Question

why should acidic food items such as curd and pickles not to be stored in metallic utensil?

Answer 1

because metal react with acid and
destroy the food

Answer 2

Hi there!

Curds & Pickles contain large amounts of Acid in the Vinegar used and Citrus juices.

These Acids (foods) will react with metal and the Metallic flavours and compounds would render the foods poisonous (perform Neutralization rxn.)
 
So All containing Acids should be stored in Sterilized Vacuum Sealed Glass jars not Metallic bowls or containers..

Cheers!