Question

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

Answer 1

Curd and sour substances should not be kept in brass or copper vessels because curd and sour substances reacts with copper and forms toxic compounds and these compounds may be unfit for human consumption and may cause food poisoning.

Answer 2

curd has lactic acid
sour substance such as lemons have citric acid.

we know acids reacts with active metals to give salt and hydrogen

therefore when curd and and sour substance are kept in bass and copper vessels they react with the elements forming respective salt and hydrogen . so we should not keep them in brass and copper vessels.