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from http://chowhound.chow.com/topics/793133 :
Low-acid foods like garlic can be a source of Clostridium botulinum bacteria which are found in soil, water, and air. Oil's oxygen-free environment is perfect for growth of this anaerobic bacteria. Garlic in oil, therefore, must be stored correctly to prevent botulism food poisoning.
Commercial garlic-in-oil mixtures are acidified to prevent bacterial growth. These products can be stored safely at room temperature. Unfortunately, acidification of garlic in homemade oil mixtures can't be recommended because no research exists to support proper procedures. Different people recommend different methods and time to acidify and it is hard to know who is right. Instead, it's best to store these hazardous oils in the refrigerator, but for a limited time only. This conflicts with the desire for long term storage.
When raw garlic is stored in oil, Clostridium botulinum bacteria can grow. These mixtures must be refrigerated to slow bacterial growth. After 3 weeks of refrigeration, the increased number of bacteria will become a food safety hazard. Therefore, these mixtures should not be refrigerated longer than 3 weeks.
When garlic is immediately removed after flavoring oil, the bacteria will not have a "food source" for growth. The flavored oil can be stored safely at room temperature.
Storage Recommendations: (According to Oregon State University Extension Service).
Raw or cooked garlic and/or herbs in oil:
These mixtures MUST be refrigerated. Do not store them longer than 3 weeks in the refrigerator. (Note. Raw garlic MAY be safely stored in vinegar at room temperature.)
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