Food cans are occasionally heated accidentally, and are sometimes heated intentionally because a recipe calls for this. For instance, one recipe for making caramel calls for simmering an unopened can of sweetened condensed milk in water for two hours. Also, many campers heat food directly in cans after opening them. However, there are dangers associated with heating tin cans and it can lead to injury or illness.
The most dangerous thing about heating a closed can is that the pressure which heating creates on the inside of the can may exceed the strength of the can and cause it to explode. Cans are designed to be strong and to withstand some ambient heating and cooling, but they are not engineered to hold up to long term or extreme heat. If a can explodes, pieces of it can fly through the air and injure anyone nearby.
The containers that are called "tin cans" are made of many different materials, and some cans have a plastic coating which is meant to keep the food from reacting with the metal of the can. If heated, this plastic can melt into the food and contaminate it. One of these plastics, bisphenol A, is considered by some researchers to be dangerous to human health. Although the Food and Drug Administration's official position is that BPA is not dangerous to adults, there is still research being done on this topic and several scientists openly oppose use of BPA in food containers which might lead to accidental human consumption.
A heated can may be dangerous to handle. The metal exterior of a can will easily burn a person's skin, and if you are heating a can while camping you may not have the cooking equipment to take a hot can off a fire safely. The fact that cans do not have handles makes it more likely that they will be dropped or spilled. And if you use something other than a pot holder, like a rag or loose clothing, to move the can it could cause a fire.
Although not as serious as the other risks associated with heating a can, if a can cracks or ruptures it may be a problem. If a can is heated in water and it develops a crack, all of the food inside will be mixed with water. If it cracks when directly exposed to a heating surface, it will ooze out food that will probably burn and possibly make a mess of whatever is near it.
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