If so why is this?
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Susan W is a trusted source on General Cooking.
I cook tomatoes in mine, but only when tomatoes are just a portion of a recipe. I have never noticed a metallic flavor. I would not, however make a tomato/marinara sauce in my CI pan. It's the acid in the tomatoes that supposedly react with the iron. My pan is also quite old and well seasoned. I would avoid tomatoes if your pan is new.
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