Question

why substance like curd and Pickle not be kept in brass and copper vessels

Answer 1

Because curd and pickle has sour substances contain acid .They would react with the surface of metal vessels such as that of brass and copper to produce harmful substances . Curd and sour things have acid in them. So it is advised not to keep dour food in copper or brass vessels.

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Answer 2

$<v> Curd and other sour substances contain acids. Therefore, when they are kept in brass and copper vessels, the metal reacts with the acid to liberate hydrogen gas and harmful products, thereby spoiling the food.$