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Getting copper out of your cocktail and petroleum off your produce

by Craig Bettenhausen
February 13, 2022 | A version of this story appeared in Volume 100, Issue 6


Cocktail copper count

A photo of a copper mug with ice and mint poking out of the top. Limes are scattered decoratively around.
Credit: Shutterstock
Moscow mule metal: The copper is classic but contaminates your cocktail.

The Moscow mule is a classic American cocktail made with vodka, ginger beer, and lime juice and traditionally served in a copper mug. Though some say the copper’s taste or thermal conductivity enhances the experience, most sources agree that the mugs are mostly a marketing shtick cooked up in the 1940s by the distiller Smirnoff to goose vodka sales.

It does look cool. But the World Health Organization sets a maximum safe ingestion limit for copper at 10 mg per day, and chemistry professors Caroline R. Pharr and John G. Rowley of Montana’s Carroll College wondered how much copper the drink’s acidic ingredients leach from the metal mugs.

So Pharr and Rowley dug into the question with a group of undergraduate students, who placed standardized Moscow mules in pure-copper mugs and measured the change in copper concentration over time using ultraviolet-visible and atomic absorption spectroscopy.

They found that it took 27 min for the copper to reach 1.3 ppm, the US Environmental Protection Agency’s threshold for copper in drinking water. A cocktail that sat for several days turned turquoise and reached 1,000 ppm. The students also observed etching on the copper surface using scanning electron microscopy and investigated a mechanism for the reaction (J. Environ. Health 2022, 84, 8).

The researchers point out that although they observed significant leaching, it would take 30 Moscow mules at 1.3 ppm to pass the World Health Organization’s safe daily intake limit. Someone drinking 30 cocktails a day probably has more pressing health concerns than excess copper.

Dutch Courage, a gin bar in Baltimore known for craft cocktails in interesting glassware, doesn’t use copper mugs of any kind for its Moscow mules, not because of copper leaching but because they do nothing for the flavor. “They are a gimmick and nuisance. Guests steal them all the time and they are like $15 per mug,” owner Brendan Dorr tells Newscripts.

Rowley says that over the course of the project, his students prepared more than 100 Moscow mules, which, sadly, were disposed of like any other chemical experimental waste. But the mixology experience wasn’t wasted. “I suspect that outside of lab, our seniors were also preparing Moscow mules,” Rowley says. “In fact, the year many of our coauthors graduated, the cocktail served at our senior dinner celebration was a Moscow mule.”


Biobased banana barrier

Photos of bananas; the ones labeled as having the coating are less overripe.
Credit: Empa
Fresh fruit: An edible cellulose coating from Empa extends the shelf life of produce.

A steel liner can keep the copper out of cocktails, but that’d be impractical for preserving produce. Researchers at the Swiss Federal Laboratories for Materials Science and Technology (Empa) are developing a cellulose coating for bananas and cucumbers to replace the plastic wrap and petroleum wax commonly used today. The coatings keep water in and oxygen out, prolonging shelf life and reducing food waste.

The Empa team makes the coating from carrot pulp left over from juice making. The researchers dry, bleach, and finely grind the cellulosic material, then suspend it in water and spray it on the produce.

Empa is testing the edible cellulose coating with the international grocery chain Lidl. In early results, the coating extended the shelf life of bananas by a full week. If all goes well, Lidl hopes to roll out the tech in all its 150 Swiss stores.

Gustav Nyström, who leads the cellulose and wood materials laboratory at Empa, says that biobased materials may not be able to beat conventional options, like plastic and wax, at keeping water vapor in and oxygen out. “On the other hand, there are other well-known environmental concerns with these materials and this extremely high performance is not always needed,” he tells Newscripts in an email. “Our hypothesis is that with our materials we will have a good enough performance for some of these fruits and vegetables that we are considering while at the same time being renewable, biodegradable, and safe.”

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