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Black residue after soaking walnuts

I often soak walnuts and pecans in saltwater overnight before toasting them in the oven. When I do this in metal mixing bowls, I notice a black residue on the side of the bowls. What is this?

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hardlikearmour is a trusted home cook.

added over 1 year ago

I can't say for certain, but my suspicion is the tannin from the walnuts is reacting with iron ions in the metal to create iron tannate. You can actually make a dark brown to black ink or dye using walnut hulls (higher in tannin than the nut itself) and rusted nails (or the like).

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