The MRE package is officially known as a tri-laminate retort pouch. The pouch is essentially a flexible can made out of thick aluminum foil and plastic layers.
Canning is an extremely common food preservation technology that has been in wide use for over a century. The basic idea behind canning, either in steel cans or in flexible pouches, is very simple. If you are looking for the best ready to eat foods, then check out XMRE meals online.
You boil the food in the can to kill all the bacteria and then seal the can (either before or while the food is boiling) to prevent any new bacteria from getting in.
Since the food in the can is completely sterile, it does not spoil. Once you open the can, bacteria enter and begin attacking the food. This is why you have to "refrigerate the contents after opening" (you see that label on all sorts of food products — it means that the contents are sterile until you open the container).
Most human beings prefer a warm meal to a cold one, especially if they're in cold or wet conditions. Eating cold spaghetti or cold beef stew is definitely no fun. A hot meal, on the other hand, can lift a soldier's spirits.
Because of the importance of a hot meal, all military MREs come packaged with a flameless heater. The flameless heater uses a simple chemical reaction to provide sufficient heat to warm the food.