Question

Why should curd and sour substances not be kept in brass and copper
vessels?​

Answer 1

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when kept in copper or brass vessels,curd(and other acidic food items) reacts with copper and forms toxic compounds which may be unfit for human consumption,and may cause food poisoning. thus it is not advisable to keep curd and other acidic or sour food items in copper or brass vessels.

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Answer 2

It is so because Copper and Brass React with the Sour (acidic) substance...