Vacuum flask cooking

A thermal cooker, or a vacuum flask cooker, is a cooking device that uses thermal insulation to retain heat and cook food without the continuous use of fuel or other heat source. It is a modern implementation of a haybox, which uses hay or straw to insulate a cooking pot.

The earliest known thermal cooker dates from the Medieval period in Europe. After heating over a fire, a hot, earthenware pot containing food was placed in another, larger pot, box or hole in the ground, insulated by hay, moss, dry leaves or other material, and covered. The heat conserved within would slowly cook the food inside, saving fuel and work. An example of this type of cooker was found in Wales by a Monmouth Archaeological Society excavation.

Medieval instructions for cooking without fire taken from an Anglo-Norman manuscript in the British Library:

Take a small earthenware pot, with an earthenware lid which must be as wide as the pot, then take another pot of the same earthenware, with a lid like that of the first; this pot is to be deeper than the first by five fingers, and wider in circumference by three; then take pork and hens and cut into fair-sized pieces, and take fine spices and add them, and salt; take the small pot with the meat in it and place it upright in the large pot, cover it with the lid and stop it with moist clayey earth, so that nothing may escape, then take unslaked lime, and fill the larger pot with water, ensuring that no water enters the smaller pot; let it stand for the time it takes to walk between five and seven leagues and then open your pots, and you will found your food indeed cooked.

In the mid 1990s steel thermal cookers were developed in Asia, consisting of two stainless steel pots, one within the other. The inner pot is used to bring the food to the boil and the insulated outer pot is used as the container to retain heat and continue the cooking process. Some use insulation material between the outer pot walls, while others use a vacuum.

A vacuum cooker (Chinese: 燜燒鍋) is a stainless steel vacuum flask. The flasks come in various sizes ranging from 20 to 40 centimetres (8 to 16 in) in diameter and 25 centimetres (10 in) tall. A removable pot, with handle and lid, fits inside the vacuum flask. The pot and contents are heated to cooking temperature, and then sealed in the flask. The flask simply reduces heat loss to a minimum, so that the food remains at cooking temperature for a long time, and cooks without continued heating. Note that the food is not cooked in a vacuum. It is cooked inside a vacuum flask. The hollow evacuated wall of the cooker thermally insulates its contents from the environment, so they remain hot for several hours.

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