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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! enter image description here

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  • What type of pan was it cooked in, aluminum magnesium by chance?
    – moscafj
    Commented Jun 21, 2018 at 11:11
  • I'm not entirely sure - but I have used this pan multiple times to bake things!
    – Apoorva Ramamurthy
    Commented Jun 21, 2018 at 11:41
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    Recipe says it has a cup of blackberries in it, could the color be from that?
    – renesis
    Commented Jun 21, 2018 at 15:14
  • Didn't add them! It contained only apples and walnuts.
    – Apoorva Ramamurthy
    Commented Jun 21, 2018 at 17:56
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    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
    Commented Jun 21, 2018 at 18:10

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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.

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  • 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
    Commented Jun 22, 2018 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
    Commented Jun 24, 2018 at 0:18

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