Vacuum flask cooking
Vacuum flask cooking was introduced to the Asian market in the mid-1990s. The vacuum cooker (燜燒鍋), often called a thermal cooker
Thermal cooking
A modern thermal cooker uses the concept of the Haybox whereby placing hay or straw around a cooking pot of heated food the meal continues to cook without fuel.-History:...

 in English, is a stainless steel vacuum flask
Vacuum flask
A vacuum flask is an insulating storage vessel which keeps its contents hotter or cooler than its surroundings. Invented by Sir James Dewar in 1892, the vacuum flask consists of two flasks, placed one within the other and joined at the neck...

. The flasks come in various sizes ranging from 20 to 40 cm (7.9 to 15.7 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.

Vacuum flasks appeal to Cantonese cooks because many Cantonese dishes
Cantonese cuisine
Cantonese cuisine comes from Guangdong Province in southern China and is one of 8 superdivisions of Chinese cuisine. Its prominence outside China is due to the great numbers of early emigrants from Guangdong. Cantonese chefs are highly sought after throughout the country...

 require prolonged braising or simmering. When these cookers were first introduced in the US, they sold very quickly in the larger Asian supermarkets. The slow cooker
Slow cooker
A slow cooker, also known as a Crock-Pot or Slo-Cooker , is a countertop electrical cooking appliance that is used for simmering, which requires maintaining a relatively low temperature compared to other cooking methods for many hours, allowing unattended cooking...

 is used for a similar purpose; but instead of minimising 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 historical equivalent of the vacuum cooker is the haybox
A hay box, straw box or fireless cooker is a cooker that utilizes the heat of the food being cooked to complete the cooking process. Food items to be cooked are heated to boiling point, and then insulated. Over a period of time, the food items cook by the heat captured in the insulated container...

, 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
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

; it is then sealed inside the vacuum flask for several hours; the flask minimises 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.


If a large part of the cooking time is spent at temperatures lower than 60 °C (as when the contents of the cooker are slowly cooling over a long period), a danger of food poisoning due to bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

l infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...

, or toxins produced by multiplying bacteria, arises. It is essential to heat food sufficiently at the outset of vacuum cooking. 60 °C throughout the dish for 10 minutes is sufficient to kill most pathogens of interest, effectively pasteurizing the dish. It is then safe to cook at lower temperature for many hours, however, the bacteria which causes deadly botulism, Clostridium botulinum, is widespread and can survive boiling and will multiply in certain foods at room temperatures in a sealed container, so foods that are not subjected to autoclaving or pressure cooking at temperature of 121 °C (250 °F) for at least 3 minutes should be assumed to contain live Clostridium botulinum and if this food reaches temperatures of 3 °C to 43 °C (38 °F to 110 °F) it should be assumed to contain Clostridium botulinum multiplying and producing deadly botulin toxin. Thus in vacuum flask cooking before the food cools to 43 °C (110 °F) it should be removed from the sealed insulated container and eaten or immediately refrigerated. If there is any question of the possibility of botulinum toxin, the food should be reheated to 80 °C (176 °F) for 20 minutes to deactivate the toxin, noting, this does NOT kill the spores producing the toxin. Pasteurization is a process for killing many heat sensitive bacteria such as the ones that sour milk, but it is ineffective on Clostridium botulinum bacteria spores.


  • A different kind of vacuum cooker is used in the candy manufacturing industry to cook candies at low air pressures.

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