Question

pickles and other source foodstuffs should not be kept in copper and brass vessel​

Answer 1

sour substances are cidic in nature,i.e, they have acids in them. So, when they are kept in brass or copper vessels, they react with them and form poisonous or better to say toxic elements which may prove harmful for human body. So they are not kept in metal containers.

is the answer for your question

Answer 2

because if we should kept these food atoms in copper or brass vessel they does reaction get rancid.

therefore pickles and other source foodstuffs should not kept in copper and brass vessels.