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?

  • Which kind of beans did you use? Nov 26, 2010 at 18:01
  • Great northern white beans. Nov 26, 2010 at 18:23

2 Answers 2


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.

  • 5
    Preparing your own indicator solution with red cabbage juice is a safe and fun little experiment.
    – Nick T
    Nov 27, 2010 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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