Why does garlic occasionally turn green? ~ Hindy Zirkind
Hindy- here are the results of my research: 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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~ Gloria Kobrin of “Kosher Cookbook”