Raw Red Onions turn blue...why?

Joined Dec 5, 2015
Where I work, we slice red onions on a meat slicer. They sometimes turn blue, they don't smell bad, just strong.

Maybe we hold them for too long...maybe the onions themselves are mature...is the slicer blade too dull? Maybe we are slicing them too thin, for the application?

Any ideas?



Staff member
Joined Mar 29, 2002
Usually this is an acid/base reaction with red onions or red cabbage.
In this particular case it's probably oxidation and some trace iron from the blade. Or at least the blade is catalyzing the reaction.
Joined Dec 1, 2015
French onion soup with red onions is delicious but will turn a weird grey-blue during cooking.  It's an acid base imbalance that is corrected by a 1-3 teaspoons of vinegar.  Best of all the vinegar can add a flavor, like sherry or balsamic.
Joined Aug 21, 2004
Red onions can be prone to blue mold rot depending upon how picked and or stored. It shows up as dry powdery spots especially on the skins and outside layers. Care needs to be taken to make sure that these layers are removed before cutting and prepping the onions. Otherwise when you slice you are exposing the remaining layers to the fungus.

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