Why do red onions turn blue or green when cooking sometimes?

Question

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? – Dr. belisarius Nov 26 '10 at 18:01 — Dr. belisarius, Nov 26 '10 at 18:01
Great northern white beans. – Chris AtLee Nov 26 '10 at 18:23 — Chris AtLee, Nov 26 '10 at 18:23

score 24 · Accepted Answer · answered Nov 26 '10 at 19:12

24

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.

answered Nov 26 '10 at 19:12

talon8

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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 03:34

score 2 · Answer 2 · edited Oct 13 '12 at 11:02

2

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

edited Oct 13 '12 at 11:02

TFD

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answered Oct 13 '12 at 02:18

cfb

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Why do red onions turn blue or green when cooking sometimes?

2 Answers2