Question

Why do curd and sour substances not kept in copper?

Answer 1

Sour substances have citrus acid in them and copper is a metal.
When acid and metal reacts it leads to liberation of hydrogen and hydrogen gas leads to the spoiling of food

Answer 2

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