If the garlic is not fully ripe and dry, it can turn green in the presence of sunlight or heat due to the formation of a chlorophyll like compound. This same reaction can happen in the presence of acidic foods like vinegar, onions, lemon juice, etc. or with copper found in some utensils and/or tap water. It is harmless and can be fixed by either letting the garlic mature (at room temperature) or heating it more until the proper color is achieved.
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My usual answer to this question is: If the garlic is not fully ripe and dry, it can turn green in the presence of sunlight or heat due to the formation of a chlorophyll like compound. This same reaction can happen in the presence of acidic foods like vinegar, onions, lemon juice, etc. or with copper found in some utensils and/or tap water. It is harmless and can be fixed by either letting the garlic mature (at room temperature) or heating it more until the proper color is achieved.
However, the "over time" portion of your question requires more information. Depending upon how long that period of time was, and if the garlic was not in sunlight or exposed to any of the above mentioned items, I would worry that it may be a type of mold. Garlic stored in oil (and not refrigerated) has been associated with rare cases of botulism, so I wouldn't take any chances in that case. (See the CDC information at this link: http://www.cdc.gov/nczved/dfbmd/disease_listing/botulism_gi.html)
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