If you’ve ever made tomato sauce in an aluminum pan, you’ve probably discovered why nonreactive materials are important when cooking acidic ingredients. Aluminum and copper are favored in the kitchen as good conductors of heat, but the presence of acids causes a chemical reaction that gives the food off-colors and metallic flavors. We’ve even seen aluminum foil dissolve partially when used to cover a tomatoey lasagne.

Cookware manufacturers have come up with several ways to take advantage of aluminum and copper’s excellent conductivity while overcoming their reactive qualities. Lining reactive pans with a nonreactive interior surface (like stainless steel, enamel, or tin) is one such strategy. Another approach is to give a stainless-steel pan an exterior base or enclosed core of copper or aluminum. Aluminum can also be made nonreactive through an electrochemical process called anodization. Anodized aluminum pans are usually very dark gray or black.

While cast iron itself is reactive, a good seasoning is often enough of a barrier to prevent the chemical interaction. Ceramic and Pyrex bakeware are also nonreactive.