It’s happened to me during classes I’ve taught where an acid, like lemon juice is added to a pan that has garlic sautéing in it. (Just ask my sous chef, Kay Hitzler, who was certain it was my chemistry and hers that somehow turned the garlic blue – yep, it seemed to happen only when she was my sous chef…).
Anyway, I’ve also had garlic turn blue when pickling veggies. The first time it happened, I threw all the lovely, tasty pickled pepper rings out – I was sure I’d poison somebody if they ate them.
Come to find out, though, that when garlic turns blue or green it’s a harmless reaction due to the garlic’s sulfur compounds reacting sometimes to a mineral like copper. or an acid like lemon juice, or even a fat, like butter. Research shows that the amount of copper need is very small and frequently found in normal water supplies. (I think that’s the culprit with me).
That’s why when the garlic turned a tiny bit bluish in my pickled green tomatoes (see the photo) I’m not going to worry about it.
More good information can be found on the Food Safety site. Check it out!