It depends on how you make it. If you add garlic cloves willy-nilly, unpeeled or peeled, whole or minced, to oil and let it stand for weeks at room temperature, yes, you’re flirting with botulism.
The lethal Clostridiurn botulinurn bacterium lives in the soil and in stream and lake sediments, among other places. It cannot flourish in extreme dryness or in air, but will thrive in a moist, airless (anaerobic) environment. And exactly those conditions can exist on the surface of a moist garlic clove smothered in oil.
Many references tell us that C. botulinurn bacteria can be killed by being heated for 10 minutes at a temperature above 175°F ( 79°C) or that they can at least be inhibited by acidic media below pH 4.6. That’s true of the active bacteria themselves, but their dormant spores, if present, can survive long periods of highly unfavorable environments such as air, dryness, and high temperatures.
In fact, the 175-degree treatment may only “heat-shock” the spores into germinating more readily. The spores are not reliably killed until subjected to a temperature of 25o°F (121°C) for several minutes, as is done in commercial canning. At home, that temperature can be reached in either an oven or a pressure cooker. But simple boiling or simmering will not do the job.
Executing the bacteria and their spores may be too little and too late, however, because it’s not the bacteria themselves that are the potential killers; it’s a neurotoxin they manufacture while multiplying. Botulinum toxin is one of the most powerful poisons known.
Virtually all references repeat the statement that the toxin is not destroyed by the heat of cooking. But that’s a precautionary oversimplification. The toxin is actually unstable to heat, but it depends on what we mean by “cooking.” Research by several groups of scientists in the 1970s showed that different amounts of toxin, different foods, different pH’s, and different acids can affect the toxin-deactivation process differently. So it is indeed prudent to assume that you can’t get rid of it by “cooking.”
The symptoms of botulin poisoning were named botulisrn when a number of people died in Germany in the late eighteenth century after eating contaminated sausage; the Latin word for sausage is botulus. Botulism is a rare occurrence, with only ten to thirty outbreaks per year in the United States, so there is hardly a galloping botulism plague going on.
But a head of garlic just might have some C. botulinurn spores lurking under its skin, where they would lie protected from air until they found themselves in an airless medium, such as when submerged in a sea of oil. There, the spores could become active and launch a reproductive orgy, even at refrigerator temperatures.
It’s the better part of valor, therefore, not to tempt fate by making your own garlic-infused oil. Commercial garlic-infused oil products are usually acidified with vinegar to thwart bacterial growth. But acidifying a solid in an oil can be tricky, so it isn’t recommended for home preparation of garlic-flavored oil.
Still want to make some? Adventurous types should make only a small amount from chopped garlic in olive oil, keep it refrigerated, and discard what isn’t used after a week or two.
