system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't contaminated metal be recycled? Wouldn't the contaminants be burned off when the metal is melted down?

I was looking at recycling resources, and it said that aluminum cans are easily recycled, but aluminum foil is usually contaminated with food and can't be easily recycled. But wouldn't any of the food be completely destroyed when it's at the temperature aluminum melts at? 
I tried to find explanations, but they just said contaminates lower the value of metal and not the reason why.
gpt
Think of what happens when you burn a pizza in the oven, or you're cooking a hot dog over a campfire and it falls into the fire and you just leave it there. It doesn't just disappear/get destroyed, it undergoes combustion which will leave behind carbon and other trace minerals in solid form. Carbon's melting point is much higher than aluminum's, and even if it weren't you'd still need to separate the carbon/other trace elements from the aluminum if you wanted pure aluminum. More leftover food, more leftover carbon/other trace minerals, less pure aluminum.

That, plus leftover food waste can help gum up sorting machines.