I have what I believe to be a copper pot.  At least it has copper on the outside.  I've cooked pasta in it many times and it works great.

However, once I kept the leftover pasta in the pot and put it in the fridge.  A day or two later when I took the pasta and pot out of the fridge, the pasta was discolored…I think it was green.  But the pasta smelled fine, and certainly gave no indication that it was spoiled.

Is this some sort of a chemical reaction?  Is it dangerous?  Is there something I can do to prevent this?

--Daniel

Copper pots that are not just for show are copper on the outside, but lined with another metal on the inside.  The reason for this is that copper itself is toxic.  According to McGee, the issue isn't necessarily the amount of copper taken into the diet as the fact that the human body has a limited capacity to excrete copper so that if the intake is higher than what can be eliminated, copper will build up in the the body.   Copper toxicity causes nausea, vomiting, abdominal pain, metallic taste, and diarrhea. Ingestion of large doses may cause stomach and intestine ulceration, jaundice, and kidney and liver damage.  Copper is also needed by the body in small amounts for enzyme production.  The USDA dietary guidelines for copper are a Recommended Daily Allowance (RDA) of 900 micrograms and a maximum daily upper limit of 10 milligrams.

The pot you have is more than likely copper lined with tin.  Stainless steel is also sometimes used to line copper pots, though less commonly.  Don't confuse this, though, with stainless steel pots that have a copper coating on the outside.  Tin is most commonly used to line copper, although it is prone to wear, has a low melting point and shouldn't be used with acidic foods.

The fact that leftover pasta cooked in the pot has turned green suggests that the tin lining on your pot is scratched or worn through, and that some of the copper is leeching into the cooking water.  As the pasta sat afterward, the copper reacted with chemicals in the food, most likely sulfur compounds, causing the characteristic green tint.

If you have tin lined copper pots, they should be inspected frequently for scratches or areas of wear that will allow the food being cooked, or the cooking liquid, to come into direct contact with the underlying copper.  If the lining looks worn or deeply scratched, check with kitchen supply stores or on the Internet for re-tinning services.

Metal utensils should never be used in copper pots lined with tin, as the tin scratches easily.  Only use plastic or wooden utensils.  Avoid overheating copper pots or placing them empty on the stovetop to preheat, as this can cause the tin lining to melt.  Wash by hand using a mild dish detergent, never an abrasive cleaner or scouring pad, as these can wear down the lining, too.  To clean the outside, use a commercial copper cleaner following instructions, or a soft cloth dipped in a paste made with vinegar and table salt.  As mentioned earlier, don't use tin lined copper pots to cook acidic foods such as tomato based sauces.  And, as you might have guessed, don't use copper pots or bowls, lined or unlined, to store food.

Copper pots or pans that are not lined with tin or stainless steel are for decorative use only and should not be used for cooking.  The one exception is that some cooks prefer to whisk egg whites in a copper bowl as it helps produce firmer peaks.  The small amount of copper that enters food through this application is not sufficient to be considered a concern.

As for the safety of the pasta you cooked, it is hard to say.  A small amount of copper is useful in the diet, but how much is in the pasta is anyone's guess, as is the amount that might have already entered your body through other foods cooked in this pan.  The first rule of safety in the kitchen is "When in doubt, throw it out!"  That's what I would do with the pasta in this case.