Iowa and many other states have adopted the FDA Model Food Code, which bans copper (and copper alloys, such as brass) from coming into direct contact with acidic foods that have a pH below 6.0. Examples of these acidic foods include fruit juice, wine, vinegar — and Moscow Mules.
The problem is serious enough that Iowa’s Alcoholic Beverages Division issued a statement against using Moscow Mule mugs with copper interiors:
The pH of a traditional Moscow Mule is well below 6.0. This means that copper mugs that have a copper interior may not be used with this beverage. However, copper mugs lined on the interior with another metal, such as nickel or stainless steel, are allowed to be used and are widely available.”
Copper is commonly used for kitchenware like pots and pans, but it can be unsafe. Foods that are acidic (pH lower than 6.0) can corrode the copper and cause the metal alloys to mix in the food or liquids.
The problem is that in recent years, copper mugs have become a trendy vessel for Moscow Mule cocktails and other mixed alcoholic beverages, where a common ingredient is highly-acidic fruit juice.
Lime juice has a pH around 2.0, which is easily acidic enough to dissolve small amounts of copper from the mug into your drink.
High levels of copper are poisonous and can cause food poisoning. The symptoms of copper poisoning include vomiting, abdominal pain, diarrhea, or jaundice (yellowing of the skin and eyes). The long-term effects of copper poisoning also include liver damage or cirrhosis.