The vacuum cooker is a stainless steel vacuum flask also known as a thermal cooker. The flasks come in various sizes ranging from 20-40 cm (8-16 in) in diameter and 25 cm (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.
Vacuum flasks appeal to cooks because many dishes require prolonged braising or simmering. The slow cooker is used for a similar purpose; but instead of minimizing heat loss, sufficient heat is applied to the non-insulated slow cooker to maintain a steady temperature somewhat below the boiling point of water. A slow cooker allows any desired cooking time; the more energy-efficient vacuum flask must cook within the time taken for the food to cool below cooking temperature.
The historic equivalent of the vacuum cooker is the hayboxes, nowadays perhaps using more modern insulating material than the original hay or straw. This works on the same principle but has much poorer heat retention. The pot is filled with food and heated to cooking temperature; it is then sealed inside the vacuum flask for several hours; the flask minimizes heat loss, keeping the food hot enough to continue cooking.
A vacuum flask cooker cooks food expending only enough energy to heat the pot and contents to cooking temperature; the food is cooked by the stored heat, as the flask drastically reduces heat lost to the environment. 10 to 20 times less energy is required to cook this way than using conventional methods without thermal insulation.
Cooking this way is convenient for people with little free time, and activities away from home. For example, a domestic user may prepare a meal, then spend many hours away from home—working at a job, for instance—returning to food ready to eat.
A large part of the cooking time is spent at a temperature which allows bacteria to grow in the food, with a danger of food poisoning due to bacterial infection, or toxins produced by bacteria. It is essential to heat food to a safe temperature before vacuum cooking—60 °C for at least 10 minutes will kill most temperature-sensitive bacteria. The food is then safe to cook at below safe temperature for several hours.
Most slow cooker or crock pot recipes can be adapted easily to use in a vacuum cooker. A good source of recipes for all retained heat cookers can be found on this weblog: http://thermalcooker.wordpress.com/category/recipes-all/