Black apple crumble!

Question

I used the apple crisp recipe from Joy of Baking and I'm quite sure I followed it to the dot (didn't add blackberries, though, only apples). But for some reason, after I took it out, let it cool a little and cut into it, I saw that the apple had turned a strange purple/black color! These were perfectly good (not rotten or bruised) apples that I bought a few days ago. Any reason that this could have happened?

EDIT: Here's a picture. I managed to salvage this one piece of cooked apple, since I had thrown out the rest of it - sorry for the crappy quality and excuse my face in the background! As you can see, half of it is tinged a strange purple/black!

What type of pan was it cooked in, aluminum magnesium by chance? — moscafj, Jun 21 '18 at 11:11
I'm not entirely sure - but I have used this pan multiple times to bake things! — Apoorva Ramamurthy, Jun 21 '18 at 11:41
Recipe says it has a cup of blackberries in it, could the color be from that? — renesis, Jun 21 '18 at 15:14
How long a time passed between cutting the apples and making the filling? What variety of apple did you use? It's a little hard to tell based on the picture but the color seems to be mostly around the edges - it could be as simple as an exaggerated color from oxidation. — logophobe, Jun 21 '18 at 18:10

dlb · Answer 1 · 2018-06-22T21:07:33.627

Walnuts are most likely your culprit. Black walnuts have more of this tendency than common cooking walnuts, but I have had it occur with both. I especially have seen in with port chops stuffed with walnuts and applesauce getting a very purple stain to them. I have seen multiple theories on why purple, but with walnut breads a common suggestion seems to be that iron reacts with the walnut to cause the purple, and some flours are higher in iron than others.

And idea, but just a suggestion, is toasting the walnuts first might reduce this tendency. In fact, here is an article from Cook's Illustrated https://www.cooksillustrated.com/how_tos/6672-preventing-purple-walnut-bread. They indicate that they believe it to be Gallic acid in the walnuts doing it, and that blanching for 1 minute before using them will reduce the effect.

ETA: Looking up Gallic Acid was interesting. It was used as a key ingredient in "Iron Gall Ink" in the 12th through 19th centuries as a standard writing and drawing ink, giving some idea of its ability to color.

Interesting! That might explain why cardamom powder can darken apple fillings more than it should by the green color alone (cardamom appears to have gallic acid and related compounds too...) — rackandboneman, Jun 22 '18 at 22:19
@rackandboneman I have always found the purple somewhat appealing, but have seen it go to black, which is not so much for me. But have seen zero effect on taste. I always knew walnuts were the source, but did not know the gallic acid part. My first thoughts were tannin, which I guess gallic acid is! — dlb, Jun 24 '18 at 00:18

Black apple crumble!

1 Answers1