State regulators are reminding Iowa bars and restaurants they can't serve beverages in solid copper cups. Iowa's adopted federal guidelines that prohibit food and beverages with a pH value below six...that includes wine, fruit juice and "Moscow Mules" that are typically served in a copper cup. High concentrations of copper are considered poisonous, especially when they come into contact with food and beverages. If the cup's lined with nickel or another metal, drinks are allowed.