Aluminum and copper, commonly used materials in pots and pans, react with (and alter the flavor and color of) acidic foods. Always use pans with a stainless steel or glass cooking surface to avoid sour tomato sauce, discolored green beans, and “off”-tasting soups. For the lightweight and even heat of aluminum and copper, combined with the nonreactive property of steel, buy aluminum or copper alloy pots clad to a steel “jacket” (inner lining).