Topic: Metals and Acids. Why is curd not stored in copper containers? Hey, wait. What are you doing? Don’t eat that curd. It was stored in a copper container. Please listen. Fine, then bear the consequences. I had warned you. Do you know why this happened? This happened because curd contains lactic acid in it. When curd is stored in containers made up of metals like copper or brass. The lactic acid present in it, reacts with the metal and forms metallic salts. These metallic salts are harmful. They make the curd unfit for human consumption. Therefore, to prevent metals from reacting with acids present in food. The metal containers are coated with a thin layer of tin. Tin is also a metal. However, it does not react with acids as rapidly as other metals do, thus keeping the food in the container safe.