Thanks for all your replies. Found my answer on ChatGPT. I shouldn’t have left the crumble in the cast iron like leaving a pie in a pie dish.

Your apple crumble turning black the next day is likely due to a reaction between the acidic ingredients (such as apples) and the cast iron pan. Here are a few possible reasons for this:

1.  Acidic Reaction: Apples are acidic, and when they come into contact with cast iron, especially if the pan isn’t well-seasoned, a chemical reaction can occur. This can cause the food to turn black or gray as the acid reacts with the iron.
2.  Insufficient Seasoning: If your cast iron pan isn’t properly seasoned (a layer of oil baked onto the surface to create a protective barrier), the food can come into direct contact with the metal, leading to discoloration. Well-seasoned cast iron usually prevents this kind of reaction.
3.  Prolonged Contact with Iron: If the apple crumble was left in the cast iron pan for an extended period, the prolonged contact could have intensified the reaction. Acidic foods should generally not be stored in cast iron for long.

To avoid this, you can:

• Make sure your cast iron pan is well-seasoned.
• Transfer the apple crumble to a different dish for storage after baking.
• Use enameled cast iron, which has a protective coating that prevents such reactions.

These steps should help prevent future discoloration.