So, I looked in the fridge and the garlic that a housemate had pickled yesterday had turned an eerie turquoise.
It smelled fine, so it was time to google an explanation. And to see whether I would gain some “I-ate-blue-garlic-today” bragging rights.
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.
The other sources of copper might be butter, lemon juice, or vinegar.
So there you have it. If your garlic turns blue, relax, it’s fine. Eat it and revel in the novelty.