Before you set out to test the warming limitations of foil on hot food, you must first consider some safety limits. Foil or any other food storage tool should not be used to keep food warm outside of refrigeration for more than two hours. Once this time limit is reached, food becomes a breeding ground for bacteria.

Once food reaches 45 degrees Fahrenheit, it becomes hospitable for bacteria that feed on food and reproduce. You won't be able to see the lysteria, staph or botulinum bacteria, but you will know about the infection when the first symptoms of food poisoning -- clammy skin and vomiting -- occur. There is no way to know when the bacteria settle, so put the foil-covered food into the refrigerator after it sits out for two hours or reaches 45 degrees Fahrenheit.

For short-term warmth, you can use foil, but it will not keep the food warm for long. Foil conducts heat, letting it pass through to outside of the food. The foil does not insulate or hold heat in without help. Use a layer of plastic wrap outside the foil or put the foil-covered food into a plastic freezer bag. They are thicker than the general storage variety of plastic bags. The plastic layer will help the foil to hold in the heat longer than a half-hour.

Instead of foil, use insulated plastic containers to keep foods warm for up to two hours, a feat that foil could not achieve alone. You can also store multiple foil-wrapped foods in an insulated place like a cooler. The cooler's dense insulation system will hold in the heat leaking out from the foil-wrapped foods. At the two-hour mark, throw out any food that hasn't been eaten. You may have to do this sooner than two hours if the food's temperature dips to 45 degrees Fahrenheit or below.