Vacuum flask cooking was introduced to the Asian market in the mid-1990s. The vacuum cooker (燜燒鍋), often called a thermal cooker in English, 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.
Vacuum flasks appeal to Cantonese cooks because many Cantonese dishes 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 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, 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 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 bacterial infection, 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.
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