does this help? (from http://www.apinchof.com/garlicqanda.htm)
I have read about garlic turning green. Mostly, the green is stated to be coming from acid. Over the many years, I have made garlic butter for our crab, and it hasn't turned green. Now, my garlic is turning green, and I don't use any acid. I have not changed my pot or utensil. I agree that it doesn't change the taste, but it's not appealing. I used the same garlic in a tomato base dish, and it didn't change color. Why all of a sudden is the garlic turning green in my butter? TH
This landed in my mailbox recently and may answer your question: "From the Food Safety.com site. Practices -- Cooking. Why does garlic turn blue-green when cooking with butter and lemon? Rating: 97.
Garlic contains sulfur compounds that might react with 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. The other sources of copper might be the butter or lemon juice. The garlic is safe to eat. To prevent this in the future, do not refrigerate garlic and store the bulbs in dry air for 32 days at above 70 F to 80 F before use to prevent formation of the green or blue-green pigments. PREPARED BY: Angela M. Fraser, Ph.D., Associate Professor/Food Safety Specialist, NC State University in July 2004"