Question

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

Answer 1

Curd and sour substances contain acids. Therefore, if we keep such food substances in brass and copper vessels, they will react with the metal to form poisonous metal compounds and liberate hydrogen gas thereby causing food poisoning and corrosion of the vessels.

Answer 2

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.