Question

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

Answer 1

when kept in copper or brass vessels,curd(and other acidic food items) reacts with copper and forms toxic compounds which may be unfit for human consumption,and may cause food poisoning.thus it is notadvisable to keep curd and other acidic or sour food items in copper or brass vessels.

Answer 2

sour substances are acidic 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.