Did you know that when you add raw garlic to an acid, it turns blue/green?
It does. I discovered this while making a spicy pineapple vinegar to go with some shrimp & white bean cakes. Not being very well versed in the science, it is best I just cut & paste rather than try to explain the reason for this phenomenon:
Garlic is known to contain sulfur compounds which can react with minute traces of copper to form copper sulfate, a blue or blue-green compound. The amount of copper needed for this reaction is very small and is frequently found in normal water supplies. Raw garlic contains an enzyme that if not inactivated by heating reacts with sulfur (in the garlic) and copper (from water or utensils) to form blue copper sulfate. The garlic is still safe to eat.
Now you know.