What you see: Red onions that turn a bluish, greenish color during cooking
What it is: A harmless reaction of a plant pigment to an alkaline (basic) environment
Eat or toss? Eat!
Once upon a time, the green thing in the image below was part of what we call a “red onion.” It was a rich, dark purple-red color, which comes from a plant pigment called anthocyanin.
Then, somewhere in New York, a chef preparing breakfast at a hotel chopped that onion and put it in a salmon scramble, along with eggs, lox and capers.
The end product was delicious, but our friend the red onion went through some kind of identity crisis along the way. Parts of it turned various shade of green and blue. The reason? Something in that salmon scramble was particularly basic (“basic” as in “alkaline” or the opposite of acidic). My money is on the egg whites, which are naturally somewhat alkaline and become more so as eggs age.
So, the scramble's alkaline environment caused the anthocyanins to change to these bluish greenish shades. Sometimes, this reaction has kind of pretty results. The triangle shaped piece of onion pictured below reminds me of a flourite crystal, but I digress.
This reaction isn’t restricted to red onions. Red cabbage, and other anthocyanin-sporting foods, behave similarly. And sometimes metals, including iron, aluminum and tin, can cause these types of color changes. If you’re intrigued, Decoding Delicious offers an in-depth explanation and instructions for playing with food pigments at home.
At the top of this post, you'll see my own experiment. I chopped a red onion and put some of the rings in a bowl with some baking soda and water. I heated them in the microwave and they quickly turned green!
Pigments, pH and beautiful food colors - Decoding Delicious
Anthocyanins - More than Nature's Colors. Journal of Biomedicine & Biotechnology.
pH Stability - American Egg Board
The National Onion Association
On Food and Cooking: The Science and Lore of the Kitchen. By Harold McGee.