A self-heating can is an enhancement of the common food can. Self-heating cans have dual chambers, one surrounding the other. The inner chamber holds the food or drink, and the outer chamber houses chemicals that undergo an exothermic reaction when combined. When the user wants to heat the contents of the can, they pull a ring on the can that breaks the barrier separating the chemicals in the outer chamber. After the heat from the reaction has been absorbed by the food, the user can enjoy a hot meal or drink.
Self-heating cans offer benefits to campers and people without access to a microwave oven, stove or camp-fire, but the technology is not yet common. This is because self-heating cans are considerably more expensive than the conventional type, and also have problems with uneven heating of their contents.
Self-heating tins started being produced around 1900 for use by mountaineers and explorers. Hiram Bingham used self-heating cans produced by the Silver's and the Grace Brothers firms during travels in 1909-1915.
In 1910, aerial pioneer Alan R. Hawley reported that, on the flight of the balloon America II they had taken "three cans of soup, self-heating with lime".
In 1941 a New York Times food column reported:
The Self-heating cans also appeared in the second World war British rations.
In 1947, the same column reported "Food in Self-Heating Cans Reappears" (their having been reserved for the military during the war). Referring to the cans as "Hotcans," the columnist noted that "Chocolate is made with milk and is delicious (65 to 72 cents). Four hamburgers in tomato sauce with mushrooms are small but good, and the sauce is ample (89 to 98 cents). Coffee tastes something like the instantly brewed type, leaving something to be desired (49 cents).. (49 cents in 1947 is approximately equivalent to $4.64 in 2005).
The technology reappeared in the USA again in 2006, when Ontech corporation released a line of self-heating drinks (such as hot cocoa and hot coffee) which use a mixture of quicklime and water to generate heat.
The source of the heat for the self heated can is an exothermic reaction that the user initiates by pressing on the bottom of the can. The can is manufactured as three containers. A container for the beverage surrounds a container of calcium oxide (quicklime) that is separated from a container of water by a thin breakable membrane. When the user pushes on the bottom of the can, a rod connected there pierces the membrane, mixing the water and calcium oxide. The resulting chemical reaction releases heat (the heat of hydration of calcium oxide) and thus warms the beverage above it.
Another way, though less effective, is copper sulfate and zinc, no gas is produced by this reaction and zinc sulfate and copper is left. Not much heat is produced by this reaction, therefore it is only good if there is a large amount of the reactants and little coffee.
The Hot-Can company in Malaysia has developed and launched in Australia in 2008 a self-heating can that is commercially viable by heating up in less than 3 minutes, with an increase in temperature of 50 - 55°C in a normal aluminum size beverage can that fits any standard filling line and at a retail price that should make it widely accepted.