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Hi, Chef James,
At the moment, I'm viewing turquoise garlic on my pot roast, and I'm curious about what kind of chemical reaction occurred.
I put my put roast in a crock pot, have it seasoned with coriander and salt, and I have smeared on a paste of chili sauce, Worcestershire sauce, a bit of flour and Dijon mustard.  I sprinkled on some chopped garlic, and after cooking a couple of hours, I checked on it, and there's the turquoise-colored garlic!  Pretty, but bizarre!
I hope you can explain what happened!  Thanks so much!
Julie W.

Hi Julie,
Blue or green garlic can have numerous causes.
Garlic contains sulfur compounds which can 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. 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.
If it is picked before it is fully mature and hasn't been properly dried it can turn and iridescent blue or green color when in the presence of acid.
A reaction between garlic's natural sulfur content and any copper in the water or in the iron, tin or aluminum cooking utensils can sometimes change the color of garlic.
Garlic will also turn green (develop chlorophyll) if exposed to an temperature change or is exposed to sunlight. Some people say it can be stored for 32 days at or above 70 - 80° F to prevent greening (but I'm not yet sure that is true).
Other reasons to cause garlic to turn blue or green:
The iodine in table salt may also cause garlic to turn blue or green.
Different varieties or growing conditions can actually produce garlic with an excess natural bluish/green pigmentation made more visible after exposure to acid (vinegar, lemon juice, etc)
Don't worry, greenish-blue color changes aren't harmful and your garlic is still safe to eat. (unless you see other signs of spoilage).

     Chef James
"The duty of a good Cuisinier is to transmit to the next generation everything he has learned and experienced."
Fernand Point, 1941



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