By Robert L. Wolke
Wednesday, March 14, 2007
This morning when I was loading dirty dishes into the sink, I poured last night's unconsumed red wine (a local Maryland Chambourcin) onto some fried egg white and it turned blue! For fun, I then mixed a little of the wine with raw egg white, and it showed only a hint of blue. I am dying to know the chemistry between the wine and the eggs.
Surprising as that must have been, there is a simple answer. Red wine is what chemists call an acid-base indicator. It was the wine, not the egg white, that changed color.
You've heard people talk about a "litmus test" to determine whether a politician is on the "right" or "wrong" side of an issue. Litmus, a dye obtained from a lichen, turns red under acidic conditions and blue under basic (often called alkaline) conditions. Thus, by its color the litmus indicates which of those two opposing chemical types a substance belongs to. (The rumor that litmus turns red in Republican states and blue in Democratic states is without foundation.)
Similarly, the colors of many foods, including grapes, cherries, berries, plums, eggplant and cabbage, are due to pigments called anthocyanins, many of which behave as acid-base indicators. Red wines are acidic enough so that their anthocyanins are in their acidic form, which is -- surprise! -- red. But if you were to add an alkaline substance such as baking soda to red wine (not recommended), it would turn purple.
The anthocyanins in red cabbage can be prevented from changing from acid red to alkaline purple during cooking by adding vinegar, wine or sour apples, as is commonly done in German kitchens.
Raw egg white (albumen) is slightly alkaline, so it can change wine's anthocyanins to their alkaline color, which is a purplish blue. When cooked, egg whites become even more alkaline. That's why you saw darker blue on the cooked whites than in the raw ones.
My wife tells me that when she was a little girl she used to make fried egg and jelly sandwiches, and that after a while, the dark red jellies or jams always turned the egg white blue. So I hurried to the kitchen, fried an egg, placed it on a slice of bread and topped it with black raspberry jam. Sure enough, wherever the egg white was in contact with the jam, the jam's stain slowly turned an unappetizing, muddy, purplish blue color.
The egg white, by the way, doesn't have enough alkalinity to react with the whole blob of jam, just the layer on its surface. Nor is it alkaline enough to affect the color of larger quantities of wine, such as with wine-poached eggs.
I prepared some meatballs by mixing ground beef, an egg, oregano, thyme, fresh bread crumbs and red onion. Next day, I threw a few into a ragu and topped some spaghetti with it. The following day, I took the meatballs out of the fridge and all the onions hugging their outsides were green! I have never seen onions turn green after two days in the fridge. Is there a logical explanation?
Of course. It's just another case of an acid-base indicator.
The anthocyanins in red onions are pale red in an acidic environment and green in an alkaline environment. As your meatballs aged, their proteins began to decompose slightly, liberating small amounts of alkaline amines, which turned your onions green.
Happy St. Patrick's Day.