COPPER IN THE KITCHEN
Concerns about Moscow mule brought to mind an old recipe for whipping egg whites
I have to admit that I had never heard of a “Moscow mule” until I came across this warning in the Washington Post: “Heads up, Moscow mule lovers: That copper mug could be poisoning you.” It turns out that the reference is to a concoction of vodka, ginger beer, lime juice and ice cubes that is traditionally served in a copper mug, probably to keep the beverage cold longer.
Why “Moscow mule?” Nobody really seems to know, but the conjecture is that vodka is associated with Moscow and that the drink gives a kick like a mule. In any case, the concern raised by the article is that acids in the drink can react with the mug and result in the leaching of copper compounds into the beverage that “could be poisoning you.”
Copper can indeed be toxic. But copper is also an essential nutrient, required by numerous enzymes in our body that are responsible for maintaining health. Whether it harms or heals depends on the dose.
We need an intake of roughly one to two milligrams a day, an amount easily met because copper compounds occur naturally in soil and water. Virtually everything we eat or drink contains trace amounts of this element. There is further exposure from water cruising through copper pipes and even from beer that is traditionally brewed in copper vessels to furnish yeast with the copper it requires for growth.
Extensive research has shown that our daily intake of copper from all sources is about two milligrams per day, well below the 10 mg per day that can cause toxic effects.
The extent of leaching from copper containers depends on the acidity of the liquid with which the metal is in contact. Unfortunately, it seems nobody has actually measured the copper content of a serving of Moscow mule, but it is known that pure water stored for 16 hours in a copper vessel results in a concentration of 0.2 mg of copper per litre. Even if we assume that an acidic beverage like Moscow mule leaches 10 times more copper, that would only result in about 0.6 mg per 300 mL (10 ounce) serving. And of course the contact time would be nowhere near 16 hours, meaning that we would be looking at a great deal less than that, nowhere near the 10 mg per day where toxic effects may occur.
By this rather reasonable estimate, we can conclude that the amount of copper in a serving of Moscow mule is hardly likely to poison you. In a bizarre twist, you can hop on the internet and find a plethora of baseless advice to optimize health by drinking from copper mugs.
All this contemplation about leaching copper did something for me, though. It triggered a childhood memory, one of a cherryspeckled pastry, in the angel food cake family, that my mother used to make. The recipe that was handed down to me describes beating egg whites into a firm foam before blending in sugar, flour, egg yolks and fresh cherries. And the beating of the egg whites had to be done in a copper bowl! Why a copper bowl? Some interesting chemistry here.
A foam is basically a stabilized dispersion of gas bubbles in a liquid or solid. Beating egg whites causes the proteins they contain to unfold and join together to form a stretchable layer around the air bubbles that are introduced by whisking. This prevents the bubbles from coalescing. Recipes that recommend copper bowls for whipping the whites actually date back some 250 years, long before there was any understanding of what was going on at the molecular level. Julia Child was big on copper bowls.
It wasn’t until the 1980s that food writer Harold McGee initiated experiments to determine what was going on. Using standard beating techniques, his experiments confirmed that egg whites whipped better in copper bowls than in glass bowls and the foam was more stable when sugar and flour were added, as required by recipes.
To prove that it was indeed dissolved copper that did the trick, further research showed that the addition of copper chloride to egg whites being whipped in a glass bowl achieved the same result as if they were beaten in a copper bowl. It wasn’t a case of any metal; iron did not do the trick.
Evidence suggests that copper ions form complexes with conalbumin, one of the main proteins in egg whites, basically making the protein film around the air bubbles more stable. The foam then becomes strong enough to withstand the assault of heavy sugar and flour particles being dropped on it. And it really does! I can confirm that, because I actually did the experiment. I whipped the egg whites in a glass bowl and in a copper bowl and the foam in the copper bowl was decidedly more firm.
Copper bowls are not cheap, so if you do not own one, there is another trick that works. Just add a small amount of lemon juice or cream of tartar before whipping. This slight increase in acidity causes the long protein molecules to unravel more effectively, which in turn allows stronger interaction between them, and that is just what is needed to form a network around air bubbles. What about the leaching of copper from the bowl? Based on the numbers from my Moscow mule estimates — totally insignificant.
For anyone still worried about being poisoned by drinking Moscow mule from a copper mug, you can just use a mug that is lined inside with tin or steel, although aficionados claim that the taste is not the same.
I would be willing to give that test a try without any concern for being poisoned by the copper. Actually, if there were to be any concern, it would be about alcohol, a chemical that is a proven carcinogen.
Copper bowls are not cheap, so if you do not own one, there is another trick that works.