Question

Why can't you keep foods like curd in metal containers ?

Answer 1

Acids are those substances which are sour in taste, and which changes blue litmus to red. The acids react readily with metals and produce hydrogen gas along with unwanted compounds which might be poisonous and hazardous to consume. Curd, pickles etc. are such types of food items.

Complete answer:
Curd contains lactic acid while other sour substances like pickles etc. contain citric acid in them, the acidic nature of these food items causes leaching of metal from the container. Although the reaction with different metals is different.
Metals placed above the hydrogen in activity series - In the activity series, when the metal is above the hydrogen, it reacts directly with the acid. For example,
\eqalign{ & 2Al + 6{H^ + } \to 2A{l^{3 + }} + 3{H_2} \cr & Zn + 2{H^ + } \to Z{n^{2 + }} + 3{H_2} \cr}
Ingestion of these metal ions can cause health problems. Aluminum tends to form and protect a coating of aluminum oxide, and zinc forms a stiff coating of zinc carbonate.
Metals placed below the hydrogen in activity series - In the activity series, the metal copper is under hydrogen, so it does not react directly with the acid. The copper pan forms a solid biofilm when in contact with dairy products. However, milk can also dissolve this biofilm and contaminate tofu and cheese with copper. The exudate is not high enough to cause problems for human health, but it can suppress the activity of lactic acid bacteria.

Note:
Not all metals are equal, and not all metals react the same in the presence of different types of food. Today's lactic acid industry allows the use of some stainless steels, such as 304,316,317, in the processing of milk and dairy products, but under very special conditions.