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Method The pot is filled with food and water and heated to cooking temperature outside of the flask on a stove, usually to boiling. It is then sealed inside the vacuum flask for several hours. The flask minimises heat loss, keeping the food hot enough to continue cooking and avoid bacterial growth for many hours.
Advantages Long cooking means more tender meat and allows cooking of beans, lentils, and brown rice. Minimizes fuel, energy use, CO₂ emissions Saves water, less evaporation. Saves food — no burning, no cleanup. Keeps flavour and nutrients in. Convenient — cooks while you are at work or sleeping or travelling. Can take travelling or to picnics. Reduces smoke, odor, humidity, grease buildup in kitchen. Easy cleanup. Safer — no power cord, the outside is not hot, spill-proof, reduces injuries. Reduces toxic fumes, which means less respiratory problems and other diseases, particularly in children.
Precautions 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. Some foods, such as kidney beans, fava beans, and many other varieties of beans contain a toxin, phytohaemagglutinin, that requires boiling at 100 °C for at least 10 minutes to break down to safe levels. The best practice is to bring briefly to a rolling boil then put the pot in the flask. This keeps it hottest longest. With big chunks of food, boil a little longer before putting into the flask.
Variants Variants of thermal cooking, though a better name would be "heat retention" cooking:
Wonderbag is an insulated bag to put around pots you already own. Haybox cooking uses hay or sawdust to provide the insulation around the pot. A different kind of vacuum cooker is used in the candy manufacturing industry to cook candies at low air pressures.