Question

why should curd and sour substance not be kept in brass and copper vessels

Answer 1

Because brass and copper vessels react with curd and sour substances and makes it harmful for us to consume

Answer 2

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 isnot advisable to keep curd and other acidic or sourfood items in copper or brass vessels.