Vacuum flask cooking is an invention introduced to the Asian market in the mid 1990s. The vacuum cooker (燜燒鍋) is a stainless steel vacuum flask. 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, is inside the vacuum flask.
Food is cooked in the pot on a regular stove at a high heat. After the food is fully cooked, the pot's lid is put on and the pot is inserted into the vacuum flask. The heavily insulated lid of the flask is closed and locked air-tight. The pot and food are left in the vacuum flask for several hours. The food continues cooking in its own heat, and stays warm.
The typical user prepares a meal in the morning, heats the meal in the pot, places the pot in the vacuum cooker and returns home after work to enjoy a hot meal. Normally, reheating is not required as the food remains hot enough for consumption after 6 to 8 hours.
The main advantages are carefree operation, and zero power consumption during the prolonged cooking process. Note however that a reduction in power consumption is due entirely to possibly greater efficency (ie, reduced loss of heat to the environment during cooking), as the same amount of heat energy is required to chemically transform uncooked food into cooked food in either case. The main difference is whether the heat escapes or goes into the chemical transformation of the food. Even given the same amount of heat energy, in the insulated environment, the prolonged chemical reaction more than cooks the food but instead breaks down the food so much to alter the texture of the dish. In order to achieve the same texture of the food, a regular stove might need to be burning for 6 hours or more. With the vacuum flask, the food is cooked on the stove for 20 minutes and then kept hot for 6 hours. The energy saving can be 10 to 20 fold.
The main disadvantage is the risk of food poisoning as the food temperature slowly decreases to levels which may allow bacteria growth in the food. The danger of food poisoning can be reduced, but not eliminated, by thoroughly cooking the food at high temperatures before putting it in the vacuum flask.
Also, it is important to buy a vacuum cooker that seals and insulates effectively, so the food temperature will be less likely to drop below a level sufficient to kill micro organisms.
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 vacuum flask approach is reminiscent of the familiar crock pot, in that food cooks unattended for extended periods. The differences are significant enough that neither is quite a replacement for the other. Haybox cooking is an earlier form of retained heat cooking.
Note that the food is NOT cooked in a vacuum. It is cooked inside a vacuum flask. The vacuum in the wall of the cooker insulates the pot, so the food in the pot remains hot over several hours. Note that a different kind of vacuum cooker is used in the candy manufacturing industry to cook candies at low air pressures. That is a different topic altogether.