Mystery - added balsamic vinegar to tomato sauce recipe and the onions turned bright blue. Wha' hopp

Discussion in 'Food & Cooking' started by mikelm, Dec 5, 2013.

  1. mikelm

    mikelm

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    My son just made Alton Brown's tomato sauce:

    http://www.foodnetwork.com/recipes/alton-brown/tomato-sauce-recipe/index.html

    He added to the recipe a couple tablespoons of white balsamic vinegar... and the white onions turned a bright blue.

    I'd be interested to know what chemistry might be involved here. Halloween's over, otherwise I might suspect witchcraft.

    Mike  /img/vbsmilies/smilies/confused.gif  
     
  2. soul vole

    soul vole

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    Red onions turn a bright pink when pickled so I'd guess whatever's going on there is happening with the onions you have. I did some searching and couldn't find an explanation, but it seems white onions don't typically change color. I'm guessing there must be something about the variety you have that's similar to red onions.
     
  3. french fries

    french fries

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    Last edited: Dec 5, 2013
  4. phatch

    phatch Moderator Staff Member

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    Well, the vinegar isn't in the recipe. And usually the cyoanthin blue shift happens when you raise the pH above 7 and shifts pink when you go acid with the pH. Adding vinegar shouldn't have raised the pH, but that's the usual culprit.
     
  5. french fries

    french fries

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    Isn't that what happens with red pigments though (red cabbage, red onion)? At least that's what I've noticed myself with red cabbage: add acid and it stays red/purple, not enough acid and it turns blue. I don't believe that's related to MikeLM's experience since he did not start with those red pigments. 
     
  6. meezenplaz

    meezenplaz

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    Sleuthing 101--process of elimination:

    What happens when you put those same onions in a bowl by themselves,

    and add your white basalmic?
     
  7. phatch

    phatch Moderator Staff Member

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    I got the word wrong. It's anthocyanin.   And white onion isn't that far behind the red onion in anthocyanin content--http://ijiit.webs.com/documents/120104-05.pdf   3rd page of the PDF< but labeled as page 27 in the document. 

    Interestingly, it can go green or yellow at the higher pH values. 

    In this older cheftalk thread, Luc H mentions that metallic ions can trigger the color shift too. Did he use an aluminum pan perhaps? 
     
  8. mikelm

    mikelm

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    Thanks all for the info.  I'm pretty sure the pan was SS, but I'll ask him tomorrow.

    Mike