Question

why is it not to use copper or brass vessels to store pickle or curd​

Answer 1

Because curd contains acid and this acid reacts with metal and produce toxic substances that's why curd or pickles are never store in metal vessels.

Answer 2

curd consists of lactobacillus acid .if it is kept in copper or brass a chemical reaction may take between the acid and metal and it turns the curd into dangerous chemical so it is not stored in metal containers like brass or copper..