Thermal cooking

History

The earliest known thermal cooker was used in medieval Europe. A hot earthenware pot of food, heated over a fire, was placed inside a larger container or a hole in the ground, insulated with hay, moss, or leaves, and covered. An example was unearthed in Wales by the Monmouth Archaeological Society. An Anglo-Norman manuscript gives a cooking time of roughly five to seven hours.

Modern thermal cookers

In the mid-1990s steel thermal cookers were developed in Asia, consisting of two stainless steel pots, one within the other. The inner pot is used to bring food to the boil, and the insulated outer pot retains heat to continue cooking. Some use insulation material between the outer pot walls, while others use a vacuum. A vacuum cooker is a stainless steel vacuum flask, ranging from 20 to 40 cm in diameter and 25 cm tall, with a removable pot that fits inside.

The pot and contents are heated to cooking temperature and then sealed in the flask, which reduces heat loss so food remains hot for several hours. Thermal cookers appeal to Cantonese cooks for prolonged braising or simmering. They sold quickly in larger Asian supermarkets when first introduced in the US. Unlike slow cookers, which apply steady heat, vacuum flasks cook within the time food takes to cool below cooking temperature.

Models with two inner pots allow cooking two items simultaneously, such as curry and rice. All thermal cookers can prepare many dishes, from soups to puddings; cakes and bread can be cooked by partly submerging the tin in boiling water.

Precautions

If a large part of cooking time is spent below 60 °C, there is a danger of food poisoning from multiplying bacteria. Heating food to 60 °C throughout for 10 minutes pasteurizes most pathogens. Some beans contain phytohaemagglutinin, requiring boiling at 100 °C for at least 10 minutes. Best practice is to bring food to a rolling boil briefly, then place the pot in the flask; boil larger chunks a little longer before sealing.

Did You Know?

• A variant called the Wonderbag is an insulated bag placed around existing pots.
• Sous-vide cooking involves cooking at temperatures under boiling, usually in a plastic bag.