Health and Medicine Question #9286

Niagara Mouse, a 67 year old n/a from Niagara-on-the-Lake asks on January 3, 2016,

Will cooking food in the metal can contaminate the food? I would like to to make ‘Dulce de Leche’, which is made by cooking Condensed Milk in its can, by placing the can in a pot of water, bringing to a boil then simmering for a couple of hours. I have read about the possibility of the can exploding and how to mitigate that, however nothing is said about heating food inside a metal can. I am concerned about that aspect of the cooking requirement. Can you comment on the possibility of the can leaching noxious chemicals into the food? I do not want to poison anyone.

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The answer

Barry Shell answered on January 3, 2016

This is what we might call a calculated risk. In my opinion the risk is extremely low, so this is something you need not worry about. However, if you want to err on the side of caution don’t make ‘Dulce de Leche’. Here are things you may wish to consider:

1. When food is canned it is heated to kill all the bacteria, so your can of evaporated milk has already been heated in the can at the time of canning. In your case you will be heating it for much longer so that is something to think about.
2. There is nothing in the metal that you need to worry about. Cans are made of steel which is mostly iron, but with trace amounts of other metals. These metals are very unlikely to come out of the steel at relatively low temperatures, such as boiling water. The alloys for metal cans are created at much higher temperatures. The amounts of toxic metals that could come out of the cooking process as you describe would be too small to be of any concern.
3. Modern cans are coated inside with a layer of epoxy resin so the food does not actually come in contact with metal. It is in contact with this thin plastic resin interior coating. About 10 years ago a compound called bisphenol-A (BPA, a component in certain plastic resins, was identified as a possible problem in cans and other plastic food containers such as water bottles. Studies showed that it could mimic certain human hormones and perhaps cause disease. Many companies have started using alternatives to BPA so this problem has diminished to some extent but it may still be a concern. You could look for brands of condensed milk that have “BPA-free” printed on the label. However, the amounts of BPA that might get into food from can linings is extremely small and is highest in acid foods such as tomatoes, not milk. Nothing was ever definitively proven about the harmful effects of BPA in humans. It *might* be harmful, but definitive large demographic studies have not been done. Right now scientists are just guessing that is could cause problems based on animal studies. Also, the chemicals that have replaced BPA may be as harmful, so who knows? Key point is: the amounts we are talking about are quite small, and unless you consumed ‘Dulce de Leche’ every day, three or four times a day in large volumes, you probably will not accumulate enough BPA to worry about. In any case you are accumulating a certain amount of BPA from many other sources in daily life such as all thermal printed cash register receipts, movie tickets, and boarding passes, etc. so skipping the ‘Dulce de Leche’ will only have a very tiny effect on your total exposure to BPA.
Do be careful about heating a sealed can, but as long as you do not allow it to get hotter than boiling water and take precautions to relieve pressure build up you should be ok. 
Bottom line: you will definitely NOT poison anyone by making ‘Dulce de Leche’. However, depending on the brand, you may be serving them extremely small quantities of a chemical (BPA) that MAY over time cause indeterminate problems. The risk is low and you have similar levels of risk with virtually any canned food.
If you want to be totally safe, just don’t do it, but if you make the ‘Dulce de Leche’ do not worry too much. I would say that regular automobile driving causes much more harm (by putting trace amounts of bad chemicals into the environment from the engine, the brake linings, and the tires) than making ‘Dulce de Leche’ once in a while.
For more reading on what scientists think about BPA check out this article in Scientific American.

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