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I cooked a pot of beans with some red onions last night. Today for lunch when I got them out of the fridge, all of the onions had turned a blue/green colour!

They still taste ok, but sure looks unappetizing!

Any ideas what's going on here?

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Which kind of beans did you use? – Dr. belisarius Nov 26 '10 at 18:01
Great northern white beans. – Chris AtLee Nov 26 '10 at 18:23
up vote 18 down vote accepted

The pigments that give onions the colour behave like a litmus test. They are red in the naturally acidic onion. They turn green/blue in an alkaline environment. It sounds like when you cooked the beans it created that alkaline environment to cause the colour change.

See Here.

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Preparing your own indicator solution with red cabbage juice is a safe and fun little experiment. – Nick T Nov 27 '10 at 3:34

Cooked red onion does turn dark blue-green colour in the fridge, but when you warm it up it turns a reddish colour again.

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