Acidic ingredients, such as citrus juice, tomatoes, vinegar, wine and many vegetables, will react with certain metals, including aluminum or cast iron. Although the reaction is harmless, it may turn a mixture gray or leave behind a metallic aftertaste. The same reaction occurs when eggs are cooked in aluminum or cast-iron pans. Therefore, recipes that include these ingredients, such as lemon curd and pastry cream, call for the use of nonaluminum pans. Stainless-steel and enamel-lined pans are excellent choices.