Garlic bulbs can pick up the bacteria that cause botulism from the soil. Storing garlic or any other low-acid vegetable in oxygen-free conditions at room temperature can encourage the growth of the toxins responsible for food-borne botulism, a dangerous illness. Eating raw garlic doesn't pose a hazard, but storing it for later use could, if you mix it with oil and keep it at room temperature.
Clostridium botulinum is a spore-forming bacterium commonly found in nature. When conditions are right, it will grow and produce a potent neurotoxin. The bacterium requires an oxygen-free environment, moisture, room temperature and low acidity to thrive. Garlic is just one of many foods that have been blamed for outbreaks of food-borne botulism. Improperly canned goods, smoked fish and even baked potatoes wrapped in foil have harbored the dangerous toxin.
Illness Due to Botulism
Botulism infection is rare, but very dangerous. Only a small amount of the toxin produced by C. botulinum can make you very sick. Symptoms include blurred vision, dizziness, difficulty in swallowing or speaking, and paralysis. Left untreated, it can be fatal.
Garlic and Botulism
Botulism spores occur commonly in nature, and are found in soil. Because garlic bulbs grow underground, they can easily pick up botulism spores. C. botulinum on garlic doesn't pose a threat until it grows in an airless, moist, warm environment, producing neurotoxin.
The Problem with Garlic in Oil
In the 1980s, health officials traced several outbreaks of food-borne botulism to improperly stored garlic in oil. Mixing garlic with oil cuts off the air supply to the garlic, creating the anaerobic conditions that botulinum spores love. After the outbreaks, the U.S. Food and Drug Administration required food processors to add acids to garlic-oil blends to inhibit the growth of bacteria.
Eating raw garlic on its own doesn't pose a threat of botulism, since it's exposed to air. Nor does heating garlic in oil, or mixing raw garlic and oil and using it right away. To preserve garlic, you can put it in vinegar, which has a high acid content that will inhibit the growth of botulism. However, it's still best to refrigerate it, which lets it keep up to four months.) You can chop garlic and freeze it, plain or with oil. If you're mixing garlic and/or herbs with oil, make a small batch, refrigerate it and use within a week. Never store garlic with oil at room temperature, and never keep it too long in the refrigerator. Chilling the oil slows down, but doesn't entirely prevent, the growth of botulinum toxins. Better yet, use commercially prepared oils with garlic.