Kitchen Mysteries is a weekly exploration of oddities surrounding cooking and food. They could be recipes that fail when they shouldn’t, conflicting advice from different sources, or just plain weirdness. If it happens in a kitchen, and you’re not sure why, send a tweet to The Food Geek to find out what’s happening.
Megan asks via the feedback form on my web site:
I was baking cookies with blood orange juice on Saturday. When I added the juice (which was a brilliant red) to the butter/egg/sugar mixture and then added the flour the batter turned a blue/grey/green color. Any idea why? The cookies taste good but the visual appeal is terrible. I made an icing with the blood orange juice and it was a lovely pink color.
I’ve been hoping someone would ask about this. A variation on this question got me started on the whole “researching food mysteries” kick. Besides, it’s just so strange!
Any fruit or vegetable with a deep red-to-purple tint, including blueberries, blood oranges, grapes, and red cabbage, contains a class of pigment molecules called anthocyanins. The anthocyanins are sensitive to alkaline pH, so anything that’s not acidic or neutral will cause them problems. The problems, specifically, are that they turn bright green.
In your case, I’m quite certain you used baking soda in your cookies, and you mentioned that you had some eggs as well. Eggs are a little alkaline and get moreso as they age, and baking soda is very alkaline. It makes a certain amount of sense that you’d use the baking soda, because you’re making an acidic dough, and baking soda will neutralize the acidity and provide lift. Generally not a bad thing, though not always necessary. An acidic dough won’t brown as easily as a neutral dough, but the flavor is appealing.
The reason your cookies weren’t uniformly green was probably because of the other pigments, especially in the orange juice, and possibly somewhat to do with the distribution of ingredients.
To solve your problem, all you have to do is either replace out your baking soda with baking powder (use 4 times the amount of baking powder as you would soda, so 2 tsp of baking powder if you’re using 1/2 tsp of baking soda). You should have enough acidity with the citrus to neutralize any eggs you have in there. If not, some more re-working might have to happen.
Another option is to use some other citrus in the cookies and keep the blood orange juice for the icing. Clearly orange juice would be a fine substitution option.
The first time I had been asked about this, it was when someone was making french toast, and the blueberries turned “frankenstein green.” That one was trickier, because there isn’t any baking soda in french toast. As I mentioned, eggs are somewhat alkaline, and there wasn’t much else to balance that except for the milk, so the most likely culprit were those eggs.
This is a great little trick for Halloween, for teaching some basics of chemistry to kids, or for finding creative ways to get house guests to leave just a little bit early. Go crazy, and have fun with it. It’s not so bad once you know what’s happening.