Freshly grated parmesan cheese - turns blue in a copper bowl?

April 3, 2013 12:28 AM

My SO and I were having some italian-themed chicken wings, and had some fresh-grated parmesan cheese on the side in a copper bowl. After our meal, we were cleaning up and transferring the leftovers to the fridge for storage, and the cheese on the bottom of the bowl had turned a light blue. Does anyone know why this happened? Given that it happened over only a few hours to just the cheese in contact with the bowl, I'm thinking it was some kind of reaction with the copper.
posted by mikurski to Food & Drink (10 answers total)

Best answer: Cu2+ + 2NH3 + 2H2O --> Cu(OH)2 + 2NH4+

Copper + ammonia --> [Cu(NH3)4(H2O)2]2+
posted by sebastienbailard at 12:39 AM on April 3, 2013 [1 favorite]

Best answer: What Sebastienbaillard said, but I don't think that was the page he meant to link to. Try this one.
posted by Kid Charlemagne at 1:16 AM on April 3, 2013 [1 favorite]

Best answer: My money would be on a form of verdigris, specifically copper (II) acetate.

Parmesan contains quite a lot of acetic acid (whether from addition as an acidifying agent or a result of natural fermentation I have no idea), which makes it seem plausible.

As a side note, I'm pleased to see that it also contains a lot of butanoic acid (aka butyric acid), because that justifies my unpopular assertion that, to me, fresh parmesan smells a lot like vomit.
posted by metaBugs at 1:38 AM on April 3, 2013 [2 favorites]

I was literally JUST (5 minutes ago!) reading how copper in water can react with certain laundry detergents to stain clothes blue. So the chemistry people can respond in more detail, but apparently "copper + certain other stuff = blue" is actually a thing.
posted by celtalitha at 1:48 AM on April 3, 2013 [1 favorite]

I understand that Roquefort is made blue by inserting copper wires in to it. There's that.
posted by taff at 3:16 AM on April 3, 2013

Bottom line is that it's a bad idea for any number of reasons (with a few notable exceptions) to cook or store something you're going to eat in unlined copper.
posted by slkinsey at 5:24 AM on April 3, 2013 [4 favorites]

I understand that Roquefort is made blue by inserting copper wires in to it. There's that.

It is highly unlikely that the blue colour comes from the copper, since high levels of copper in food are harmful (in fact, cooking in unlined copper is not recommended). My understanding was that the colour comes from the mould.
posted by firesine at 6:39 AM on April 3, 2013 [5 favorites]

The blue in blue cheeses such as roquefort is not a result of the wires being any particular metal, but from the mold (p. roqueforti) which grows in the cheese once those wires have opened its interior to oxygen. The development of this mold requires a rather wet environment unlike that of parmegiano, and rather more time than is the case here.

OP, I don't think that's what is going on here. I'd ditch the cheese because it sounds to me like a reaction with the copper like everyone else is saying.
posted by gauche at 6:50 AM on April 3, 2013

Best answer: In general it would be a good idea not to use that bowl for food. In this case, you saw the result, but in other cases where the copper is leached into the food because of acidic reactions, you might not. Copper cookware is generally lined with stainless steel or tin for this reason.
posted by beagle at 7:57 AM on April 3, 2013 [2 favorites]

Response by poster: Thanks for the feedback! We're going to use the bowls for decorative, non-food-related things.
posted by mikurski at 9:12 PM on April 3, 2013