Copper mugs are popular for some cocktails, but you may want to think twice before using one.

Iowa's Alcoholic Beverages Division has released an advisory bulletin on copper mugs and alcohol. It notes that the Food and Drug Administration (FDA) prohibits copper from coming into direct contact with foods that have a pH below 6.0. So when cocktails like the popular Moscow mule are served in copper, food poisoning is possible.

It's the acids that are the problem. (Anything below a pH of 7 is considered acidic.) According to the bulletin, a traditional Moscow mule — which contains vodka, fresh lime juice and ginger beer — is "well below 6.0." If a copper mug has a copper interior and it comes in contact with a beverage like that, the copper may leach into the drink. If the concentration of copper is high enough, it could cause foodborne illness. The risk is great enough that Iowa is limiting the "use of copper and copper alloys as a food contact surface." Copper and copper alloys, like brass, should not come in contact with food or liquids with a pH below 6.

The end of the copper mug tradition?

pistachio-mule A mule is still a mule even if it's in a rocks glass. (Photo: Robin Shreeves)

Traditionally, Moscow mules are served in copper mugs because the metal keeps the drink nice and cold. Does the copper problem mean we'll have to drink our mules a little warmer now? Not necessarily. If a copper mug's interior is lined with another metal like stainless steel, it's fine to serve beverages with a pH level below 6.0 in them.

If a 100 percent copper mug is the only option, though, it would be safer to drink this refreshing vodka drink out of another type of drinkware, like the rocks glass that the Pistachio Ginger Mule above is served in. The cocktail will taste just as good no matter what you're drinking it from.

Robin Shreeves ( @rshreeves ) focuses on food from a family perspective from her home base in New Jersey.