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Thermal cooking, also called vacuum flask cooking or fireless cooking, is a technique where an insulated chamber is used to slowly cook a pot of food without a heat source over the course of several hours.
All varieties of thermal cooking follow the same basic method. A pot is filled with food and water and heated to boiling on a stove or other source of heat. The pot is then sealed inside an insulating compartment for several hours. The insulation minimises heat loss from the pot, keeping the food hot enough to continue cooking and avoid bacterial growth for many hours without an external heat source.
A variety of different insulating mechanisms are available to facilitate thermal cooking. Vacuum flask-type cookers typically consist of a vacuum-insulated flask with a removable pot insert. The pot insert can be used on the stovetop to heat the food before sealing in the insulated flask. Some types are designed to reflect infrared heat back at the internal pot, effectively reducing heat loss. A wonderbag is a type of insulated bag that can be used to enclose most pots a cook already owns—unlike the vacuum sets, any pot can be used. Similarly, a historical insulating mechanism called a haybox used hay or sawdust to insulate the cooking pot.
The long, slow cooking possible with thermal cooking or a slow-cooker results in more tender meat, and it easily allows cooking of beans, lentils, and brown rice. However, the main advantages of thermal cooking compared to using a slow-cooker include minimized fuel usage, convenience and ease of transportation, and minimized risk of burning.
Certain precautions should be taken when using the thermal cooking technique.
Because thermal cooking does not use a source of external heat during the insulation stage, there is a risk of the food cooling and entering the "danger zone" (i.e. about 40–140°F or 4–60°C). If the food spends more than 4 hours in the danger zone, bacteria can proliferate and cause food poisoning. As such, it is essential to heat food sufficiently before removing the external heat source, sealing the pot, and insulating it. Cooking the entire pot at above 60°C (140°F) for at least 10 minutes is sufficient to kill most pathogens of interest, effectively pasteurizing the dish before sealing and insulating it for slow cooking.
Some foods, such as kidney beans, fava beans, and many other varieties of beans contain the toxin phytohaemagglutinin. These foods need to be boiled at 100°C (212°F) for at least 10 minutes to break the toxin down to safe levels. The best practice when thermal cooking these foods consists of boiling the food for the required amount of time before sealing and insulating the pot. With big chunks of food, boil a little longer before putting into the flask.