The Fruit and Vegetable Blues
Red onions—as well as other red produce, including cabbage and cherries—are rich in pigments called anthocyanins. When they’re cooked with acid, their color intensifies, but when combined with an alkaline component, they can turn a startling bluish-green color. Since eggs—specifically egg whites—are basic, ranging from 7.6 to 9.5 on the pH scale, they are most likely responsible for the blue color in your frittata.
Just for fun, we decided to see if we could reverse the color change once a fruit or vegetable with anthocyanin turns blue. We sautéed red cabbage and then added a pinch of baking soda to turn it blue. We found that a splash of vinegar brought its red color right back. This trick may not have a practical application, but it might impress your friends.